Here is a simple circuit that can be used as a tachometer. The circuit is basically a frequency to current converter which converts the incoming signal into a proportional current to drive the meter. The deflection on the ammeter will be proportional to the frequency of the incoming signal. For using this circuit as an automobile tachometer, the input terminal A should be connected to the spark plug cable and terminal B should be connected to the vehicles ground.
For calibrating the circuit, set R2 at 25K and R4 at 5K.Power up the circuit and feed the input terminal with a 60Hz square wave form your function generator. Adjust R2 so that the meter shows 0.36 mA (equal to 3600rpm). Now disconnect the input signal and adjust R3 so that the meter shows 0mA.Now connect the 60Hz signal again and if the meter does not show 0.36mA adjust R4.A completely calibrated circuit will show 0mA at 0Hz and 0.36mA at 60Hz.
- The circuit can be assembled on a Vero board.
- I have not tested this circuit on any automobile. So use this circuit on your automobile at your own responsibility.
- Anyway, the test using the signals from my function generator was satisfactory.
- The ignition voltage from the spark plug terminal is in the Kilo volt rage. The engine must be OFF while making connections and you must be very careful to avoid shock hazards. Try this circuit on your automobile only if you have sufficient knowledge and experience on automobile electricals. I have no responsibility on any mishap.
- The circuit can be powered from 12V DC.
- The bridge D3 can be made using four 1N4007 diodes.
- M1 can be a 1mA FSD ammeter.