Now we repeat the same process all over again. Figure 2-82 shows this in graphical form. Note that after 5 seconds, the capacitor has acquired 11.92 volts, which is 99% of the power supply voltage. This should be close enough to satisfy anyone’s real-world requirements.
Time
in secs
V1:
Charge on capacitor
V2:
12 – V1
V3:
63% of V2
V4:
V1 + V3
0
0.00
12.00
7.56
7.56
1
7.56
4.44
2.80
10.36
2
10.36
1.64
1.03
11.39
3
11.39
0.61
0.38
11.77
4
11.77
0.23
0.15
11.92
5
11.92
If you try to verify these numbers by measuring the voltage across the capacitor as it charges, remember that because your meter steals a little current, there will be a small discrepancy that will increase as time passes. For practical purposes, the system works well enough.
Figure 2-82. A capacitor starts with 0 volts. After 1 time constant it adds 63% of the available voltage. After another time constant, it adds another 63% of the remaining voltage difference, and so on.
Experiment 10: Transistor Switching
You will need:
AC adapter, breadboard, wire, and meter.
LED. Quantity: 1.
Resistors, various.
Pushbutton, SPST. Quantity: 1.
Transistor, 2N2222 or similar. Quantity: 1.
Potentiometer, 1 megohm, linear.
A transistor can switch a flow of electricity, just like a relay. But it’s much more sensitive and versatile, as this first ultra-simple experiment will show.
We’ll start with the 2N2222 transistor, which is the most widely used semiconductor of all time (it was introduced by Motorola in 1962 and has been in production ever since).
First, you should get acquainted with the transistor. Because Motorola’s patents on the 2N2222 ran out long ago, any company can manufacture their own version of it. Some versions are packaged in a little piece of black plastic; others are enclosed in a little metal “can.” (See Figure 2-83.) Either way, it contains a piece of silicon divided into three sections known as the collector, the base, and the emitter. I’ll describe their function in more detail in a moment, but initially you just need to know that in this type of transistor, the collector receives current, the base controls it, and the emitter sends it out.
Figure 2-83. A typical transistor is packaged either in a little metal can or a molded piece of black plastic. The manufacturer’s data sheet tells you the identities of the three wire leads, relative to the flat side of a black plastic transistor or the tab that sticks out of a metal-can transistor.
Use your breadboard to set up the circuit shown in Figure 2-85. Be careful to get the transistor the right way around! (See Figure 2-84.) For the three brands I have mentioned in the shopping list, the flat side should face right, if the transistor is packaged in black plastic, or the little tab should face toward the lower left, if the transistor is packaged in metal.
Figure 2-84. The 2N2222 transistor may be packaged in either of these formats. Left: RadioShack or Fairchild. Right: STMicroelectronics (note the little tab sticking out at the lower-left side). If you use a different brand, you’ll have to check the manufacturer’s data sheet. Insert the transistor in your breadboard with the flat side facing right, as seen from above, or the tab pointing down and to the left, seen from above.
Figure 2-85. The transistor blocks voltage that reaches it through R1. But when pushbutton S1 is pressed, this tells the transistor to allow current to pass through it. Note that transistors are always identified with letter Q in wiring diagrams and schematics.
S1: Pushbutton, momentary, OFF (ON)
R1: 180Ω
R2: 10K
R3: 680Ω
Q1: 2N2222 or similar
D1: LED
Initially, the LED should be dark. Now press the pushbutton and the LED should glow brightly. Electricity is following two paths here. Look at the schematic in Figure 2-86, which shows the same circuit more clearly. I’ve shown positive at the top and negative at the bottom (the way most schematics do it) because it helps to clarify the function of this particular circuit. If you view the schematic from the side, the similarity with the breadboard layout is easier to see.
Figure 2-86. This shows the same circuit as the breadboard diagram in Figure 2-85.
Through R1, voltage reaches the top pin (the collector) of the transistor. The transistor only lets a tiny trickle of it pass through, so the LED stays dark. When you press the button, voltage is also applied along a separate path, through R2 to the middle pin (the base) of the transistor. This tells the transistor to close its solid-state switch and allow current to flow out through its third pin (the emitter), and through R3, to the LED.
You can use your meter in volts DC mode to check the voltage at points in the circuit. Keep the negative probe from the meter touching the negative voltage source while you touch the positive probe to the top pin of the transistor, the middle pin, and the bottom pin. When you press the button, you should see the voltage change.
Fingertip Switching
Now here’s something more remarkable. Remove R2 and the pushbutton, and insert two short pieces of of wire as shown in Figure 2-87. The upper piece of wire connects with the positive voltage supply; the lower piece connects with the middle pin of the transistor (its base). Now touch the tip of your finger to the two wires. Once again, the LED should glow, although not as brightly as before. Lick the tip of your finger, try again, and the LED should glow more brightly.
Never Use Two Hands
The fingertip switching demo is safe if the electricity passes just through your finger. You won’t even feel it, because it’s 12
