3-Mar-2015: Dusk-to-Dawn Light

DEMONSTRATION

At the beginning of class, we had a demonstration in which a current was applied across a hot dog with LEDs connected to it. Some were positioned perpendicularly from the hot dog, while others were positioned parallel to it. The ones that were positioned parallel to the hot dog lit up from the current, while the perpendicular LEDs did not. This is because the ones that were parallel with the hot dog were actually in series with the hot dog/circuit, while the perpendicular ones were parallel to the hot dog/circuit. As a result, the parallel LEDs received the current, while the perpendicular ones did not (at least not enough).

PURPOSE

The purpose of this experiment was to become familiar a type of variable resistor known as the photocell and a current-controlled current source called a bipolar junction transistor (BJT).

PRE-LAB

Figure 1: Mathematical process for pre-lab

Before starting our experiment, we solved for the theoretical values of the voltage across the photocell (Vb). We did this by applying KVL across the circuit displayed within the red circle in Figure 1. Also included in Figure 1 is the mathematical process for solving for Vb when we assumed the resistance of the photocell to be 5kΩ. We found this value to be 1.667. We followed the same process to solve for Vb when the resistance of the photocell was assumed to be 20 kΩ. We found this value to be 3.333 V.

PROCEDURES

After solving for the theoretical values in the pre-lab, we set up the circuit as shown below in Figure 2. The photocell is labeled with the red circle, the BJT with the green circle, the fixed resistor with the purple circle, and the LED with the blue. In addition, the black and red clips represent the 5 V voltage source.

Figure 2: Set-up of our circuit consisting of a photocell, a BJT, a fixed resistor,
an LED, and a voltage source

Next, we applied 5 V to the circuit and measured the voltage across the photocell and the LED when the photocell was allowed to receive light and when it was not. In the pre-lab, we assumed that the resistance of the photocell was 20 kΩ when the light was on and 5 kΩ when it was off. The measured values are shown below in Figure 3.

Figure 3: Measured voltages across the photocell and the LED, respectively

When the photocell was exposed to light, the LED remained off. However, when the light was cut off from the photocell, the LED lit up. This is due to the fact that the photocell's resistance is much higher when it is exposed to light. As a result, the photocell behaves like a on/off switch that is regulated by light. A video of this process is shown below.

CONCLUSION

In this experiment, we became familiar with two circuit elements: the BJT and the photocell. This will be a valuable lesson because these are commonly used in circuits. Moreover, this experiment allowed us to apply what we learned about KVL and voltage division.

Furthermore, we compared these measured values to the values that we found in the pre-lab by calculating the percent difference between the expected values and measured values. We found the percent difference to be 10.6 percent for when the light was off and 94.0 percent when the light was on. We noticed that the percent difference for when the light was on was incredibly high. We believe this may be due to the fact that resistance of the photocell was not actually 20 kΩ when it was exposed to light. It also could have been due to a mathematical error in our calculations.