Signal Conditioning Help Please

Discussion in 'The Projects Forum' started by remout, Apr 18, 2011.

  1. remout

    Thread Starter Member

    Apr 18, 2011
    I'm building a tachometer for a small gas engine. I wrapped a solid core wire around the spark plug wire and put a scope on it. I got 3 volts AC RMS (not sure if I'm using the correct terminology, very new at this) with a frequency that made sense for the engine RPM.

    My next step is to convert this into a 5 volt DC signal. My idea is:

    From the induced voltage wire run a diode and then into the base of a transistor. This transistor would have 5 volts into the collector coming from an Arduino board. The emitter would go to a pin that is monitored in the scan cycle.

    Am I headed in the right direction?

  2. ifixit

    AAC Fanatic!

    Nov 20, 2008
    Hi Remout,

    I would suggest starting with a circuit like the one attached. It will provide some protection against wayward sparks that could blow up your Adruino. Better to blow up a cheap transistor than an Arduino. The circuit also amplifies the signal to 5 volts and no more.

    During each ignition event there may be multiple pulses produced. You can use software to recognise only the first pulse and ignore the rest for a suitable fixed period of time.

    You may have to change the value of R1 or use more turns to get a good reliable pulse output. D1 clamps the negative part of the pulse to ground. D2 provide additional insurance against damage to the Arduino.

    remout likes this.
  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
    How about using the pickup and part of the circuit from something like this to feed your Ardino?

    The timing light uses the plug pulse to trigger the xenon flash bulb. But instead of amplifying the pickup signal to that level it could be hacked to make the pulse to the Ardino.
    remout likes this.