Elenco Basic Electronic Experiments - Manual (Page 43)

43
EXPERIMENT #28: THE LIGHTHOUSE
Connect the circuit. Notice that the transformer is being used as two coils (inductors) here. Also notice that two transformer
taps 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.
+9V
100
μ
F
1M Ω
S
P
(1M Ω = brown-black-green-gold)