Elenco Basic Electronic Experiments - Manual (Page 57)

57
EXPERIMENT #40: BLINKING LIGHTS
Take a look at the schematic. This circuit configuration is a type of oscillator called an astable multivibrator. What do you
think it will do? Connect the circuit, noting that the transistor bases are not connected although their wires cross in the
schematic. Initially set the variable resistor (VR) to its minimum value (turn it to the left). Press the switch and hold it down.
One LED is on while the other is off, and they change about every second. What do you think will happen as you turn the
knob on the VR? The right LED stays on longer than the left one.
In this circuit, one transistor is always on while the other is off. In this type of oscillator there is no inductor, the frequency
is controlled only by the resistors and capacitors. The 100k
Ω and 10μF determine how long the left transistor is on and
the 3.3k Ω, VR, and 100μF determine how long the right transistor is on. If you want to experiment with changing part
values, go ahead. But don’t replace the capacitors with the smaller disc ones (you’ll see why in the next experiment).
Blinking lights like this are often used to attract people’s attention.
Note that battery
connection point
has changed.
+9V
100k
Ω 1k
Ω
3.3k
Ω 470
Ω
100
μ
F
+
10
μ F
+
Note positions of
LED and transistor
flat sides