Level: Basic. Objective: Build a circuit that activates an LED when light levels drop.
Objective and use case
In this practical case, you will design and assemble a light-sensing circuit that automatically turns on an LED when the environment becomes dark. This circuit uses a photodiode to control an NPN transistor acting as a switch.
- Real-world applications:
- Automatic street lighting systems.
- Emergency corridor lights that activate during power outages (if light absent).
- Battery-saving garden solar lights.
- Security systems triggered by shadows or obstruction of light beams.
- Expected outcome:
- Bright light: The LED remains OFF; the voltage at the transistor base is low.
- Darkness: The LED turns ON; the voltage at the transistor base rises above 0.7 V.
- Transition: The circuit reacts to the absence of light (dark sensor logic).
- Target audience: Beginners and electronics students.
Materials
- V1: 5 V DC supply, function: main power source.
- R1: 100 kΩ resistor, function: base pull-up resistor (sets sensitivity).
- R2: 330 Ω resistor, function: LED current limiting.
- D1: Generic silicon photodiode, function: light sensor.
- Q1: 2N2222 (or BC547) NPN Transistor, function: electronic switch.
- D2: Red LED, function: visual output indicator.
Wiring guide
This guide uses specific node names to define the connections clearly.
* Nodes defined: VCC (5 V), GND (0 V), V_BASE (Control voltage), V_COL (Collector voltage).
- V1 (Source): Connect positive terminal to
VCCand negative terminal toGND. - R1 (Bias): Connect between
VCCandV_BASE. - D1 (Photodiode):
- Connect the Cathode to
V_BASE. - Connect the Anode to
GND. - Note: The photodiode is used in reverse bias mode.
- Connect the Cathode to
- Q1 (Transistor):
- Connect the Base to
V_BASE. - Connect the Emitter to
GND. - Connect the Collector to
V_COL.
- Connect the Base to
- R2 & D2 (Output Loop):
- Connect R2 between
VCCand the Anode of D2. - Connect the Cathode of D2 to
V_COL(the Collector of Q1).
- Connect R2 between
Conceptual block diagram
Schematic
[ INPUT / SENSING ] [ LOGIC / SWITCHING ] [ OUTPUT / LOAD ]
[ VCC (5 V) ] [ VCC (5 V) ]
| |
| |
v v
[ R1: 100k Bias ] --(Pull Up)--+ [ R2: 330 Ohm ]
| |
| |
(V_BASE) v
| [ D2: Red LED ]
| |
+--(Control Sig)--> [ Q1: NPN Base ] |
| [ ] |
| [ Q1 Collector ] <--(Sink Current)-----+
[ D1: Photodiode ] --(Sensor)--+ [ ] (V_COL Node)
(Reverse Biased) | [ Q1 Emitter ]
| | |
| | |
v | v
[ GND ] +---------------------> [ GND ]
Measurements and tests
Perform the following validation steps using a multimeter to ensure the circuit works as intended.
- Check Supply Voltage:
- Measure between
VCCandGND. It should read approximately 5 V.
- Measure between
- Test Light Condition (LED OFF):
- Shine a bright light directly on the photodiode D1.
- Measure voltage at
V_BASE. It should be low (typically < 0.5 V) because the photodiode conducts current to ground. - Observe D2 (LED). It should be OFF.
- Measure voltage at
V_COL. It should be close toVCC(High) as the transistor is in cutoff.
- Test Dark Condition (LED ON):
- Cover D1 completely with your hand or a dark cap.
- Measure voltage at
V_BASE. It should rise above 0.6 V – 0.7 V. - Observe D2 (LED). It should turn ON.
- Measure voltage at
V_COL. It should drop to near 0 V (saturation voltage ~0.2 V).
SPICE netlist and simulation
Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)
* Practical case: Simple twilight switch
* --- Power Supply ---
* V1: 5 V DC supply
V1 VCC 0 DC 5
* --- Input Stage (Light Sensor) ---
* R1: 100 kΩ resistor (Base pull-up)
* Connect between VCC and V_BASE
R1 VCC V_BASE 100k
* D1: Generic silicon photodiode
* Connect the Cathode to V_BASE, Anode to GND (Reverse Bias)
* SPICE Syntax: Dname Anode Cathode Model
D1 0 V_BASE D_GENERIC
* OPTICAL STIMULUS SIMULATION
* The photodiode generates a photocurrent flowing from Cathode to Anode
* (Reverse current) proportional to light intensity.
* We simulate this with a Current Source (I_LIGHT) in parallel with D1.
* ... (truncated in public view) ...Copy this content into a .cir file and run with ngspice.
* Practical case: Simple twilight switch
* --- Power Supply ---
* V1: 5 V DC supply
V1 VCC 0 DC 5
* --- Input Stage (Light Sensor) ---
* R1: 100 kΩ resistor (Base pull-up)
* Connect between VCC and V_BASE
R1 VCC V_BASE 100k
* D1: Generic silicon photodiode
* Connect the Cathode to V_BASE, Anode to GND (Reverse Bias)
* SPICE Syntax: Dname Anode Cathode Model
D1 0 V_BASE D_GENERIC
* OPTICAL STIMULUS SIMULATION
* The photodiode generates a photocurrent flowing from Cathode to Anode
* (Reverse current) proportional to light intensity.
* We simulate this with a Current Source (I_LIGHT) in parallel with D1.
* Logic:
* 0uA = Dark (Night) -> Base High -> Q1 ON -> LED ON
* 100uA = Light (Day) -> Base Low -> Q1 OFF -> LED OFF
* Waveform: Dark (0uA) transitioning to Light (100uA)
I_LIGHT V_BASE 0 PULSE(0 100u 100u 100u 100u 2m 5m)
* --- Switching Stage ---
* Q1: 2N2222 NPN Transistor
* Base to V_BASE, Emitter to GND, Collector to V_COL
* SPICE Syntax: Qname Collector Base Emitter Model
Q1 V_COL V_BASE 0 2N2222
* --- Output Stage ---
* R2: 330 Ω resistor
* Connect between VCC and the Anode of D2 (Node V_LED_ANODE)
R2 VCC V_LED_ANODE 330
* D2: Red LED
* Connect Anode to V_LED_ANODE, Cathode to V_COL
D2 V_LED_ANODE V_COL LED_RED
* --- Models ---
* Standard NPN Transistor Model
.model 2N2222 NPN (IS=1E-14 VAF=100 BF=200 IKF=0.3 XTB=1.5 BR=3 CJC=8E-12 CJE=25E-12 TR=46.91E-9 TF=411.1E-12 ITF=0.6 VTF=1.7 XTF=3 RB=10 RC=1 RE=0.1)
* Generic Red LED Model (Approx 1.8V-2V drop)
.model LED_RED D (IS=93.2p RS=42m N=3.73 BV=5 IBV=10u CJO=2.97p VJ=0.75 M=0.333 TT=4.32u)
* Generic Silicon Diode Model for Photodiode (Dark characteristics)
.model D_GENERIC D (IS=1N N=1 RS=0.1)
* --- Analysis Directives ---
* Transient analysis to show the switching behavior
.tran 10u 5m
* Print required voltages to verify operation
* V(V_BASE): Sensor voltage (High = Dark, Low = Light)
* V(V_COL): Output state (Low = LED ON, High/Floating = LED OFF)
.print tran V(V_BASE) V(V_COL)
.op
.endSimulation Results (Transient Analysis)
Show raw data table (534 rows)
Index time v(v_base) v(v_col) 0 0.000000e+00 7.119659e-01 4.863696e-01 1 1.000000e-07 7.119659e-01 4.863696e-01 2 2.000000e-07 7.119659e-01 4.863696e-01 3 4.000000e-07 7.119659e-01 4.863696e-01 4 8.000000e-07 7.119659e-01 4.863696e-01 5 1.600000e-06 7.119659e-01 4.863696e-01 6 3.200000e-06 7.119659e-01 4.863696e-01 7 6.400000e-06 7.119659e-01 4.863696e-01 8 1.280000e-05 7.119659e-01 4.863696e-01 9 2.280000e-05 7.119659e-01 4.863696e-01 10 3.280000e-05 7.119659e-01 4.863696e-01 11 4.280000e-05 7.119659e-01 4.863696e-01 12 5.280000e-05 7.119659e-01 4.863696e-01 13 6.280000e-05 7.119659e-01 4.863696e-01 14 7.280000e-05 7.119659e-01 4.863696e-01 15 8.280000e-05 7.119659e-01 4.863696e-01 16 9.280000e-05 7.119659e-01 4.863696e-01 17 1.000000e-04 7.119659e-01 4.863696e-01 18 1.006859e-04 7.117420e-01 5.075675e-01 19 1.020576e-04 7.110644e-01 5.716214e-01 20 1.044620e-04 7.094358e-01 7.222583e-01 21 1.068767e-04 7.077111e-01 8.743413e-01 22 1.096009e-04 7.056321e-01 1.048175e+00 23 1.150494e-04 7.009675e-01 1.400214e+00 ... (510 more rows) ...
Common mistakes and how to avoid them
- Reversing the Photodiode:
- Error: Connecting the Anode to Base and Cathode to Ground.
- Result: The diode acts like a standard forward-biased diode, clamping the Base to ~0.7 V permanently or conducting fully, preventing the switching logic.
- Solution: Ensure the Cathode (marked with a line or flat side) connects to the positive side (Base) for reverse bias operation.
- Incorrect Transistor Pinout:
- Error: Swapping Collector and Emitter on the 2N2222.
- Result: The gain is significantly reduced, and the LED may not turn on fully or the transistor might overheat.
- Solution: Verify the pinout (E-B-C) in the datasheet before insertion.
- Wrong Resistor Value for R1:
- Error: Using a very low value (e.g., 1 kΩ) for R1.
- Result: The photodiode current cannot pull the voltage down enough in bright light, keeping the LED ON permanently.
- Solution: Use a high value (100 kΩ to 330 kΩ) to allow the small photocurrent to control the voltage divider effectively.
Troubleshooting
- Symptom: LED is always ON, even in bright light.
- Cause: R1 is too small, or the ambient light is not strong enough to generate sufficient photocurrent.
- Fix: Increase R1 to 220 kΩ or 330 kΩ, or bring the light source closer.
- Symptom: LED is always OFF, even in total darkness.
- Cause: Photodiode is shorted, R1 is open, or Transistor is blown.
- Fix: Check continuity on R1. Remove D1; if LED turns on, D1 was shorted or installed backward (forward biased).
- Symptom: LED glows dimly in the dark.
- Cause: Q1 is not fully saturating.
- Fix: Decrease R2 slightly (ensure it stays above 220 Ω) or check if V1 is actually 5 V.
Possible improvements and extensions
- Sensitivity Adjustment: Replace R1 with a 500 kΩ potentiometer (in series with a 10 kΩ safety resistor) to manually tune the light level at which the LED triggers.
- Hysteresis/Clean Switching: Add a second transistor or a Schmitt Trigger (e.g., 74HC14) between the photodiode node and the driver transistor to prevent the LED from flickering at the «twilight» threshold.
More Practical Cases on Prometeo.blog
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Carlos Núñez Zorrilla
Electronics & Computer Engineer
Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).
