Tachometer Circuit.

A simple tachometer circuit using one NPN transistor. (12/04/2007)


Basically, this is a frequency-current converter. It is used as frequency meter. Calibrating it can be used as tachometer. Please be aware this is an experimental project, I didn't use it connected on any vehicle but it should work fine after calibration.


It can be connected directly to the voltage from the car. The input signal can be connected directly to any source of frequency. I believe it will work fine if it is connected directly to the spark plug line. Be sure not to use germanium diodes for this one. Any silicon diode should work fine.


The diodes at the input reduces the voltage to a safe level ( 0.7 Volts). DO NOT CONNECT the circuit without doing the calibration first.


bridge diode The bridge diode that is connected to the miliamperimeter can be replaced by 4 silicon diodes as the picture shows.


The miliamperimeter can be replaced by a multi-meter set to measure mA.


To calibrate this circuit first:
  1. Adjust R1 and R2 at the middle point.
  2. Apply power (12 Volts) and apply 60Hz at the input signal.
  3. Adjust R1 until it measures 0.360 (3,600 RPM)
  4. Stop the signal (0 hz) and adjust R3 until the meter indicates 0.
  5. Apply 60Hz again and adjust R1.
  6. If the meter doesn't show "0" at 0Hz and 0.36 at 60Hz, adjust R2 and repeat the calibration.


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