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Touch switch
13
Behind door number 13, you will find a
second transistor of type BC547. Along with
the first transistor, it should now provide
even more amplification. Both collector
terminals are directly connected, and the
emitter of the first transistor leads to the base
of the second transistor. This circuit is called
a Darlington circuit. With this a touch switch
will be built here. If you touch the cable and
the resistor with your finger at the same time,
a very small, harmless and imperceptible
current flows through your finger, which is
amplified to such an extent that both LEDs
are switched on.
The speaker is again connected to the
collector of the transistors. And sometimes
you will hear strange sounds. Just touch
the base cable. Depending on where you
are, you may hear a crackling, humming or
buzzing sound from the speaker. The noise
can become even stronger if two people
touch both wires. It comes from the electrical
wiring in the room. If you also move your
feet on the floor, you can sometimes see the
LEDs flashing or flickering. It indicates the
electric charge of your body through friction
on the shoes.
The Darlington Circuit
The connection of two transistors as shown in the circuit diagram is
called a Darlington circuit. Two transistors amplify more than one.
This is especially true for this circuit, where the already amplified
current is amplified again by a second transistor. The name comes
from its inventor, Sidney Darlington, who came up with the idea
in 1952. Both collectors are connected, and the emitter of the first
transistor flows to the base of the second transistor. The Darlington
circuit behaves like a single transistor with huge gain.
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