Experiment 10: Transistor Switching
74 Chapter 2
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.
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 open
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 volts DC from a power supply of 1 amp or less.
But it’s not a good idea to put the finger of one hand on one wire, and the finger of
your other hand on the other wire. This would allow the electricity to pass through
your body. Although the chance of hurting yourself this way is extremely small, you
should never allow electricity to run through you from one hand to the other. Also,
when touching the wires, don’t allow them to penetrate your skin.R1
R3
S1
Q1
R2
Figure 2-86. This shows the same circuit as
the breadboard diagram in Figure 2-85.