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A simple pulse proportional control circuit from the 1960s.
The 50k variable resistor controls the on time and the off time for the relay allowing pulse proportional control of the load.
The 20k variable resistor allows some control of the switching speed.
A slight theoretical problem is when a capacitor is fully charged and the transistor collector it is connected to is pulled to ground, the base of the other transistor can be pulled down below its maximum reverse base emitter voltage. In practice this is not a major problem but can easily be avoided by adding a couple of diodes to the circuit.
If the supply voltage to an astable multivibrator is greater the maximum reverse base emitter voltage of the transistors add protecting diodes.
Pulse proportional control circuit with cleaner square wave output.
For higher supply voltages the protecting diode are still needed.
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