LAB #08: Dusk to Dawn Light

OVERVIEW:
The purpose of this lab was to sketch, analyze, and build a light-sensor circuit using a BJT, an LED, and a photoresistor.

PROCEDURE:
Pre-Lab:
As illustrated in the ENGR 44 Lab book, use KVL on the outer loop of the circuit in Figure 1 to determine to determine the predicted voltage difference Vb when the resistance across the photoresistor is:

• R = 8.25k (uncovered)
• R = 11.37k (covered)

Lab:

1. Construct the circuit given in the lab book, measure the base voltage of the BJT, Vb and the voltage across the LED, Vd for both resistances in the pre-lab.
2. Calculate the percent difference between your predicted values and measured values for Vb.

Physical Circuit

ANALYSIS:
Pre-Lab:
Predicted Voltage Values:
A) Vb = 2.26 V (uncovered, R = 8.25k)
B) Vb = 2.66 V (covered, R = 11.37k)

Pre-Lab Calculations w/ Circuit Schematic

Lab:
Actual Voltage Values:
A) Vb = 2.3 V (uncovered, R = 8.25k)
B) Vb = 2.53 V (covered, R = 11.37k)

C) Vd = 1.86 V (uncovered, R = 8.25k)
D) Vd = 1.78 V (covered, R = 11.37k)

Sample: Vd Measurement on DMM

Sample: Vb Measurement on DMM

Part 2:
Percent error between predicted and measured voltage:
Error in Vb (uncovered): (|2.26 - 2.3| / 2.26)* 100 = 1.76%
Error in Vb (covered): (|2.66 - 2.53| / 2.66)* 100 = 4.89%

CONCLUSION:
Our measured values for the voltage difference Vb matched relatively close to their corresponding predicted values. During the pre-lab, we used KVL to predict the values of Vb and we found these values to be 2.26 V when the photoresistor was uncovered and 2.66 V when the photoresistor was covered. For the actual lab, we measured Vb (uncovered) = 2.3 V and Vb (covered) = 2.66 V, which corresponds to a percent error of 1.76% and 4.89%  respectively. This uncertainty is likely due to the internal resistance of the DMM, which may have caused our measured voltage values to be slightly off. Another possible source of error is the release of thermal energy due to the resistors.