Dusk-to-Dawn Light

In this experiment, a photocell was used in conjunction with an LED and a 2N3904 Bipolar Junction Transistor to create a light that automatically illuminates once the ambient light is dimmed.We began by employing KVL to determine the voltage across the photocell when it has a resistance of 5K Ohms and 20K Ohms (in a light and dark environment, respectively). 

The calculated values were as follows: 

Vb of photocell in the dark (20K Ohms) => 3.34 V

Vb of photocell in the light (5K Ohms) =>  1.665 V

We then built the circuit that we had been analyzing, to experimentally test it. The following is the schematic for  the circuit:

Before connecting the photocell though, we measured the resistance of the photocell both covered and uncovered, and found them to be 82.8k Ohms and 2.48k Ohms respectively. We also measured the resistance of the 10k Ohm resistor and determined it to be 9.90k Ohms. These measurements are important, as they were expected to cause a rather significant change in the operating voltages of our circuit. The completed circuit appeared as follows:

We then tested the circuit, and as the video shows, it worked!

We then measured the base voltages from the BJT to ground, as well as the voltage across the LED both when the LED was illuminated, and when it wasn't illuminated. These are the results:

LED off

Voltage from BJT to ground: 1.01V plus/minus .03V

Voltage across LED : 0.65V plus/minus .03V

LED illuminated

Voltage from BJT to ground: 2.76V plus/minus .01V

Voltage across LED : 2.12V plus/minus .03V

The voltages of the BJT to ground vary from our calculated values, but it is to be expected; the values that we calculated were based on entirely different values of resistance for the photocell, so it makes sense that they were different. They were however somewhat close, and the voltages in both cases increased when the LED was illuminated, so it's safe to say that the circuit behaved as expected. Perhaps with recalculation (using experimentally determined values of resistance), voltages that are significantly closer to the experimental values would be determined.