Chapter 4, Two Projects and Then Some More Video Solutions, Electronic Circuits for the Evil Genius: 64 Lessons with Projects

Problem 1

Set the pot so the resistance between legs $\mathrm{A}$ and C, the center leg, is close to 50 kilo-ohms. Figure L12-3 shows how the voltage and current move in the different conditions for this setup.

Salamat Ali

Numerade Educator

02:25

Problem 2

Now set your circuit under a fairly good light. Measure and record the voltage at the test point shown in Figure L12-4. ______ V

Khoobchandra Agrawal

Numerade Educator

14:57

Problem 3

Now cover the LDR with a heavy, dark pen cap. Measure and record the voltage at the test point again. ______ V
Did the LED output change at all?
So here's a major question: Do you recall how much voltage is used from $\mathrm{V}+$ to ground?
■ Does it depend on the amount of voltage available? NO!
■ Does it depend on the number or types of parts in the circuit? NO!
By definition, voltage used between $\mathrm{V}$ + and ground doesn't relate to any circuit variables.
The question is like asking, "How much distance is there between this altitude and sea level?'
The answer is - whatever the altitude is all the distance!
By definition, ground is 0 volts.
■ How much voltage is used between $\mathrm{V+}$ and ground?
■The answer will always be All the voltage is always used between $V+$ and ground.

Brandy Heflin

Numerade Educator

Problem 4

Okay. Figure L12-5 will help explain how all the voltage in this circuit is used.
Remember the idea of the waterfall when we first looked at resistors. As the size of the load increased, the amount of voltage used increased proportionately. The same thing happens with two resistor loads in this circuit. More detail is given in Figure L12-6.
The pot uses some voltage because it is set near $50,000 \mathrm{ohm}$ here.
The LDR uses a small amount of the voltage in the light because it has a small resistance. When it is in the dark, the LDR has a great deal of resistance. The base of the transistor reacts to the voltage available at that point where the LDR and pot connect. It becomes obvious which situation provides more voltage to the transistor's base.

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Problem 5

Set the pot so the resistance between legs A and $\mathrm{C}$, the center leg, is close to $0 \mathrm{ohms}$. Figure L12-6 shows how the voltage available to the transistor base is identical in both the light and the dark. The potentiometer uses none of the voltage, so the base of the transistor is exposed to nearly full voltage in both circumstances.

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01:30

Problem 6

Now set your circuit under a fairly good light. Measure and record the voltage at the test point shown in Figure L12-6. _____ V

Thomas Thompson

Numerade Educator

Problem 7

Now cover the LDR with a heavy, dark pen cap. Measure and record the voltage at the test point again. _____ V
Did the LED output change at all? Did you expect the output to change?

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Problem 8

Set the pot so the resistance between legs A and C, the center leg, is close to 100 kiloohms. Figure L12-7 shows how the voltage is responding to the changing resistance of the LDR, changing the voltage available to the base of the transistor. The pot here is adjusted to twice as much resistance as before, so it will use twice as much of the voltage available. The LDR must be set into nearly complete darkness or you will get muddy results.

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01:22

Problem 9

Now set your circuit under a fairly good light. Measure and record the voltage at the test point. _____V

Saman Zulfiqar

Numerade Educator

Problem 10

Now cover the LDR with a heavy, dark pen cap. Measure and record the voltage at the test point again. _____V
Did the LED output change at all?

Check back soon!