Light/Temp Switch (using a Darlington Pair)

The light attached to the original circuit increases its brightness as its surroundings get darker. This is a practical circuit that uses a Darlington pair as a transistor amplifier to function, and has a few variants that can be created with small modifications (you could reverse the effect of the circuit or make it sensitive to temperature instead of light).

1. Matrix perforated board
2. Variable resistor (100kΩ)
3. Resistor (1kΩ)
4. LDR (light sensor, required) and/or thermistor (optional)
5. Bulb (6V, 0.06A)
6. 6V batteries
7. NPN transistors (BC108 and BFY51)

A Darlington pair requires either 2 BJT transistors (either both PNP or NPN). There is a configuration that can be made with one PNP and one NPN transistor called the Sziklai Darlington pair.

This project uses 2 NPN transistors (BC108 and BFY51). Each transistor has 3 leads: the base, emitter and collector. In an NPN transistor, the base is used to control the flow of the current which flows through from the emitter to the collector. To figure out which lead is which (this varies based on the transistor you use) I've attached labeled pictures of the transistors used in this circuit.

The key component of this circuit is made up of the two transistors using a Darlington configuration. In this, the base of one transistor and emitter of the other are connected, as well as both connectors. It's usually used when a circuit can benefit from a high current gain as it works better than using a single transistor, or to increase the sensitivity to the current. In this project, it is used to amplify the current in the circuit so the bulb lights up.

The picture above of the TinkerCAD breadboard simulation uses a regular 100k resistor instead of a variable resistor, so in the simulation the brightness of the circuit's surroundings doesn't affect the brightness of the LED.

Alternatively, the LDR can be replaced with a thermistor to make the circuit sensitive to temperature instead of light and the LDR can be swapped with the other resistor to reverse the effect of the circuit.