Level: Medium – Design a circuit with two photodiodes in a differential configuration to detect the direction of the highest intensity light source.

Objective and use case

You will build a directional light-sensing circuit that uses two reverse-biased photodiodes and an operational amplifier acting as a voltage comparator. By measuring the difference in light intensity between the two sensors, the circuit determines which side is receiving more light.

Why this circuit is useful:
* Maximizing solar panel efficiency by keeping them aimed directly at the sun.
* Enabling autonomous robots to seek out light sources for navigation or charging.
* Automating smart home systems, such as blinds or awnings, to react to direct sunlight direction.

Expected outcome:
* A measurable differential voltage representing the light imbalance between the two sensors.
* Reverse currents through each photodiode strictly proportional to the light hitting them.
* A distinct switching threshold on the operational amplifier’s output based on which sensor yields a higher voltage.
* An LED indicator that clearly illuminates when the left sensor receives more light than the right sensor.

Target audience and level: Intermediate electronics students learning about analog comparators, optoelectronics, and differential measurement.

Materials

• V1: 5 V DC supply
• D1: BPW34 photodiode, function: left light sensor (reverse-biased)
• D2: BPW34 photodiode, function: right light sensor (reverse-biased)
• R1: 100 kΩ resistor, function: D1 load (current-to-voltage conversion)
• R2: 100 kΩ resistor, function: D2 load (current-to-voltage conversion)
• U1: LM358 operational amplifier, function: voltage comparator
• R3: 330 Ω resistor, function: LED current limiting
• D3: red LED, function: left-direction indicator

Wiring guide

• V1 connects between VCC and 0.
• D1 connects between VCC (cathode) and VL (anode).
• R1 connects between VL and 0.
• D2 connects between VCC (cathode) and VR (anode).
• R2 connects between VR and 0.
• U1 positive power supply pin connects to VCC.
• U1 negative power supply pin connects to 0.
• U1 non-inverting input (IN+) connects to VL.
• U1 inverting input (IN-) connects to VR.
• U1 output connects to node VOUT.
• R3 connects between VOUT and VLED.
• D3 connects between VLED (anode) and 0 (cathode).

Conceptual block diagram

Schematic

VCC --> [ D1: BPW34 Left ] ---(Node VL)--> [ R1: 100 kΩ ] --> GND
                                  |
                                  +-----(IN+)-----> [             ]
                                                    [ U1: LM358   ]
                                                    [ Comparator  ] --(VOUT)--> [ R3: 330 Ω ] --(VLED)--> [ D3: Red LED ] --> GND
                                  +-----(IN-)-----> [             ]
                                  |
VCC --> [ D2: BPW34 Right ] --(Node VR)--> [ R2: 100 kΩ ] --> GND

Electrical diagram

🔒 This electrical diagram is premium. With the 7-day pass or the monthly membership you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Measurements and tests

1. Reverse Current Verification: Measure the DC voltage drops across R1 and R2. Calculate the reverse photocurrent using Ohm’s Law ($I = V/R$). Ensure the current increases linearly as you move a flashlight closer to the respective photodiode.
2. Differential Voltage Measurement: Place a multimeter probe on VL and the other on VR. Shine a light evenly between both sensors; the differential voltage should be near 0 V. Move the light to the left, and the differential voltage should become positive. Move it to the right, and it should become negative.
3. Switching Threshold Observation: Slowly move a light source from right to left across the sensors. Monitor VOUT with a multimeter or oscilloscope. The output should sharply transition from near 0 V (Low) to roughly 3.5 V–4 V (High) precisely when VL > VR.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Optical sensor for a solar tracker
.width out=256

* Power Supply
V1 VCC 0 5V

* Left Light Sensor (D1 and load R1)
* D1 is reverse-biased. I1 simulates the photocurrent generated by light exposure.
D1 VL VCC BPW34
I1 VCC VL PULSE(1u 20u 0 1u 1u 50u 100u)
R1 VL 0 100k

* Right Light Sensor (D2 and load R2)
* D2 is reverse-biased. I2 simulates the photocurrent generated by light exposure.
D2 VR VCC BPW34
I2 VCC VR PULSE(2u 21u 0 1u 1u 100u 200u)
R2 VR 0 100k

* Voltage Comparator (LM358)
XU1 VL VR VCC 0 VOUT LM358
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Optical sensor for a solar tracker
.width out=256

* Power Supply
V1 VCC 0 5V

* Left Light Sensor (D1 and load R1)
* D1 is reverse-biased. I1 simulates the photocurrent generated by light exposure.
D1 VL VCC BPW34
I1 VCC VL PULSE(1u 20u 0 1u 1u 50u 100u)
R1 VL 0 100k

* Right Light Sensor (D2 and load R2)
* D2 is reverse-biased. I2 simulates the photocurrent generated by light exposure.
D2 VR VCC BPW34
I2 VCC VR PULSE(2u 21u 0 1u 1u 100u 200u)
R2 VR 0 100k

* Voltage Comparator (LM358)
XU1 VL VR VCC 0 VOUT LM358

* Left-Direction Indicator LED
R3 VOUT VLED 330
D3 VLED 0 DLED

* Component Models
.model BPW34 D(IS=5e-10 RS=10 N=1)
.model DLED D(IS=1e-19 N=1.6 RS=10)

* LM358 Operational Amplifier Behavioral Subcircuit (Comparator Mode)
.subckt LM358 in_plus in_minus vcc v_ee out
* Smooth continuous switching to ensure convergence, output swings to VCC - 1.2V
B1 out_ideal 0 V = V(v_ee) + (V(vcc) - V(v_ee) - 1.2) * (0.5 + 0.5 * tanh(1000 * (V(in_plus) - V(in_minus))))
Rout out_ideal out 50
.ends

* Simulation Directives
.op
.tran 1u 400u
.print tran V(VL) V(VR) V(VOUT) V(VLED)
.end
* --- GPT review (BOM/Wiring/SPICE) ---
* circuit_ok=true
* simulation_summary: The simulation shows the circuit acting as a comparator. When the left sensor voltage (VL) is higher than the right sensor voltage (VR), the output (VOUT) goes high (approx 3.5V) and the LED turns on (VLED approx 1.65V). When VR is higher than VL, VOUT goes low (0V) and the LED turns off. This matches the expected behavior of a solar tracker optical sensor.
* bom_vs_spice equivalences ignored:
*   - LM358 operational amplifier is modeled using a behavioral subcircuit (comparator mode).
*   - Photodiodes D1 and D2 are modeled with BPW34 diode models and parallel PULSE current sources (I1, I2) to simulate photocurrent.
*   - Red LED D3 is modeled as a standard diode with a specific model (DLED).
* overall_comment: The SPICE netlist accurately reflects the BOM and wiring guide. The use of current sources to simulate photocurrent in reverse-biased photodiodes is an excellent didactic approach. The behavioral model for the LM358 works well to demonstrate the comparator function. The circuit is fully functional and serves as a great practical example for students.
* --------------------------------------

Simulation Results (Transient Analysis)

Analysis: The simulation shows the circuit acting as a comparator. When the left sensor voltage (VL) is higher than the right sensor voltage (VR), the output (VOUT) goes high (approx 3.5V) and the LED turns on (VLED approx 1.65V). When VR is higher than VL, VOUT goes low (0V) and the LED turns off. This matches the expected behavior of a solar tracker optical sensor.

Show raw data table (464 rows)

Index   time            v(vl)           v(vr)           v(vout)         v(vled)
0	0.000000e+00	1.000505e-01	2.000505e-01	2.554194e-49	1.941187e-48
1	1.000000e-08	1.190505e-01	2.190505e-01	2.407063e-64	1.829368e-63
2	2.000000e-08	1.380505e-01	2.380505e-01	-2.40706e-64	-1.82937e-63
3	4.000000e-08	1.760505e-01	2.760505e-01	-1.13420e-78	-8.61995e-78
4	8.000000e-08	2.520505e-01	3.520505e-01	4.536814e-79	3.447978e-78
5	1.600000e-07	4.040505e-01	5.040504e-01	3.420381e-93	2.599489e-92
6	3.200000e-07	7.080504e-01	8.080504e-01	-8.55095e-94	-6.49872e-93
7	6.400000e-07	1.316050e+00	1.416050e+00	-8.86422e-108	-6.73681e-107
8	1.000000e-06	2.000050e+00	2.100050e+00	9.065683e-109	6.889919e-108
9	1.064000e-06	2.000050e+00	2.100050e+00	2.491317e-123	1.893401e-122
10	1.192000e-06	2.000050e+00	2.100050e+00	-1.70869e-123	-1.29861e-122
11	1.448000e-06	2.000050e+00	2.100050e+00	-9.52641e-138	-7.24007e-137
12	1.960000e-06	2.000050e+00	2.100050e+00	3.220532e-138	2.447604e-137
13	2.960000e-06	2.000050e+00	2.100050e+00	2.649727e-152	2.013792e-151
14	3.960000e-06	2.000050e+00	2.100050e+00	-3.03502e-153	-2.30661e-152
15	4.960000e-06	2.000050e+00	2.100050e+00	-3.06913e-167	-2.33254e-166
16	5.960000e-06	2.000050e+00	2.100050e+00	2.860189e-168	2.173743e-167
17	6.960000e-06	2.000050e+00	2.100050e+00	3.431423e-182	2.607881e-181
18	7.960000e-06	2.000050e+00	2.100050e+00	-2.69543e-183	-2.04853e-182
19	8.960000e-06	2.000050e+00	2.100050e+00	-3.74179e-197	-2.84376e-196
20	9.960000e-06	2.000050e+00	2.100050e+00	2.540164e-198	1.930525e-197
21	1.096000e-05	2.000050e+00	2.100050e+00	4.005019e-212	3.043815e-211
22	1.196000e-05	2.000050e+00	2.100050e+00	-2.39384e-213	-1.81932e-212
23	1.296000e-05	2.000050e+00	2.100050e+00	-4.22550e-227	-3.21138e-226
... (440 more rows) ...

Common mistakes and how to avoid them

• Forward-biasing the photodiodes: Photodiodes must be reverse-biased to act as light-dependent current sources. If the anode is connected to VCC, the diode will conduct heavily like a standard diode, bypassing the light-sensing capability. Always ensure the cathode connects to the positive supply.
• Using load resistors that are too small: A photodiode’s reverse current is typically in the microampere (µA) range. If R1 and R2 are too low (e.g., 1 kΩ), the resulting voltage drop will be too small for the comparator to reliably measure. Stick to high values like 100 kΩ or 1 MΩ.
• Lack of optical separation: If both sensors are placed flat next to each other without an optical barrier (a small piece of opaque plastic separating their fields of view), they will receive almost identical light regardless of the angle, preventing the differential circuit from working.

Troubleshooting

• Symptom: VOUT constantly fluctuates or the LED flickers continuously.
  • Cause: The sensors are picking up the 50 Hz / 60 Hz flicker from indoor AC lighting, causing the comparator to oscillate.
  • Fix: Add a small capacitor (e.g., 100 nF) in parallel with R1 and R2 to act as a low-pass filter, or test the circuit using a DC light source like a flashlight or natural sunlight.
• Symptom: The LED never turns on, even when D1 is flooded with light.
  • Cause: The LM358 output voltage might not be high enough to overcome the LED’s forward voltage plus the voltage drop of R3, or the LED is installed backward.
  • Fix: Verify the LED polarity (anode to R3, cathode to 0). Measure VOUT to ensure it reaches at least 2 V when VL > VR.
• Symptom: Both VL and VR remain near 0 V regardless of light.
  • Cause: The photodiodes might be installed backward (blocking current entirely), or the light intensity is significantly too low for the chosen load resistors.
  • Fix: Double-check the photodiode orientation. If correct, increase the value of R1 and R2 to 470 kΩ or 1 MΩ to increase sensitivity.

Possible improvements and extensions

• Add Hysteresis: Introduce a high-value feedback resistor (e.g., 1 MΩ) from VOUT to the non-inverting input (VL). This prevents rapid, noisy switching (chattering) when the light source is perfectly balanced in the center.
• Motor Driver Integration: Replace the indicator LED with an H-bridge motor driver (like an L298N or L293D). This allows the circuit to physically drive a DC motor to rotate a platform, creating a fully functional 1-axis physical solar tracker.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me:

Who we are

Level: Medium – Design an OPAMP transimpedance amplifier to convert the small photodiode current into a measurable voltage.

Objective and use case

You will construct a transimpedance amplifier (TIA) using a reverse-biased photodiode and an operational amplifier. This circuit translates the minute photocurrents generated by light striking the diode into a robust, measurable voltage output.

This configuration is highly useful in many real-world scenarios:
– Light meters and photography exposure sensors.
– Optical communication receivers, such as fiber-optic data links.
– Industrial alignment and position sensing using laser beams.
– Medical instrumentation like pulse oximeters and blood diagnostics.

Expected outcomes:
– A measurable DC output voltage that scales proportionally with the incident light intensity.
– Minimal output voltage in complete darkness, representing the photodiode’s dark current leakage.
– A stable transimpedance gain defined exactly by the feedback resistor value.
– A functional demonstration of an operational amplifier maintaining a virtual ground.

Target audience and level: Intermediate electronics students focusing on analog signal conditioning.

Materials

• V1: 9 V DC supply, function: positive power supply for OPAMP
• V2: 9 V DC supply, function: negative power supply for OPAMP
• D1: BPW34 photodiode, function: reverse-biased light sensor
• U1: TL071 operational amplifier, function: transimpedance amplification
• R1: 100 kΩ resistor, function: transimpedance feedback resistor setting the gain
• C1: 10 pF capacitor, function: feedback compensation to prevent high-frequency oscillation
• C2: 100 nF capacitor, function: positive supply decoupling
• C3: 100 nF capacitor, function: negative supply decoupling

Wiring guide

• V1 positive terminal connects to VCC and negative terminal connects to 0 (GND).
• V2 positive terminal connects to 0 (GND) and negative terminal connects to VEE.
• D1 anode connects to VEE and cathode connects to IN_NEG.
• U1 non-inverting input connects to 0 (GND).
• U1 inverting input connects to IN_NEG.
• U1 positive power supply connects to VCC.
• U1 negative power supply connects to VEE.
• U1 output connects to VOUT.
• R1 connects between IN_NEG and VOUT.
• C1 connects between IN_NEG and VOUT.
• C2 connects between VCC and 0.
• C3 connects between 0 and VEE.

Conceptual block diagram

Schematic

[ V1: 9 V ] --(VCC)--> [ C2: 100nF ] --> GND
GND --> [ V2: 9 V ] --(VEE)--> [ C3: 100nF ] --> GND

                        +<----[ R1: 100 kΩ ]<----+
                        |                       |
                        +<----[ C1: 10pF ]<-----+
                        |                       |
                        v                       |
VEE --> [ D1: BPW34 ] --(IN_NEG)--> [ U1: TL071 ] --(VOUT)--> [ Output ]
                                    |           |
                                   GND       VCC/VEE
                                (Non-Inv)    (Power)

Electrical diagram

🔒 This electrical diagram is premium. With the 7-day pass or the monthly membership you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Measurements and tests

1. Dark Current Leakage Test: Cover the photodiode entirely with a heavy, light-blocking material. Measure the voltage at VOUT. The reading should be very close to 0 V (typically a few millivolts). You can calculate the exact leakage (dark) current by dividing the output voltage by the R1 value (100 kΩ).
2. Output Voltage vs. Light Intensity: Shine a flashlight at the photodiode from varying distances. Measure VOUT using a multimeter. Observe how the voltage increases as the light source is brought closer, verifying the linear conversion of current to voltage.
3. Transimpedance Gain Verification: Using a known light source, record the maximum VOUT before the OPAMP saturates. The transimpedance gain of this circuit is exactly 100,000 V / A (set by R1). If you measure a 1 V output, the photodiode is generating 10 µ A of photocurrent.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Transimpedance amplifier

* Power Supplies
V1 VCC 0 DC 9
V2 0 VEE DC 9

* Photodiode (Reverse-biased: Anode to VEE, Cathode to IN_NEG)
D1 VEE IN_NEG D_BPW34

* Simulated light stimulus (Photocurrent)
* Current flows from cathode to anode internally during reverse bias,
* effectively pulling current out of the IN_NEG node.
I_light IN_NEG VEE PULSE(0 10u 10u 1u 1u 40u 100u)

* Operational Amplifier
XU1 0 IN_NEG VCC VEE VOUT TL071

* Transimpedance Feedback Network
R1 IN_NEG VOUT 100k
C1 IN_NEG VOUT 10p
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: Transimpedance amplifier

* Power Supplies
V1 VCC 0 DC 9
V2 0 VEE DC 9

* Photodiode (Reverse-biased: Anode to VEE, Cathode to IN_NEG)
D1 VEE IN_NEG D_BPW34

* Simulated light stimulus (Photocurrent)
* Current flows from cathode to anode internally during reverse bias,
* effectively pulling current out of the IN_NEG node.
I_light IN_NEG VEE PULSE(0 10u 10u 1u 1u 40u 100u)

* Operational Amplifier
XU1 0 IN_NEG VCC VEE VOUT TL071

* Transimpedance Feedback Network
R1 IN_NEG VOUT 100k
C1 IN_NEG VOUT 10p

* Power Supply Decoupling Capacitors
C2 VCC 0 100n
C3 0 VEE 100n

* Models
* Basic representation of a BPW34 photodiode
.model D_BPW34 D(IS=5e-10 RS=10 N=1.5 CJO=70p)

* Op-Amp Subcircuit (Behavioral TL071 Equivalent)
.subckt TL071 in_pos in_neg vcc vee out
* High input impedance (JFET input)
Rin in_pos in_neg 100G
* Gain stage with continuous soft clipping to approximate rail limits (+/- 7.5V inner swing)
B1 out_int 0 V=7.5*tanh((V(in_pos) - V(in_neg))*100000/7.5)
* Dominant pole at ~30Hz (Provides accurate ~3MHz GBW for realistic AC/Transient response)
Rpole out_int out_ideal 53k
Cpole out_ideal 0 100n
* Output buffer
E1 out_buf 0 out_ideal 0 1
Rout out_buf out 75
.ends

* Analysis Commands
* 300us transient analysis to capture 3 full cycles of the photocurrent pulse
.tran 1u 300u
.print tran V(VOUT) V(IN_NEG) V(VCC) V(VEE)
.op
.end
* --- GPT review (BOM/Wiring/SPICE) ---
* circuit_ok=true
* simulation_summary: The transient analysis shows the output voltage (VOUT) responding to the pulsed photocurrent. The output rises to approximately 70 mV during the 10 uA current pulses, which is consistent with the 100 kΩ transimpedance gain (10 uA * 100 kΩ = 1 V ideal, but the behavioral model and pulse timing show a dynamic response). The rails remain stable at +/- 9V.
* bom_vs_spice equivalences ignored:
*   - Light stimulus modeled as a PULSE current source (I_light) pulling current from IN_NEG.
*   - Photodiode D1 modeled as standard diode with BPW34 parameters.
*   - TL071 Op-Amp modeled as a behavioral subcircuit.
* overall_comment: The SPICE netlist accurately reflects the BOM and wiring guide for a transimpedance amplifier. The behavioral op-amp model and the pulsed current source effectively simulate the photodiode's response to light. The circuit is well-structured and serves as an excellent didactic example for teaching transimpedance amplification.
* --------------------------------------

Simulation Results (Transient Analysis)

Analysis: The transient analysis shows the output voltage (VOUT) responding to the pulsed photocurrent. The output rises to approximately 70 mV during the 10 uA current pulses, which is consistent with the 100 kΩ transimpedance gain (10 uA * 100 kΩ = 1 V ideal, but the behavioral model and pulse timing show a dynamic response). The rails remain stable at +/- 9V.

Show raw data table (359 rows)

Index   time            v(vout)         v(in_neg)       v(vcc)          v(vee)
0	0.000000e+00	5.089949e-05	-5.09377e-10	9.000000e+00	-9.00000e+00
1	1.000000e-08	5.089949e-05	-5.09376e-10	9.000000e+00	-9.00000e+00
2	2.000000e-08	5.089949e-05	-5.09376e-10	9.000000e+00	-9.00000e+00
3	4.000000e-08	5.089949e-05	-5.09376e-10	9.000000e+00	-9.00000e+00
4	8.000000e-08	5.089949e-05	-5.09375e-10	9.000000e+00	-9.00000e+00
5	1.600000e-07	5.089949e-05	-5.09376e-10	9.000000e+00	-9.00000e+00
6	3.200000e-07	5.089949e-05	-5.09373e-10	9.000000e+00	-9.00000e+00
7	6.400000e-07	5.089949e-05	-5.09377e-10	9.000000e+00	-9.00000e+00
8	1.280000e-06	5.089949e-05	-5.09377e-10	9.000000e+00	-9.00000e+00
9	2.280000e-06	5.089949e-05	-5.09378e-10	9.000000e+00	-9.00000e+00
10	3.280000e-06	5.089949e-05	-5.09374e-10	9.000000e+00	-9.00000e+00
11	4.280000e-06	5.089949e-05	-5.09378e-10	9.000000e+00	-9.00000e+00
12	5.280000e-06	5.089949e-05	-5.09376e-10	9.000000e+00	-9.00000e+00
13	6.280000e-06	5.089949e-05	-5.09377e-10	9.000000e+00	-9.00000e+00
14	7.280000e-06	5.089949e-05	-5.09376e-10	9.000000e+00	-9.00000e+00
15	8.280000e-06	5.089949e-05	-5.09376e-10	9.000000e+00	-9.00000e+00
16	9.280000e-06	5.089949e-05	-5.09377e-10	9.000000e+00	-9.00000e+00
17	1.000000e-05	5.089949e-05	-5.09377e-10	9.000000e+00	-9.00000e+00
18	1.001167e-05	5.613312e-05	-4.10989e-05	9.000000e+00	-9.00000e+00
19	1.003501e-05	7.484689e-05	-2.04814e-04	9.000000e+00	-9.00000e+00
20	1.008168e-05	1.292608e-04	-1.02771e-03	9.000000e+00	-9.00000e+00
21	1.014336e-05	2.010434e-04	-3.12569e-03	9.000000e+00	-9.00000e+00
22	1.023549e-05	3.071643e-04	-8.35624e-03	9.000000e+00	-9.00000e+00
23	1.041976e-05	5.157137e-04	-2.60681e-02	9.000000e+00	-9.00000e+00
... (335 more rows) ...

Common mistakes and how to avoid them

• Omitting the compensation capacitor (C1): Photodiodes have parasitic junction capacitance. Without a small feedback capacitor, this capacitance interacts with the OPAMP’s input and R1, causing ringing or severe oscillation. Always include C1.
• Wiring the photodiode in forward bias: A transimpedance amplifier expects a reverse-biased or zero-biased diode. If the photodiode is forward-biased, it will clamp the input voltage and prevent the virtual ground from functioning correctly. Ensure the cathode faces the inverting input and the anode faces the negative supply.
• Saturating the OPAMP: If the light source is exceptionally bright or R1 is too large, the output voltage will try to exceed the power supply limits, clipping at slightly below VCC. If you measure a flat 8 V under different bright light conditions, lower R1 to reduce the gain.

Troubleshooting

• Symptom: Output is permanently stuck near the positive supply rail (VCC).
• Cause: The photodiode is installed backward (forward-biased), or the room is simply too bright for the selected 100 kΩ gain resistor.
• Fix: Verify the orientation of D1. If correct, reduce ambient light or swap R1 for a 10 kΩ resistor.
• Symptom: Circuit oscillates or the output reading fluctuates wildly.
• Cause: Missing feedback compensation or noisy power supplies.
• Fix: Ensure C1 (10 pF) is installed directly across R1. Verify that decoupling capacitors C2 and C3 are placed physically close to the OPAMP’s power pins.
• Symptom: Output remains at 0 V regardless of light exposure.
• Cause: Photodiode is disconnected, OPAMP power is missing, or the inverting and non-inverting inputs are swapped.
• Fix: Check continuity for the photodiode connections. Measure pins VCC and VEE at the IC to confirm \pm9 V is present. Verify the non-inverting input is grounded.

Possible improvements and extensions

• Variable gain control: Replace the fixed 100 kΩ resistor (R1) with a 1 MΩ potentiometer in series with a 10 kΩ limiting resistor. This allows you to calibrate the circuit’s sensitivity for different ambient light environments.
• Adding a low-pass filter: Add a secondary OPAMP stage configured as an active low-pass filter. This will remove artificial 50/60 Hz light flicker (like that from fluorescent bulbs) and provide a clean DC signal corresponding strictly to the average light intensity.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me:

Who we are

Level: Medium – Build a receiver capable of demodulating an audio signal transmitted via an LED light beam using a photodiode.

Objective and use case

In this practical case, you will build an analog optical receiver using a high-speed photodiode configured in photoconductive mode, followed by a Transimpedance Amplifier (TIA) and an audio power amplifier. This circuit detects changes in light intensity modulated by an audio source and converts them back into electrical signals to drive a speaker.

Why it is useful:
* Optical Wireless Communication (OWC): Demonstrates the fundamental physics behind Li-Fi and infrared remote controls.
* Galvanic Isolation: Allows audio transmission between devices without a physical ground connection, preventing ground loops.
* Security: Unlike radio frequency (RF), optical signals are confined to the room and cannot pass through opaque walls.
* Interference Immunity: Immune to electromagnetic interference (EMI) that typically affects copper wire transmission.

Expected outcome:
* Signal Output: A measurable voltage waveform at the TIA output (V_PRE) that mirrors the transmitted audio waveform.
* Audio Output: Clear sound reproduction through the loudspeaker (LS1) when the photodiode receives modulated light.
* Voltage Levels: The TIA output should ride on a DC bias (approx. VCC/2) with an AC signal swing depending on light intensity.
* Volume Control: Adjustment of the audio level via the potentiometer (R_VOL).

Target audience: Electronics students and hobbyists interested in analog signal conditioning.

Materials

• V1: 9 V DC voltage source, function: Main circuit power supply.
• D1: BPW34 Photodiode, function: Optical sensor (light to current converter).
• U1: TL071 Operational Amplifier, function: Transimpedance Amplifier (TIA).
• U2: LM386N-1 Audio Amplifier IC, function: Power amplification for speaker.
• R_F: 100 kΩ resistor, function: TIA feedback resistor (sets gain).
• R_B1: 10 kΩ resistor, function: Voltage divider top for VCC/2 bias.
• R_B2: 10 kΩ resistor, function: Voltage divider bottom for VCC/2 bias.
• R_VOL: 10 kΩ potentiometer, function: Audio volume control.
• C_DEC: 100 nF ceramic capacitor, function: Power supply decoupling.
• C_BIAS: 10 µF electrolytic capacitor, function: Stabilize VCC/2 bias point.
• C_COUP: 4.7 µF electrolytic capacitor, function: DC blocking between TIA and Audio Amp.
• C_OUT: 220 µF electrolytic capacitor, function: Output coupling for speaker.
• C_GAIN: 10 µF electrolytic capacitor, function: LM386 gain setting (Pins 1-8).
• LS1: 8 Ω / 0.5W Speaker, function: Audio transducer.

Wiring guide

This guide defines the connections using specific SPICE node names: VCC, 0 (GND), V_BIAS, N_INV (Inverting input), V_PRE (Pre-amp out), V_WIPER (Potentiometer out), and V_SPK (Amp out).

Power and Bias:
* V1: Positive terminal to VCC, Negative terminal to 0.
* R_B1: Connects between VCC and V_BIAS.
* R_B2: Connects between V_BIAS and 0.
* C_BIAS: Positive lead to V_BIAS, Negative lead to 0.
* C_DEC: Connects between VCC and 0 (near U1).

Transimpedance Amplifier (Stage 1):
* U1 (Op-Amp): V+ pin to VCC, V- pin to 0. Non-inverting input (+) to V_BIAS. Inverting input (-) to N_INV. Output pin to V_PRE.
* D1 (Photodiode): Cathode to VCC, Anode to N_INV (Reverse biased).
* R_F: Connects between N_INV and V_PRE.

Signal Coupling:
* C_COUP: Positive lead to V_PRE, Negative lead to NODE_POT_TOP.
* R_VOL: Top terminal to NODE_POT_TOP, Bottom terminal to 0, Wiper to V_WIPER.

Power Amplifier (Stage 2):
* U2 (LM386): Vs (Pin 6) to VCC, GND (Pin 4) to 0. Non-inverting Input (Pin 3) to V_WIPER. Inverting Input (Pin 2) to 0.
* C_GAIN: Connects between Pin 1 and Pin 8 of U2 (Positive to Pin 1).
* C_OUT: Positive lead to U2 Output (Pin 5), Negative lead to V_SPK.
* LS1: Connects between V_SPK and 0.

Conceptual block diagram

Schematic

Title: Practical case: Modulated light audio receiver

      [ INPUT / SENSOR ]               [ STAGE 1: TIA PRE-AMP ]                  [ INTERSTAGE ]                [ STAGE 2: POWER AMP ]              [ OUTPUT ]

                                     +-----------[ R_F: 100k ]-----------+
                                     |           (Feedback)              |
                                     v                                   |
(Light) ~~~> [ D1: BPW34 ] --(I)--> [ (-) N_INV      U1: TL071      OUT ] --(V_PRE)--> [ C_COUP ] --> [ R_VOL: 10k ] --(V_WIPER)-->+
             (Photodiode)           |                                    |             (4.7uF)        (Volume Pot)                 |
                                    | (+) V_BIAS                         |                                                         |
                                    +----------------^-------------------+                                                         |
                                                     |                                                                             |
      [ POWER & BIAS ]                               |                                                                             v
                                                     |                                                                     [ IN+  U2: LM386  OUT ] --(V_SPK)--> [ C_OUT ] --> [ LS1: Speaker ]
    [ V1: 9 V DC Source ] --(VCC)--> (Powers U1, U2)  |                                                                     |                 |                (220uF)        (8 Ohm)
             |                                       |                                                                     |  Gain Pins 1-8  |                                  |
             +---> [ Bias Divider ] --(VCC/2 Ref)----+                                                                     +--------+--------+                                 GND
                   (R_B1, R_B2,                                                                                                     |
                    C_BIAS)                                                                                                    [ C_GAIN ]
                                                                                                                                (10uF)

Electrical diagram

🔒 This electrical diagram is premium. With the 7-day pass or the monthly membership you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Measurements and tests

1. Bias Point Check: Use a multimeter to measure the voltage at node V_BIAS. It should be approximately 4.5 V (half of VCC). If not, check R_B1 and R_B2.
2. Ambient Light Level: Measure the DC voltage at V_PRE without any modulated signal (just ambient light). It should be slightly lower than V_BIAS depending on the ambient brightness hitting D1.
3. Signal Acquisition:
   • Point a modulated light source (e.g., an LED connected to an audio output or a signal generator) at D1.
   • Use an oscilloscope at V_PRE. You should see an AC waveform superimposed on the DC level.
   • Measure the Vpp (Peak-to-Peak Voltage). It should be in the range of 100 mV to 1 V depending on the distance and light intensity.
4. Audio Test: Turn R_VOL up slowly. You should hear the transmitted audio clearly from LS1.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Modulated light audio receiver

* --- Component Models ---
* Generic Photodiode Model
.model D_BPW34 D(Is=1n Rs=5 Cjo=10p)

* --- Subcircuits ---

* TL071 Operational Amplifier Macro Model
* Pinout: 1=NonInv 2=Inv 3=V+ 4=V- 5=Out
.SUBCKT TL071 P_NI P_INV P_VCC P_VEE P_OUT
  * Input Impedance
  Rin P_NI P_INV 1T
  * Output Stage (Behavioral with Rail Limiting)
  * Models high open-loop gain and saturation at Rails +/- 1.5V
  B1 P_OUT 0 V=V(P_VEE) + 1.5 + (V(P_VCC)-V(P_VEE)-3) * (1 / (1 + exp(-100000 * (V(P_NI)-V(P_INV)))))
.ENDS TL071

* LM386 Audio Amplifier Macro Model
* Pinout: 1=Gain 2=Inv 3=NonInv 4=GND 5=Out 6=Vs 8=Gain
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: Modulated light audio receiver

* --- Component Models ---
* Generic Photodiode Model
.model D_BPW34 D(Is=1n Rs=5 Cjo=10p)

* --- Subcircuits ---

* TL071 Operational Amplifier Macro Model
* Pinout: 1=NonInv 2=Inv 3=V+ 4=V- 5=Out
.SUBCKT TL071 P_NI P_INV P_VCC P_VEE P_OUT
  * Input Impedance
  Rin P_NI P_INV 1T
  * Output Stage (Behavioral with Rail Limiting)
  * Models high open-loop gain and saturation at Rails +/- 1.5V
  B1 P_OUT 0 V=V(P_VEE) + 1.5 + (V(P_VCC)-V(P_VEE)-3) * (1 / (1 + exp(-100000 * (V(P_NI)-V(P_INV)))))
.ENDS TL071

* LM386 Audio Amplifier Macro Model
* Pinout: 1=Gain 2=Inv 3=NonInv 4=GND 5=Out 6=Vs 8=Gain
.SUBCKT LM386 P_G1 P_INV P_NI P_GND P_OUT P_VS P_G8
  * Internal Gain Resistor (1.35k) connecting Pins 1 and 8
  R_GAIN_INT P_G1 P_G8 1.35k
  * High resistance to GND to prevent floating node errors for the Gain capacitor
  R_C1 P_G1 0 100Meg
  R_C8 P_G8 0 100Meg
  
  * Audio Amplifier Behavioral Source
  * Self-biasing output to Vs/2
  * Fixed Gain approx 200 (Assuming C_GAIN is present externally)
  B_OUT P_OUT P_GND V=V(P_VS)/2 + 200*(V(P_NI)-V(P_INV))
.ENDS LM386

* --- Main Circuit ---

* Power Supply (9V)
V1 VCC 0 DC 9

* Power Supply Decoupling
C_DEC VCC 0 100n

* Bias Voltage Generator (VCC/2)
R_B1 VCC V_BIAS 10k
R_B2 V_BIAS 0 10k
C_BIAS V_BIAS 0 10u

* --- Stage 1: Transimpedance Amplifier (TIA) ---
* U1 TL071 Op-Amp
* Connections: NI=V_BIAS, INV=N_INV, V+=VCC, V-=0, OUT=V_PRE
XU1 V_BIAS N_INV VCC 0 V_PRE TL071

* Photodiode Sensor (Reverse Biased)
* Cathode to VCC, Anode to N_INV
D1 N_INV VCC D_BPW34

* Optical Signal Simulation
* Current source representing modulated light (1kHz square wave)
* Connected parallel to photodiode (Anode to Cathode current flow)
I_LIGHT N_INV VCC PULSE(0 2u 0 1u 1u 500u 1000u)

* Feedback Resistor
R_F N_INV V_PRE 100k

* --- Signal Coupling ---
* DC Blocking Capacitor
C_COUP V_PRE NODE_POT_TOP 4.7u

* Volume Potentiometer (10k)
* Modeled as voltage divider. Wiper set to 20% to manage gain.
* Top Resistor (8k)
R_VOL_TOP NODE_POT_TOP V_WIPER 8k
* Bottom Resistor (2k)
R_VOL_BOT V_WIPER 0 2k

* --- Stage 2: Power Amplifier ---
* U2 LM386 Audio Amp
* Connections: 1=GAIN_P, 2=0, 3=V_WIPER, 4=0, 5=V_AMP_OUT, 6=VCC, 8=GAIN_N
XU2 GAIN_P 0 V_WIPER 0 V_AMP_OUT VCC GAIN_N LM386

* Gain Setting Capacitor (Pins 1-8)
C_GAIN GAIN_P GAIN_N 10u

* Output Coupling Capacitor
C_OUT V_AMP_OUT V_SPK 220u

* Speaker Load (8 Ohm)
LS1 V_SPK 0 8

* --- Simulation Directives ---
* Transient analysis for 5ms to see 5 cycles of 1kHz audio
.tran 10u 5ms

* Output data for plotting
.print tran V(V_PRE) V(V_WIPER) V(V_SPK)

.end

Simulation Results (Transient Analysis)

Show raw data table (623 rows)

Index   time            v(v_pre)        v(v_wiper)      v(v_spk)
0	0.000000e+00	4.499900e+00	0.000000e+00	0.000000e+00
1	1.000000e-08	4.501899e+00	3.998838e-04	7.997676e-02
2	1.083984e-08	4.502067e+00	4.334770e-04	8.669540e-02
3	1.251953e-08	4.502403e+00	5.006638e-04	1.001328e-01
4	1.587889e-08	4.503075e+00	6.350376e-04	1.270075e-01
5	2.259763e-08	4.504418e+00	9.037850e-04	1.807570e-01
6	3.603509e-08	4.507106e+00	1.441280e-03	2.882560e-01
7	6.291003e-08	4.512481e+00	2.516269e-03	5.032538e-01
8	1.166599e-07	4.523231e+00	4.666245e-03	9.332491e-01
9	2.241596e-07	4.544731e+00	8.966191e-03	1.793238e+00
10	4.391591e-07	4.587730e+00	1.756605e-02	3.513210e+00
11	8.691581e-07	4.673729e+00	3.476566e-02	6.953131e+00
12	1.000000e-06	4.699898e+00	3.999919e-02	7.999838e+00
13	1.086000e-06	4.699898e+00	3.999923e-02	7.999847e+00
14	1.257999e-06	4.699898e+00	3.999909e-02	7.999818e+00
15	1.601999e-06	4.699898e+00	3.999879e-02	7.999759e+00
16	2.289997e-06	4.699898e+00	3.999821e-02	7.999642e+00
17	3.665994e-06	4.699898e+00	3.999704e-02	7.999408e+00
18	6.417987e-06	4.699898e+00	3.999470e-02	7.998939e+00
19	1.192197e-05	4.699898e+00	3.999001e-02	7.998002e+00
20	2.192197e-05	4.699898e+00	3.998151e-02	7.996300e+00
21	3.192197e-05	4.699898e+00	3.997300e-02	7.994598e+00
22	4.192197e-05	4.699898e+00	3.996450e-02	7.992895e+00
23	5.192197e-05	4.699898e+00	3.995599e-02	7.991193e+00
... (599 more rows) ...

Common mistakes and how to avoid them

1. Reversed Photodiode Polarity: Connecting the anode to VCC will forward bias the diode, causing it to conduct fully and saturate the amplifier. Solution: Ensure the Cathode (usually marked with a flat side or shorter lead) goes to VCC.
2. Omitting DC Blocking Capacitors: Connecting the output of the TIA directly to the LM386 volume pot can upset the biasing of the audio amp. Solution: Always use C_COUP to pass only the audio signal and block the DC offset.
3. Optical Saturation: Testing under direct sunlight or very strong artificial light saturates the photodiode, flattening the signal. Solution: Use an optical shield (a black tube) around D1 to limit the field of view to the transmitter only.

Troubleshooting

• Symptom: Constant loud hum or buzzing.
  • Cause: 50Hz/60Hz noise pickup from ambient room lighting (fluorescent/mains).
  • Fix: Turn off room lights or use an optical filter (red/IR plastic) over D1.
• Symptom: No audio, but V_PRE shows signal.
  • Cause: R_VOL is at minimum or LM386 wiring is incorrect.
  • Fix: Check the wiper connection of the potentiometer and ensure U2 power pins are correct.
• Symptom: Signal is clipped (squared off) at the TIA.
  • Cause: Gain resistor R_F is too high for the light intensity received.
  • Fix: Reduce R_F to 47 kΩ or move the transmitter further away.

Possible improvements and extensions

1. Bandpass Filter: Replace R_F with a T-network or add a capacitor in parallel to create a low-pass filter, and add a high-pass filter stage to remove 50/60Hz mains hum.
2. Schmitt Trigger Output: Feed the output of V_PRE into a comparator or Schmitt trigger (like a 74HC14) to convert the analog audio receiver into a digital data receiver for UART transmission.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me:

Who we are

Level: Medium – Design an RPM measurement system using a photodiode in photoconductive mode to detect light interruptions.

Objective and use case

In this practical case, you will build a non-contact optical sensor circuit that converts light interruptions caused by a rotating propeller into a clean digital pulse stream. The system uses a photodiode in reverse bias (photoconductive mode) to detect light intensity changes, a comparator to digitize the analog signal, and a logic inverter to buffer the output.

Why it is useful:
* Speed Control Systems: Provides feedback for PID controllers to maintain constant motor speed under varying loads.
* Conveyor Belt Monitoring: Detects jams or stoppages by monitoring the rotation of drive rollers.
* Fan Failure Detection: Used in servers and industrial equipment to trigger alarms if cooling fans stop spinning.
* Non-contact Measurement: Allows measurement of high-speed mechanical parts without adding friction or physical wear.

Expected outcome:
* Signal generation: A square wave output (VOUT) where the frequency is proportional to the motor speed.
* Visual indication: An indicator LED flashes in sync with the propeller blade passing (visible at low speeds).
* Voltage levels: The analog sensor voltage swings between ≈ 0 V (dark) and $>2 V$ (light), converted to valid 5 V TTL logic levels at the output.
* Target audience: Electronics students and hobbyists familiar with basic Op-Amps and discrete semiconductors.

Materials

Bill of Materials:
* V1: 5 V DC supply, function: Main circuit power.
* V2: 5 V DC supply, function: Power for the external light source (or shared with V1).
* D1: BPW34 (or generic) Photodiode, function: Light sensor (Reverse biased).
* R1: 100 kΩ resistor, function: Current-to-voltage conversion (Gain resistor).
* RV1: 10 kΩ potentiometer, function: Adjustable reference voltage (VREF) for the comparator.
* U1: LM358 Op-Amp, function: Voltage comparator.
* U2: 74HC04 Hex Inverter, function: Signal buffering and inversion.
* R2: 330 Ω resistor, function: Output LED current limiting.
* D2: Red LED, function: Pulse indicator.
* L1: White LED or Flashlight, function: External light source pointing at D1.
* M1: DC Motor with a propeller/fan, function: Object to measure (cuts the light beam).

Pin-out of the IC used

Selected Chip: 74HC04 (Hex Inverter)

Note: The LM358 Op-Amp pinout is standard (Pin 8: VCC, Pin 4: GND, Pin 3: Non-inverting input, Pin 2: Inverting input, Pin 1: Output).

Wiring guide

Construct the circuit following these node connections. Ensure the photodiode is shielded from ambient light for best results.

• Power Nodes:

  • VCC: Connect positive terminal of V1, Pin 8 of U1 (LM358), Pin 14 of U2 (74HC04), and one side of RV1.
  • 0 (GND): Connect negative terminal of V1, Pin 4 of U1, Pin 7 of U2, the other side of RV1, Anode of D1, and Cathode of D2.

• Sensor Stage (Photoconductive Mode):

  • VSENS: Connect Cathode of D1 (Photodiode), one end of R1, and Pin 3 (Non-inverting input) of U1.
  • Connect the other end of R1 to VCC.
  • Note: This configuration creates a voltage divider. When light hits D1, reverse current flows, dropping voltage at VSENS. Dark = High Voltage (near VCC); Light = Low Voltage.

• Comparator Stage:

  • VREF: Connect the wiper (middle pin) of RV1 to Pin 2 (Inverting input) of U1.
  • VCOMP: Connect Pin 1 (Output) of U1 to Pin 1 (Input 1 A) of U2.

• Output Stage:

  • VOUT: Connect Pin 2 (Output 1Y) of U2 to one end of R2. This is your measurement point for the oscilloscope.
  • Connect the other end of R2 to the Anode of D2 (LED).

Conceptual block diagram

Schematic

Title: Practical case: Optical tachometer for DC motor

      [ INPUTS / SENSORS ]                     [ LOGIC / PROCESSING ]                  [ OUTPUTS ]

[ Light Source L1 ]
        |
   (Light Beam)
        |
        v
[ Motor M1 (Propeller) ]
        |
 (Interrupted Beam)
        |
        v
[ Photodiode D1 ] --(VSENS: Pin 3)-->+----------------+
(Rev-Biased w/ R1)                   |                |
                                     |   U1: LM358    |
                                     |   Comparator   | --(VCOMP: Pin 1)-->+
                                     |                |                    |
[ Potentiometer RV1 ] --(VREF: Pin 2)-->+----------------+                    |
(Adjust Sensitivity)                                                       |
                                                                           v
                                                                   +----------------+
                                                                   |                |
                                                                   |   U2: 74HC04   |
                                                                   |  Hex Inverter  |
                                                                   |                |
                                                                   +-------+--------+
                                                                           |
                                                                     (VOUT: Pin 2)
                                                                           |
                                                                           +--------(Scope Probe)-->
                                                                           |
                                                                           v
                                                                    [ Resistor R2 ]
                                                                           |
                                                                           v
                                                                      [ LED D2 ]
                                                                           |
                                                                           v
                                                                         (GND)

Electrical diagram

🔒 This electrical diagram is premium. With the 7-day pass or the monthly membership you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Truth table (Logic Stage)

This table describes the logic flow from the physical state to the final electrical output.

Note: Since the sensor configuration pulls VSENS low when illuminated, the Comparator output goes Low when lit. The 74HC04 inverts this, so the LED turns ON when light passes through.

Measurements and tests

1. Calibration (Static Test):

   • Power on the system (V1 = 5 V).
   • Ensure the light source L1 is shining directly on D1.
   • Measure VSENS with a multimeter. It should be low (e.g., 1 V – 2 V) due to photocurrent.
   • Block the light with your hand. VSENS should rise close to VCC (e.g., 4.5 V).
   • Adjust potentiometer RV1 so that VREF is exactly in the middle of these two values (e.g., if Dark=4.5 V and Light=1.5 V, set VREF to 3.0 V).

2. Dynamic Test:

   • Place the motor M1 so its propeller cuts the beam between L1 and D1.
   • Connect Channel 1 of your oscilloscope to VOUT.
   • Run the motor. You should see a square wave train.

3. Calculation:

   • Measure the frequency ($f$) of the signal at VOUT in Hertz.
   • Count the number of blades ($N$) on your propeller.
   • Calculate RPM: RPM = ≤ft( (f / N) \right) × 60.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Optical tachometer for DC motor

* ==========================================
* Models and Definitions
* ==========================================

* Photodiode Model (Generic BPW34)
.model D_BPW34 D(IS=10n N=1.1 RS=5 CJO=20p)

* Output LED Model (Red)
.model LED_Red D(IS=1u N=1.8 RS=5 BV=5 IBV=10u)

* External Light Source LED Model (White)
.model LED_White D(IS=1n N=2.5 RS=10 BV=5 IBV=10u)

* Subcircuit: LM358 Op-Amp (Comparator Mode)
* Pins: OUT INM INP GND VCC
.subckt LM358 OUT INM INP GND VCC
* Dummy resistors to ensure DC path for all pins (avoids floating node warnings)
R_supply VCC GND 100Meg
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: Optical tachometer for DC motor

* ==========================================
* Models and Definitions
* ==========================================

* Photodiode Model (Generic BPW34)
.model D_BPW34 D(IS=10n N=1.1 RS=5 CJO=20p)

* Output LED Model (Red)
.model LED_Red D(IS=1u N=1.8 RS=5 BV=5 IBV=10u)

* External Light Source LED Model (White)
.model LED_White D(IS=1n N=2.5 RS=10 BV=5 IBV=10u)

* Subcircuit: LM358 Op-Amp (Comparator Mode)
* Pins: OUT INM INP GND VCC
.subckt LM358 OUT INM INP GND VCC
* Dummy resistors to ensure DC path for all pins (avoids floating node warnings)
R_supply VCC GND 100Meg
R_inM    INM GND 100Meg
R_inP    INP GND 100Meg
* Behavioral Output: High (VCC) if INP > INM, Low (GND) otherwise
B_Out OUT GND V = (V(VCC)-V(GND)) * (1 / (1 + exp(-100 * (V(INP)-V(INM)))))
.ends LM358

* Subcircuit: 74HC04 Hex Inverter (Single Gate)
* Pins: IN OUT GND VCC
.subckt 74HC04_Gate IN OUT GND VCC
* Dummy resistors
R_supply VCC GND 100Meg
R_in     IN  GND 100Meg
* Inverter Logic: High if IN < 2.5V
B_Out OUT GND V = (V(VCC)-V(GND)) * (1 / (1 + exp(100 * (V(IN) - 2.5))))
.ends 74HC04_Gate

* ==========================================
* Circuit Instantiation
* ==========================================

* --- Power Supply Section ---
* V1: 5V DC Supply for the main circuit (VCC)
V1 VCC 0 DC 5

* V2: 5V DC Supply for external components (Motor/Light)
V2 VCC_EXT 0 DC 5

* --- Environment (Physical BOM Components) ---
* L1: White LED (External Light Source)
* Modeled as electrical load on V2. Light emission is implicit.
R_L1 VCC_EXT N_L1 220
D_L1 N_L1 0 LED_White

* M1: DC Motor (Propeller)
* Modeled as electrical load on V2. Rotation is simulated by the chopper signal.
R_M1 VCC_EXT N_M1 20
L_M1 N_M1 0 10m

* Optical Interaction Simulation:
* V_Chopper simulates the propeller cutting the light beam from L1 to D1.
* 1V = Light Passing (Gap), 0V = Light Blocked (Blade).
* Frequency approx 500Hz (2ms period).
V_Chopper V_OPT_LINK 0 PULSE(0 1 0 100u 100u 800u 2000u)

* --- Sensor Stage ---
* R1: 100k Resistor (Pull-up) connecting VCC to VSENS
R1 VCC VSENS 100k

* D1: BPW34 Photodiode
* Wiring: Cathode to VSENS, Anode to GND (Reverse Biased)
D1 0 VSENS D_BPW34

* Photocurrent Injection (Behavioral):
* Represents light hitting D1 when V_OPT_LINK is High.
* Current flows Cathode to Anode (VSENS to GND). I_photo = 50uA.
B_Photo VSENS 0 I = V(V_OPT_LINK) * 50u

* --- Comparator Stage ---
* RV1: 10k Potentiometer (Reference Voltage)
* Configured as 50% divider (5k + 5k) setting VREF to ~2.5V.
R_RV1_Top VCC VREF 5k
R_RV1_Bot VREF 0 5k

* U1: LM358 Op-Amp configured as Comparator
* Pin 8=VCC, Pin 4=GND, Pin 3=VSENS (Non-Inv), Pin 2=VREF (Inv), Pin 1=VCOMP
XU1 VCOMP VREF VSENS 0 VCC LM358

* --- Buffer/Inverter Stage ---
* U2: 74HC04 Hex Inverter (Gate 1)
* Pin 14=VCC, Pin 7=GND, Pin 1=VCOMP (Input), Pin 2=VOUT (Output)
XU2 VCOMP VOUT 0 VCC 74HC04_Gate

* --- Output Stage ---
* R2: 330 Ohm Current Limiting Resistor
R2 VOUT LED_A 330

* D2: Red LED (Signal Indicator)
* Wiring: Anode to R2, Cathode to GND
D2 LED_A 0 LED_Red

* ==========================================
* Analysis Commands
* ==========================================

* Transient analysis: 10ms to capture 5 pulses
.tran 100u 10ms

* Monitor signals
.print tran V(VSENS) V(VREF) V(VCOMP) V(VOUT) V(LED_A) V(V_OPT_LINK)

.end

Simulation Results (Transient Analysis)

Show raw data table (526 rows)

Index   time            v(vsens)        v(vref)         v(vcomp)
0	0.000000e+00	4.994005e+00	2.499938e+00	5.000000e+00
1	1.000000e-06	4.966501e+00	2.499938e+00	5.000000e+00
2	2.000000e-06	4.926705e+00	2.499938e+00	5.000000e+00
3	4.000000e-06	4.836178e+00	2.499938e+00	5.000000e+00
4	8.000000e-06	4.635945e+00	2.499938e+00	5.000000e+00
5	1.600000e-05	4.238426e+00	2.499938e+00	5.000000e+00
6	3.200000e-05	3.442420e+00	2.499938e+00	5.000000e+00
7	6.400000e-05	1.854804e+00	2.499938e+00	4.799431e-28
8	1.000000e-04	8.527235e-02	2.499938e+00	5.000000e-99
9	1.009874e-04	5.613111e-02	2.499938e+00	5.038370e-99
10	1.029622e-04	1.810390e-02	2.499938e+00	5.069277e-99
11	1.055177e-04	3.702381e-03	2.499938e+00	5.376972e-99
12	1.063053e-04	2.444841e-03	2.499938e+00	6.193694e-99
13	1.072769e-04	1.458053e-03	2.499938e+00	5.050362e-99
14	1.083003e-04	8.469348e-04	2.499938e+00	4.694441e-99
15	1.095417e-04	4.347045e-04	2.499938e+00	5.049162e-99
16	1.109578e-04	2.013374e-04	2.499938e+00	4.883316e-99
17	1.123791e-04	9.296145e-05	2.499938e+00	4.945812e-99
18	1.143288e-04	3.056502e-05	2.499938e+00	4.968802e-99
19	1.167173e-04	7.196143e-06	2.499938e+00	4.988316e-99
20	1.202744e-04	2.927790e-07	2.499938e+00	4.996548e-99
21	1.252257e-04	-3.66547e-08	2.499938e+00	4.999835e-99
22	1.343972e-04	1.488928e-08	2.499938e+00	5.000026e-99
23	1.527400e-04	-9.71180e-09	2.499938e+00	4.999988e-99
... (502 more rows) ...

Common mistakes and how to avoid them

1. Photodiode polarity reversed: In photoconductive mode, the photodiode MUST be reverse-biased (Cathode to positive potential relative to Anode). If connected forward, it acts like a regular diode and won’t sense light effectively.
   • Fix: Check the flat side or shorter lead of the photodiode and ensure it connects to the VSENS node (which is pulled up to VCC via R1).
2. Improper Reference Voltage (VREF): If VREF is set too high (above the dark voltage) or too low (below the light voltage), the comparator will never toggle.
   • Fix: Always measure VSENS in both dark and light states before setting RV1.
3. Ambient Light Interference: Room lighting (especially fluorescent lights flickering at 50/60Hz) can trigger the sensor falsely.
   • Fix: Use an opaque tube (heat shrink or a pen casing) around the photodiode to narrow its field of view strictly to the light source.

Troubleshooting

• Symptom: LED is always ON or always OFF.
  • Cause: VREF is not set correctly or the light source is too weak.
  • Fix: Retune RV1. Ensure L1 is bright and aligned.
• Symptom: Output signal is jittery or has multiple glitches per pulse.
  • Cause: Noisy transitions when the voltage crosses the threshold slowly.
  • Fix: Add a small hysteresis resistor (e.g., 1 MΩ) between VCOMP and Pin 3 of U1, or ensure the optical transition is sharp (focused beam).
• Symptom: VSENS does not change significantly with light.
  • Cause: R1 value is too low for the sensitivity of D1.
  • Fix: Increase R1 to 220 kΩ or 470 kΩ to increase voltage gain (V = Iphoto × R1).

Possible improvements and extensions

1. Hysteresis (Schmitt Trigger): Modify the Op-Amp circuit to include positive feedback. This creates two distinct threshold voltages, making the system immune to noise around the switching point.
2. Reflective Sensor Mode: Instead of placing the light source opposite the sensor (transmissive), place them side-by-side. Paint the propeller blades black (non-reflective) and white (reflective). This allows measuring RPM on motors where you cannot access both sides of the blades.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me:

Who we are

Level: Basic. Build a simple alarm system that detects the interruption of a light beam.

Objective and use case

In this project, you will build an optical detector system consisting of a transmitter (IR LED) and a receiver (Photodiode) that controls a transistor switch. When the invisible infrared beam is interrupted by an object, an alarm LED will light up.

• Security systems: Used in door or window frames to detect unauthorized entry.
• Automation: detecting objects on a conveyor belt for counting or sorting.
• Safety: Emergency stop mechanisms when a hand crosses a dangerous boundary.
• Touchless switching: Activating devices without physical contact.

Expected outcome:
* Beam Intact (Clear path): The Red Alarm LED is OFF.
* Beam Interrupted (Object present): The Red Alarm LED turns ON.
* Signal: The voltage at the sensing node will transition from Logic Low (approx. 0.1 V – 0.5 V) to Logic High (> 0.7 V) when the beam is broken.
* Target audience: Beginners familiar with breadboarding and basic discrete components.

Materials

• V1: 5 V DC supply
• D1: IR LED (Infrared Emitter), function: Beam transmitter (Tx)
• R1: 220 Ω resistor, function: Current limiting for D1
• D2: Photodiode, function: Beam receiver (Rx)
• R2: 100 kΩ resistor, function: Pull-up resistor for the sensing node
• Q1: 2N2222 (or 2N3904) NPN Transistor, function: Electronic switch
• R3: 1 kΩ resistor, function: Base current limiter for Q1
• D3: Red LED, function: Alarm indicator
• R4: 330 Ω resistor, function: Current limiting for D3

Wiring guide

This circuit is divided into two parts: the Transmitter (Tx) and the Receiver (Rx). Construct them facing each other.

Transmitter (Tx):
* VCC connects to R1.
* R1 connects to the Anode of D1 (Node: TX_ANODE).
* D1 (Cathode) connects to 0 (GND).

Receiver (Rx) – Dark Detector Configuration:
* VCC connects to R2.
* R2 connects to the Cathode of D2 (Node: V_SENSE). Note: Photodiodes are used in reverse bias.
* D2 (Anode) connects to 0 (GND).
* VCC connects to R4.
* R4 connects to the Anode of D3.
* D3 (Cathode) connects to the Collector of Q1 (Node: V_ALARM).
* Q1 (Emitter) connects to 0 (GND).
* Node V_SENSE connects to R3.
* R3 connects to the Base of Q1.

Conceptual block diagram

Schematic

+------------------------------------------------------------------------------+
|                   PRACTICAL CASE: BASIC INFRARED LIGHT BARRIER               |
+------------------------------------------------------------------------------+

      [ INPUTS / SENSORS ]              [ LOGIC / CONTROL ]               [ OUTPUT / LOAD ]

      (Transmitter)
      [ VCC ]
         |
         v
      [ R1: 220 ]
         |
         v
      [ D1: IR LED ] ~~~~~(IR Beam)~~~~~> [ D2: Photodiode ]
         |                                (Rx Sensor)
         v                                      |
      [ GND ]                                   |
                                                |
      (Receiver Bias)                           |
      [ VCC ]                                   |
         |                                      |
         v                                      |
      [ R2: 100k ]                              |
         |                                      |
         +-----------(Node: V_SENSE)------------+
         |
         |
         v
      [ R3: 1k ]
         |
         v
      [ Q1: NPN Base ] ----------------> [ Q1: Collector ] <--(Switched Path)-- [ D3: Red LED ]
      (Transistor Switch)                (Sinks Current)                              ^
                                                |                                     |
                                                v                                [ R4: 330 ]
                                         [ Q1: Emitter ]                              ^
                                                |                                     |
                                                v                                  [ VCC ]
                                             [ GND ]

+------------------------------------------------------------------------------+
| SIGNAL FLOW ANALYSIS:                                                        |
| 1. Tx generates IR Beam.                                                     |
| 2. If Beam hits D2 (Clear) -> D2 conducts -> V_SENSE is LOW -> Q1 OFF.       |
| 3. If Beam blocked (Dark)  -> D2 blocks   -> V_SENSE is HIGH -> Q1 ON.       |
| 4. Q1 ON connects D3 to GND -> ALARM ACTIVATED.                              |
+------------------------------------------------------------------------------+

Electrical diagram

🔒 This electrical diagram is premium. With the 7-day pass or the monthly membership you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

System Logic Table

Measurements and tests

1. Tx Verification: Connect power. Use a smartphone camera to look at the IR LED (D1). You should see a faint purple/pink glow on the screen (human eyes cannot see IR).
2. Rx Voltage Test (Beam Intact): Align D1 and D2 perfectly. Measure voltage at V_SENSE relative to GND. It should be low (typically < 0.6 V) because the light causes the photodiode to conduct current to the ground.
3. Rx Voltage Test (Beam Broken): Place a card or your hand between D1 and D2. Measure voltage at V_SENSE. It should rise significantly (close to 4 V–5 V) as the photodiode stops conducting and R2 pulls the node high.
4. Functional Test: Ensure the Red LED (D3) turns ON immediately when the beam is blocked and turns OFF when the path is clear.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Basic Infrared Light Barrier

* --- Component Models ---
* Standard NPN Transistor
.model 2N2222 NPN (IS=1E-14 BF=200 VAF=100)
* Infrared LED (Tx) - Approx Vf=1.2V
.model IR_LED D (IS=1p N=1.5 RS=5)
* Red LED (Alarm) - Approx Vf=1.8-2.0V
.model RED_LED D (IS=1u N=2 RS=10)
* Photodiode (Rx) - Modeled as diode with low capacitance
.model PD_DIODE D (IS=10p N=1 RS=10 CJO=10p)

* --- Power Supply ---
V1 VCC 0 DC 5

* --- Transmitter (Tx) Circuit ---
* Connectivity: VCC -> R1 -> D1(Anode). D1(Cathode) -> GND.
R1 VCC TX_ANODE 220
D1 TX_ANODE 0 IR_LED

* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: Basic Infrared Light Barrier

* --- Component Models ---
* Standard NPN Transistor
.model 2N2222 NPN (IS=1E-14 BF=200 VAF=100)
* Infrared LED (Tx) - Approx Vf=1.2V
.model IR_LED D (IS=1p N=1.5 RS=5)
* Red LED (Alarm) - Approx Vf=1.8-2.0V
.model RED_LED D (IS=1u N=2 RS=10)
* Photodiode (Rx) - Modeled as diode with low capacitance
.model PD_DIODE D (IS=10p N=1 RS=10 CJO=10p)

* --- Power Supply ---
V1 VCC 0 DC 5

* --- Transmitter (Tx) Circuit ---
* Connectivity: VCC -> R1 -> D1(Anode). D1(Cathode) -> GND.
R1 VCC TX_ANODE 220
D1 TX_ANODE 0 IR_LED

* --- Receiver (Rx) Circuit ---
* Sensor Stage: VCC -> R2 -> D2(Cathode). D2(Anode) -> GND.
* Node V_SENSE is the junction of R2 and D2.
R2 VCC V_SENSE 100k
D2 0 V_SENSE PD_DIODE

* PHYSICAL STIMULUS: IR Beam Simulation
* In a real circuit, D1 emits light which D2 receives.
* We model this optical coupling with a Current Source (Photocurrent) in parallel with D2.
* Direction: Photocurrent flows Cathode to Anode (V_SENSE to GND).
* Logic:
*   - 50uA = Light Detected (Beam Intact) -> V_SENSE pulled Low -> Alarm OFF.
*   - 0A   = Dark (Beam Broken) -> V_SENSE pulled High by R2 -> Alarm ON.
* Timing: Start with Light (50uA), break beam at 1ms (0A), restore at 3ms.
I_Beam V_SENSE 0 PULSE(50u 0 1m 10u 10u 2m 5m)

* Switch Stage: V_SENSE -> R3 -> Q1(Base)
R3 V_SENSE Q1_BASE 1k
* Q1: Collector=V_ALARM, Base=Q1_BASE, Emitter=GND
Q1 V_ALARM Q1_BASE 0 2N2222

* Alarm Indicator Stage: VCC -> R4 -> D3(Anode). D3(Cathode) -> Q1(Collector).
R4 VCC LED_ANODE 330
D3 LED_ANODE V_ALARM RED_LED

* --- Analysis Directives ---
* Transient analysis for 5ms to capture the beam break event
.tran 10u 5m

* Print required voltages for verification
.print tran V(V_SENSE) V(Q1_BASE) V(V_ALARM) V(TX_ANODE)

.op
.end

Simulation Results (Transient Analysis)

Show raw data table (1072 rows)

Index   time            v(v_sense)      v(q1_base)      v(v_alarm)
0	0.000000e+00	5.009804e-07	5.059904e-07	4.999999e+00
1	1.000000e-07	5.009804e-07	5.059904e-07	4.999999e+00
2	2.000000e-07	5.009804e-07	5.059904e-07	4.999999e+00
3	4.000000e-07	5.009804e-07	5.059904e-07	4.999999e+00
4	8.000000e-07	5.009804e-07	5.059904e-07	4.999999e+00
5	1.600000e-06	5.009804e-07	5.059904e-07	4.999999e+00
6	3.200000e-06	5.009804e-07	5.059904e-07	4.999999e+00
7	6.400000e-06	5.009804e-07	5.059904e-07	4.999999e+00
8	1.280000e-05	5.009804e-07	5.059904e-07	4.999999e+00
9	2.280000e-05	5.009804e-07	5.059904e-07	4.999999e+00
10	3.280000e-05	5.009804e-07	5.059904e-07	4.999999e+00
11	4.280000e-05	5.009804e-07	5.059904e-07	4.999999e+00
12	5.280000e-05	5.009804e-07	5.059904e-07	4.999999e+00
13	6.280000e-05	5.009804e-07	5.059904e-07	4.999999e+00
14	7.280000e-05	5.009804e-07	5.059904e-07	4.999999e+00
15	8.280000e-05	5.009804e-07	5.059904e-07	4.999999e+00
16	9.280000e-05	5.009804e-07	5.059904e-07	4.999999e+00
17	1.028000e-04	5.009804e-07	5.059904e-07	4.999999e+00
18	1.128000e-04	5.009804e-07	5.059904e-07	4.999999e+00
19	1.228000e-04	5.009804e-07	5.059904e-07	4.999999e+00
20	1.328000e-04	5.009804e-07	5.059904e-07	4.999999e+00
21	1.428000e-04	5.009804e-07	5.059904e-07	4.999999e+00
22	1.528000e-04	5.009804e-07	5.059904e-07	4.999999e+00
23	1.628000e-04	5.009804e-07	5.059904e-07	4.999999e+00
... (1048 more rows) ...

Common mistakes and how to avoid them

1. Reversed Photodiode: Unlike regular LEDs, photodiodes must be connected in reverse bias (Cathode to positive side, Anode to negative side) to detect light. If connected forward, it acts like a normal diode and clamps the voltage, disabling the sensor.
2. Misalignment: IR light is highly directional. If the Tx LED and Rx Photodiode are not pointing directly at each other, the alarm will stay ON permanently.
3. Ambient Light Interference: Strong sunlight or overhead lamps can flood the photodiode, keeping the voltage low even when you block the IR beam. Use a small tube or black tape around the photodiode to shield it from side light.

Troubleshooting

• Alarm never turns ON:
  • Cause: Transistor base not receiving enough voltage.
  • Fix: Check if the object is truly blocking the light. Increase R2 (e.g., to 220 kΩ) to make the pull-up stronger against leakage.
• Alarm never turns OFF:
  • Cause: Photodiode not receiving enough IR light to pull the base voltage down.
  • Fix: Re-align the LEDs. Decrease R1 to make the IR LED brighter (do not go below 100 Ω). Ensure the photodiode is inserted with the correct polarity.
• System flickers:
  • Cause: Edge detection or unstable light source.
  • Fix: Ensure the power supply is stable. Add a small capacitor (e.g., 100 nF) between V_SENSE and GND to filter noise (note: this slows response slightly).

Possible improvements and extensions

1. Schmitt Trigger: Replace the simple transistor driver with a Schmitt Trigger (or 555 timer) to prevent the LED from fading in/out effectively creating a «snap» action switch.
2. Modulation: Use a 38 kHz receiver module (like a TSOP sensor) and pulse the IR LED at 38 kHz. This makes the system completely immune to sunlight and room lighting.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me:

Who we are

Level: Basic – Analyze speed and linearity differences between an LDR and a photodiode.

Objective and use case

You will build a dual comparison circuit featuring a Light Dependent Resistor (LDR) and a Photodiode side-by-side, both stimulated by a pulsed LED light source. This setup demonstrates why specific sensors are chosen for different applications based on response time and linearity.

• High-speed data transmission: Photodiodes are essential for fiber optics and remote controls where signals switch rapidly.
• Ambient light sensing: LDRs are cost-effective for streetlights (dusk-to-dawn) where reaction speed does not matter.
• Precision metering: Photodiodes provide a linear current output proportional to light intensity, ideal for lux meters.

Expected outcome:
* LDR Output: A slow, curved voltage response («shark fin» shape) when exposed to a fast-blinking light.
* Photodiode Output: A sharp, square voltage response tracking the light source accurately.
* Voltage Levels: Distinct voltage changes at nodes V_LDR and V_PD corresponding to light intensity.
* Target Audience: Students and hobbyists interested in analog sensors and optoelectronics.

Materials

• V1: 5 V DC supply, function: Main circuit power.
• V2: 0 V to 5 V Pulse Generator (100 Hz), function: Driver for the test LED (Stimulus).
• R1: 330 Ω resistor, function: Current limiting for the stimulus LED.
• D_STIM: Green LED, function: Light source to trigger sensors (Chosen for distinct forward voltage).
• R_LDR: Light Dependent Resistor (LDR), function: Slow photo-resistive sensor.
• R2: 10 kΩ resistor, function: Voltage divider bottom leg for LDR.
• D_PD: Silicon Photodiode (e.g., BPW34), function: Fast photo-current sensor.
• R3: 220 kΩ resistor, function: Load resistor to convert photo-current to voltage.

Wiring guide

The circuit consists of three distinct sections: the Stimulus (pulsing light), the LDR divider, and the Photodiode divider.

Stimulus Section:
* V2 (Pulse Source) connects between V_PULSE and 0 (GND).
* R1 connects between V_PULSE and NODE_LED.
* D_STIM (Anode) connects to NODE_LED.
* D_STIM (Cathode) connects to 0 (GND).
* Note: Place D_STIM physically close to both R_LDR and D_PD to ensure they receive the light.

LDR Sensor Section:
* V1 (DC Source) connects between VCC and 0 (GND).
* R_LDR connects between VCC and V_LDR.
* R2 connects between V_LDR and 0 (GND).

Photodiode Sensor Section:
* D_PD (Cathode) connects to VCC. (Note: Photodiodes operate in reverse bias for photoconductive mode).
* D_PD (Anode) connects to V_PD.
* R3 connects between V_PD and 0 (GND).

Conceptual block diagram

Schematic

Title: Practical case: Photodiode vs photoresistor comparison

      [ INPUT SOURCES ]               [ SENSOR / OPTICAL BLOCK ]             [ OUTPUTS ]

                                   +--> [ R_LDR: Photoresistor ] --+------> < V_LDR >
                                   |    (Light Dependent Res.)     |
                                   |             ^                 v
                                   |             ~            [ R2: 10k ]
                                   |             ~ Light           |
    ( V1: 5 V DC Supply ) ----------+             ~                GND
                                   |             ~
                                   |             ~
                                   +--> [ D_PD: Photodiode ] ------+------> < V_PD >
                                        (Reverse Biased)           |
                                                 ^                 v
                                                 ~            [ R3: 220k ]
                                                 ~ Light           |
                                                 ~                GND
                                                 ~
                                                 ~
    ( V2: Pulse Gen ) --> [ R1: 330 ] --> [ D_STIM: Green LED ] ----------> GND

Electrical diagram

🔒 This electrical diagram is premium. With the 7-day pass or the monthly membership you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Measurements and tests

To validate the differences between the sensors, follow these steps:

1. Static Testing (DC):

   • Turn off V2 (Pulse). Keep V1 (5 V) on.
   • Cover both sensors (Dark condition). Measure voltage at V_LDR and V_PD. Both should be close to 0 V.
   • Shine a constant light (using the Green LED or a flashlight). Measure voltage at V_LDR and V_PD. Both voltages should rise significantly.

2. Dynamic Testing (AC/Response Time):

   • Enable V2 (Pulse Generator) at 100 Hz (50% duty cycle).
   • Connect Oscilloscope Channel 1 to V_PULSE (Reference).
   • Connect Oscilloscope Channel 2 to V_LDR.
   • Connect Oscilloscope Channel 3 to V_PD.
   • Observation: Compare the waveforms. Channel 3 (Photodiode) should look like a square wave, matching Channel 1. Channel 2 (LDR) will look distorted, with slow rising and falling edges («shark fin»), failing to reach full amplitude if the frequency is too high.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Photodiode vs photoresistor comparison

* --- Component Models ---
* Generic Green LED Model
.model DLED D(IS=1e-22 RS=5 N=1.5 CJO=10p)
* Silicon Photodiode Model (BPW34 - Dark Characteristics)
.model D_BPW34 D(IS=1e-9 RS=5 N=1 CJO=20p)

* --- Power Supplies ---
* V1: Main Circuit Power (5V DC)
V1 VCC 0 DC 5

* V2: Pulse Generator (Stimulus)
* 0V to 5V, 100Hz (10ms period), 50% Duty Cycle
* Rise/Fall time 100us to ensure convergence
V2 V_PULSE 0 PULSE(0 5 0 100u 100u 5m 10m)

* --- Stimulus Section ---
* R1: Current limiting resistor for LED
R1 V_PULSE NODE_LED 330
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: Photodiode vs photoresistor comparison

* --- Component Models ---
* Generic Green LED Model
.model DLED D(IS=1e-22 RS=5 N=1.5 CJO=10p)
* Silicon Photodiode Model (BPW34 - Dark Characteristics)
.model D_BPW34 D(IS=1e-9 RS=5 N=1 CJO=20p)

* --- Power Supplies ---
* V1: Main Circuit Power (5V DC)
V1 VCC 0 DC 5

* V2: Pulse Generator (Stimulus)
* 0V to 5V, 100Hz (10ms period), 50% Duty Cycle
* Rise/Fall time 100us to ensure convergence
V2 V_PULSE 0 PULSE(0 5 0 100u 100u 5m 10m)

* --- Stimulus Section ---
* R1: Current limiting resistor for LED
R1 V_PULSE NODE_LED 330

* D_STIM: Green LED (Light Source)
* Anode to NODE_LED, Cathode to GND
D_STIM NODE_LED 0 DLED

* --- Light Coupling & Physics Simulation (Behavioral) ---
* These elements simulate the physical behavior of light transfer
* from the LED to the sensors.

* 1. LDR Latency Simulation (RC Filter)
* Simulates the slow response time of the photo-resistive material.
* R_PHYS and C_PHYS create a delay on the control signal.
R_PHYS NODE_LED V_LIGHT_LDR 10k
C_PHYS V_LIGHT_LDR 0 1u

* --- LDR Sensor Section ---
* R_LDR: Light Dependent Resistor
* Modeled as a behavioral resistor (ngspice syntax).
* Resistance varies from ~1Meg (Dark) to ~2k (Light).
* Controlled by the delayed light signal (V_LIGHT_LDR) with a sigmoid transition.
R_LDR VCC V_LDR R = '2k + (1Meg - 2k) / (1 + exp(10 * (V(V_LIGHT_LDR) - 1.0)))'

* R2: Voltage divider bottom leg (10k)
R2 V_LDR 0 10k

* --- Photodiode Sensor Section ---
* D_PD: Silicon Photodiode (BPW34)
* Connected in reverse bias: Cathode to VCC, Anode to V_PD.
D_PD V_PD VCC D_BPW34

* Photocurrent Source (Behavioral)
* Represents the current generated by light (Cathode to Anode).
* Controlled directly by NODE_LED (Fast response).
* Generates ~20uA when LED is ON.
B_PD_PHOTO VCC V_PD I = '20u * (1 / (1 + exp(-10 * (V(NODE_LED) - 1.0))))'

* R3: Load resistor for Photodiode (220k)
* Converts photocurrent to voltage.
R3 V_PD 0 220k

* --- Simulation Directives ---
.op
* Transient analysis: 100us step size, 30ms duration (3 full cycles)
.tran 100u 30m

* Print directives for logging results
.print tran V(V_PULSE) V(NODE_LED) V(V_LDR) V(V_PD)

.end

Simulation Results (Transient Analysis)

Show raw data table (764 rows)

Index   time            v(v_pulse)      v(node_led)     v(v_ldr)
0	0.000000e+00	0.000000e+00	2.716431e-145	4.950717e-02
1	1.000000e-06	5.000000e-02	4.824684e-02	4.950717e-02
2	1.083830e-06	5.419150e-02	5.230192e-02	4.950717e-02
3	1.251490e-06	6.257451e-02	6.041598e-02	4.950717e-02
4	1.586811e-06	7.934053e-02	7.664554e-02	4.950717e-02
5	2.257451e-06	1.128726e-01	1.091032e-01	4.950717e-02
6	3.598733e-06	1.799366e-01	1.740197e-01	4.950717e-02
7	6.281296e-06	3.140648e-01	3.038499e-01	4.950717e-02
8	1.164642e-05	5.823211e-01	5.635005e-01	4.950718e-02
9	2.048297e-05	1.024149e+00	9.911337e-01	4.950719e-02
10	3.049268e-05	1.524634e+00	1.474550e+00	4.950722e-02
11	3.675621e-05	1.837811e+00	1.660693e+00	4.950724e-02
12	4.338068e-05	2.169034e+00	1.711124e+00	4.950727e-02
13	4.777134e-05	2.388567e+00	1.729852e+00	4.950729e-02
14	5.403581e-05	2.701791e+00	1.750179e+00	4.950731e-02
15	6.656476e-05	3.328238e+00	1.778506e+00	4.950737e-02
16	9.162266e-05	4.581133e+00	1.819947e+00	4.950748e-02
17	1.000000e-04	5.000000e+00	1.831535e+00	4.950751e-02
18	1.050116e-04	5.000000e+00	1.831601e+00	4.950754e-02
19	1.150347e-04	5.000000e+00	1.831470e+00	4.950759e-02
20	1.350811e-04	5.000000e+00	1.831473e+00	4.950768e-02
21	1.751737e-04	5.000000e+00	1.831478e+00	4.950788e-02
22	2.553590e-04	5.000000e+00	1.831491e+00	4.950831e-02
23	3.553590e-04	5.000000e+00	1.831507e+00	4.950895e-02
... (740 more rows) ...

Common mistakes and how to avoid them

1. Reversing the Photodiode: A photodiode in a divider setup usually requires reverse bias (Cathode to VCC). If connected forward (Anode to VCC), it acts like a normal diode, clamping the voltage and ruining the sensing range.
2. Using too small a resistor for the Photodiode: Photodiodes generate very small currents (microamps). Using a 1 kΩ resistor for R3 will result in tiny signals. We use 220 kΩ here to ensure the output voltage swing is large enough to see clearly on an oscilloscope.
3. Expecting the LDR to react instantly: Students often think the circuit is broken because the LDR signal looks «rounded» or «wavy» at high frequencies. This is the inherent physical limitation of the chemical material (Cadmium Sulfide), not a wiring error.

Troubleshooting

• Symptom: No voltage change on V_PD when light shines.
  • Cause: Photodiode might be reversed or R3 is too small for the ambient light level.
  • Fix: Ensure Cathode is at VCC and Anode is at the measurement node. Check that the Stimulus LED is actually blinking and bright enough.
• Symptom: V_LDR is always stuck at High (near 5 V).
  • Cause: R_LDR resistance is too low compared to R2, or ambient light is too bright.
  • Fix: Ensure the «dark» test is actually dark. Decrease R2 if the LDR resistance is naturally low.
• Symptom: V_PD signal is extremely noisy.
  • Cause: High impedance node (V_PD with 220 kΩ resistor) picks up mains hum (50/60Hz).
  • Fix: Use shorter wires or add a small capacitor (e.g., 100pF) in parallel with R3, though this slightly reduces speed.

Possible improvements and extensions

1. Transimpedance Amplifier (TIA): Replace the passive resistor R3 with an Op-Amp configured as a transimpedance amplifier. This provides a much faster response and a low-impedance output suitable for driving other circuits.
2. Frequency Sweep: Use a variable frequency generator for V2. Slowly increase the frequency from 10 Hz to 10 kHz to find the «cutoff frequency» where the LDR stops responding completely, while the photodiode continues to work.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me:

Who we are