Today we learned abut voltage/current dividers, a lab using a bipolar junction transistor (BJT) as a switch, and how DMM's work to measure voltage and current.
The objective of this lab is to create a circuit that will make an led turn on when there is a small amount of ambient light, and turn off when there is a high ambient light intensity. This circuit used a LDR to vary the resistance with respect to the amount of light it perceives, and a BJT (2N3904) to act as a switch. We will assume the LDR resistance value at a high amount of light is 5k ohms and 20k ohms with low light.
The picture above shows the diagram of the circuit we built. The 10 ohm resistor in the picture is actually a 10k ohm resistor. As shown above, we calculated the voltage across the LDR to be 1.67V for high light levels and 3.33V for low light levels.
This is the voltage measured at a high light level.
This is the voltage that we measured at a low light. The measured values are off by about one volt at high light levels and half a volt at low light levels. We believe this to be due to our original assumption that the LDR is at 5k and 20k ohms respectively. As for using the BJT as a switch, if works quite fine. Check the link below for a video of the circuit in action!
We learned how the voltmeter measures voltage by a connection as a parallel to the circuit. Such that the original circuit not be affected by the voltmeter, the voltmeter has a very high resistance which is what is represented by the equation above. R_m is the internal resistance of the meter.
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