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The 555 timer can be configured in a variety of ways. When used in a tachometer circuit it is configured as a monostable vibrator, ie a device that has one stable state on its output terminal. Let's consider that state as "OFF" .
If a trigger pulse is sent to the input it will turn "ON" for a fixed period of time set by the circuit time constant then turn "OFF" again.
Consider a single cylinder four stroke petrol engine firing once every two revolutions and revolving at 1000 rpm. A strong pulse will be generated from its ignition circuit 500 times a minute - or every 120 milliseconds. With some
pulse shaping and amplitude limiting, a pulse can be derived from this to trigger the monostable vibrator from "OFF" to "ON"
Let's assume that the monostable time constant components are chosen to make the fixed width output pulse be 20 Milli seconds and a current measuring meter is connected to the output and adjusted to read one fifth of full scale deflection - ideally marked "1000" on its scale.
Now as the speed of the engine is increased, there will be more and more 20 Milli second pulses at the output and the meter reading will rise linearly in proportion to the engine speed because the meter movement will respond to the average current.
In this particular case, with a 20 millisecond output pulse, when 5000 rpm is reached the trigger pulses are now arriving every 24 Milli seconds and the 555 output will be "ON" for 83% of the time. The meter will have reached full scale deflection.
Above 5000 rpm this example would soon become unstable and inaccurate.
Pulses from other types of rotating machinery can be derived optically or magnetically to trigger a tachometer like this.
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