Dusk-to-Dawn Light

This lab creates a circuit that operates as a "dusk-to-dawn" light. It turns a light on when the ambient light level goes below a certain level.
Circuit

BJT is a current controlled current source. The collector–emitter current can be viewed as being controlled by the base–emitter current (current control), or by the base–emitter voltage (voltage control). 

The calculation shows that the voltage of the photocell is 5/3 V when the resistance is 5K ohms. The voltage is 10/3 V when the resistance is 20K ohms. 

There is a 10K Ohms resistance connected in the circuit.

When the photocell does not receive enough light, the LED will be turned on. Likewise, if the photocell receives enough light, the LED remains off. Here is a failed trial that shows exactly the opposite due to the negative voltage mistake.

After we fixed the voltage problem, it works in the intended way.

Summary:

The photocell works as a light-sensitive resistor. It has high resistance when it received limited light, but the resistance drops as it receives more light. By using the Bipolar Junction Transistor in the circuit, it can operate as a trigger for dusk-to-dawn light.