Topics Discussed
A demonstration of completing a circuit with a hot dog was
shown in the beginning of class. A 120 V potential difference was placed across
the hot dog, and the hot dog acted as a wire in the circuit. It was predicted
that the hot dog would slowly cook, and this is indeed the case. LED lights
were placed across the hot dog, some parallel and some perpendicular to the
hot dog. (Fig. 1) It was predicted that the LEDs parallel to the hot dog would light up,
since they were in the direction of the potential difference, while those
perpendicular would experience no potential difference and thus remain off.
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Fig. 1 |
Circuits of varying
complexities were introduced in class. More practice with dependent sources was
done, as well as an introduction to voltage dividers that involved an abundance
of in class group work. The equation of voltage dividers and resistors was
derived, and more examples reinforced the concept. (
Fig. 1 &
Fig. 2)
|
Fig. 2
A circuit with an LED is shown, instructing us to find the range of possible resistors |
|
Fig. 3
A circuit with a current controlled current source (CCCS) is shown. |
Dusk-to-Dawn Light Lab
In the dusk-to-dawn light lab, our goal was to create a circuit which would cause an LED light to turn off and on as its surroundings became more and less luminous. A BJT (Bipolar Junction Transistor) was used as a switch in this lab, and a photocell was used as the device which would react to how luminous our environment was, and would obtain a certain resistance accordingly. This varying resistance is what would allow, or not allow, current to pass through the LED light. To create the circuit, it was first determined how voltage would be affected when the photocell obtained different resistances (Fig. 4).
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Fig. 4 |
We first tested the photocell to see the varying resistances we would get at high and low light situations. We found that at its highest level of resistance under direct flash, the photocell was capable of up to 60k ohms as read by our DMM. At the lowest level of light, we measured a resistance of roughly 100 ohms.
We proceeded to create the circuit, following the diagram given in the lab manual. Using a bread board, BJT, photocell, 10k ohm resistor, and wires as needed, our circuit was operational. (Fig. 5 & Fig. 6)
|
Fig. 5 |
|
Fig. 6 |
As intended, the led light turned on in low light environments, and off in high level light environments. We had calculated the voltage across the photocell (potentiometer) to be between .05V and 4.3V at the lowest and highest resistance respectively. (Fig. 7) We then measured the voltage drop across the photocell using our analog device. (Fig. 8)
|
Fig. 7 |
|
Fig. 8 |
Video demonstration:
Summary
Having made the circuit work under the expected conditions, the voltages measured across the photocell were much different then we had initially calculated. As seen in Fig. 8, the voltages ranged from 0.32V in the light, and only 0.72V in the dark. Although this was initially surprising, a possible explanation for this difference may lie when accounting for a possible lower resistance across the photocell then initially expected. This lower resistance would in turn cause a lower voltage reading across the photocell.
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