Practical case: Simple twilight switch

Simple twilight switch prototype (Maker Style)

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 VCC and negative terminal to GND.
  • R1 (Bias): Connect between VCC and V_BASE.
  • D1 (Photodiode):
    • Connect the Cathode to V_BASE.
    • Connect the Anode to GND.
    • Note: The photodiode is used in reverse bias mode.
  • Q1 (Transistor):
    • Connect the Base to V_BASE.
    • Connect the Emitter to GND.
    • Connect the Collector to V_COL.
  • R2 & D2 (Output Loop):
    • Connect R2 between VCC and the Anode of D2.
    • Connect the Cathode of D2 to V_COL (the Collector of Q1).

Conceptual block diagram

Conceptual block diagram — 74HC14 Photodiode
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

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 ]
Schematic (ASCII)

Measurements and tests

Perform the following validation steps using a multimeter to ensure the circuit works as intended.

  1. Check Supply Voltage:
    • Measure between VCC and GND. It should read approximately 5 V.
  2. 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 to VCC (High) as the transistor is in cutoff.
  3. 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) ...

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* 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
.end

Simulation Results (Transient Analysis)

Simulation 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

  1. 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.
  2. 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.
  3. 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

  1. 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.
  2. 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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Quick Quiz

Question 1: What is the primary objective of the circuit described in the text?




Question 2: Which component acts as the electronic switch in this circuit?




Question 3: What happens to the LED when the environment is bright?




Question 4: What is the function of the 330 Ω resistor (R2)?




Question 5: Which voltage threshold at the transistor base is mentioned for turning the LED ON?




Question 6: What real-world application is listed for this type of circuit?




Question 7: What is the role of resistor R1 (100 kΩ)?




Question 8: What is the function of the V1 component in the materials list?




Question 9: How does the circuit behave during the transition from light to dark?




Question 10: Who is the target audience for this circuit project?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

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

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