|GCSE Timing 555 555 Astable 555 Monostable >555 Motor Speed< Mark Space|
Timing 555 Motor Speed
Pulsed switching circuits can be quite tunefull.
If full power were applied to the motors when they were not spinning, the current would be so great that the power grid would fail. There might also be some wheel spin - on a train! and damage to the motors not to mention toppled grannies. Once the motors are running, there is Back EMF which reduces the current to a safer level. This is why the power has to be increased gradually.
This speed controller works in the same way.
This screen shot shows the astable (Blue) producing fixed width monostable pulses (Red) with about 20% duty cycle.
This screen shot shows the astable (Blue) producing fixed width monostable pulses (Red) with about 90% duty cycle.
Adjust the potentiometer to change the frequency and vary the bulb brightness.
A thermistor in a voltage divider generates a temperature dependent voltage. This is amplified and used to control the 555 timer. The frequency and mark space ratio (duty cycle) vary depending on the control voltage. This makes a cooling fan run fast if it's hot or slowly if it's cool. The potentiometer is used to adjust the threshold temperature where the circuit starts to operate. This is a 12 Volt circuit (depending on the motor).
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