Light and Dark Detector
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About this MicroSim
A light-dependent resistor and a fixed resistor form a voltage divider. The divider's output drives a transistor base through a resistor, and the transistor switches an LED and a buzzer.
Sliding the light level changes the divider voltage smoothly, but the output switches abruptly. That gap between a continuous input and an on-or-off output is the whole point of the circuit, and it is what makes a transistor a switch rather than a dimmer.
The Circuit
9 V rail → LDR (0–100 kΩ) → divider junction → R2 4.7 kΩ → ground
divider junction → RB 4.7 kΩ → Q1 base
B+ rail → RL 470Ω → D1 → Q1 collector; BZ1 in parallel with D1
Q1 emitter → ground
| Component | Value | Purpose |
|---|---|---|
| LDR | 0–100 kΩ | resistance falls as light rises |
| R2 | 4.7 kΩ | the divider's lower leg — sets the trip point |
| RB | 4.7 kΩ | limits base current |
| Q1 | NPN | switches the load |
| RL | 470 Ω | current-limiting for the LED |
| D1, BZ1 | red LED, buzzer | the two outputs, in parallel |
Where the trip point comes from
The transistor needs about 0.7 V at its base. The divider delivers 9 × 4.7 kΩ / (RLDR + 4.7 kΩ), so it reaches 0.7 V when the LDR is around 55 kΩ — a bit over half of its dark resistance. That is why the LED switches near the middle of the slider. Change R2 and the trip point moves, which is exactly how you calibrate a real detector.
How to Use It
- Press Start, then drag Light level slowly from 0% upward.
- Watch the divider voltage climb steadily while the LED stays dark.
- Somewhere near the middle, the LED and buzzer snap on together.
- Keep going to 100%. Note that the LED does not get brighter — once the transistor is saturated, the load resistor sets the current, not the light.
What the Animation Shows
Below the threshold, a small base current flows but the collector path carries nothing, so the load wires show no dots. Above it, the collector wires come alive. The scope makes the same point numerically: the blue divider-voltage trace is a smooth ramp, and the red LED-current trace is a step.
Lesson Plan
Grade Level, Subject and Topic
High school. Electronics. Voltage dividers, transistor switching, sensor interfacing.
Learning Objective
Students will be able to explain how a voltage divider converts a sensor's resistance into a voltage, and predict the light level at which a transistor switch turns on, by sweeping the light level and reading the divider voltage against the 0.7 V base threshold.
Activities
Find the threshold
Sweep slowly and record the light percentage at which the LED comes on. Then read the divider voltage at that instant. Students should find ≈0.7 V every time, whatever path they took to get there.
Predict the calibration change
Ask: if we replaced R2 with a 10 kΩ resistor, would the light have to be brighter or dimmer to trip the circuit? Reason it out from the divider formula before checking.
Why doesn't it dim?
At 60% and at 100% light the LED current is identical. Ask students to explain using the collector loop: once the transistor saturates, what sets the current?
Assessment
- What is the divider voltage in complete darkness, and why is the LED off?
- The base sits at 0.72 V with the LED on. Why doesn't it rise to 5 V?
- Turn this into a dark detector instead. What single change do you make?
Model Limitations
This simulation solves the circuit in DC steady state. The transistor is modelled as a switch with a saturation drop, not with a current gain (beta) curve, so it is suitable for switching lessons but not for amplifier design. There is no transient response, no capacitance, no inductance and no AC. Component values are ideal.
References
- Push Button and LED Circuit — LEDs and current-limiting resistors on their own
- Series vs Parallel Explorer — the parallel loads used here, in isolation