Sensor Circuits – Potential Divider with LDR and Thermistor

What it shows

Automatic night lights, fans that start when a room gets hot and frost alarms all rely on a potential divider. A sensor whose resistance changes with light or temperature is connected in series with a variable resistor, and the voltage at the junction, Vout = Vs × Rbottom / (Rtop + Rbottom), controls a switch. A transistor starts to conduct when its base reaches about 0.7 V; a comparator switches when Vout passes a reference voltage. Putting the sensor at the top or the bottom of the divider reverses the action, and the variable resistor sets the threshold.

How to use

Choose the Sensor, the Sensor position, the Switch and the Load. Move the Light or Temperature slider, or drag across either graph, and watch the sensor resistance, Vout and the load current change. Adjust Variable resistor to shift the threshold and Supply Vs to change the supply voltage. Press Swap sensor ↕ to invert the behaviour of the circuit.

Parameters you can change

• Sensor Light-dependent resistor (LDR), Thermistor (NTC)
• Sensor position in the divider Bottom (between Vout and 0 V), Top (between +Vs and Vout)
• Variable resistor value 0.1–100 kΩ
• Supply voltage Vs 5–12 V
• Load LED, Fan, Buzzer
• Switching device NPN transistor, Comparator

Questions to explore

1. Why does placing the LDR in the bottom half of the divider make the lamp switch on in the dark?
2. To make the lamp switch on earlier at dusk, should you increase or decrease the variable resistor?
3. With the comparator, what is the sensor resistance at the switching point, and why?