615611 Electronics Workshop 2 Manual - Thames & Kosmos

23 Playground (I)
Let’s play a little with the circuits in this section, because the
results can be fun. You’ve already developed the knowledge
you’ll need to understand how they work.
Nostalgia on rails
Model railroad systems are one of the last places where the
“good old” steam locomotives run. Unfortunately, the only
thing humming there is an electric motor. But that can be
fi xed.
225. With the circuit shown in Figure 230, you’ll
be close to the real thing. Turn up the potentiom-
225 eters and listen to all the noises! The base-emitter
section of T1 will be powered with a higher
blocking voltage than it’s designed for. That means
it keeps breaking through, with R1 limiting the currents
to approximate values. That makes a nice rushing sound.
This noise voltage is led back to the AMP via the G1 of T2,
and emitted as noise through SP. In parallel, G2 receives the
charge-discharge voltage of 555 that appears at C3. It oscillates
as an astable multivibrator at a rhythm of more or less
once per second. Have fun!
Figure 320. 555 gets a head of steam: sounds just like a locomotive.
Figure 321. Layout for Figure 320.
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