This project is a simple pseudo-random LED selector circuit based on an astable multivibrator.
Two LEDs alternate extremely quickly, and when the button is pressed, the oscillation stops and only one LED remains illuminated.
Because the oscillation is running too fast for the human eye to follow, it is impossible to predict which LED will be on when the circuit is stopped. The result therefore behaves like a simple electronic coin toss.
How does it work ?
The main part of the circuit is an astable multivibrator built with two transistors.
An astable multivibrator has no stable state. It continuously switches between two states, causing the two LEDs to turn on and off alternately.
Transistors Q3 and Q4 act as electronic switches that freeze the astable multivibrator.
Their bases are tied together to a common node, which is controlled by SW1.
When the button is pressed, Q3 and Q4 are driven into saturation. They then behave roughly like closed switches.
This closes the two feedback paths of the astable multivibrator and prevents the normal alternating action of the oscillator.
Electronics component Fun Kit (with breadboard & power supply module)
General purpose transistors - BC547 (NPN) & BC557 (PNP) pack
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