Elenco Electronic Playground 50-in-1 Experiments - Manual (Page 43)

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Connect the wires according to the Wiring Checklist.
Notice that the transformer is being used as two coils
(inductors) here. Also notice that transformer springs 23
and 24 are not connected although their wires cross in
the schematic. Press the switch and hold it down for a
while. The LED blinks every few seconds, like a tiny
lighthouse!
Notice that the LED blinks at a constant rate. This circuit
is called an oscillator. It uses feedback. Feedback is
when you adjust the input to something based on what its
output is doing. The collector signal is fed back to the
base through a coil (part of the transformer) and the
100
μF capacitor. If you disconnect this feedback path
then the LED will be on continuously, because the
feedback is what turns the transistor on and off. The rate
at which the transistor is turned on and off is called the
frequency and is controlled by the resistor, capacitor, and
coil in the circuit. You can speed up the frequency (the
LED blink rate) by changing the resistor or capacitor to
smaller values. Try replacing the 1M
Ω resistor with the
100K
Ω resistor and see what happens.
Feedback is necessary for this circuit to work, but in
some cases it can be harmful. In an auditorium or
concert hall you sometimes hear a microphone scream
when it is located too close to the speaker. In this case
the sound from the speaker is feeding back into the
microphone.
EXPERIMENT #26: The Lighthouse
Wiring Checklist:
o 27-to-56
o 54-to-55-to-24
o 23-to-36
o 37-to-53-to-18
o 19-to-4
o 3-to-25
o 20-to-26
Schematic