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MG MGB Technical - Circuit for rev counter

I've just built a circuit for my rev counter and I think I'm getting a lot of 'noise' on the input signal because the monostable driving the meter appears to be double triggered. Anyone done this before? Any ideas on how to cure it? My reference for the original circuit was from an article entitled 'Tiger Tach Rebuild', Sept 2000, and generally it works well at high revs (2000 plus) but starts to show double the revs at below that.
I'm using a Lumenition opto system which is why I changed to a voltage input pulse from the original current loop.
Richard Coombs

Hi Richard,

Without the schematic for the circuit, I can't give you expilcit instructions.
I have attached the schematic for the Smiths voltage sensing tach, which has some pretty heavy input filtering.

A bit late now for you, but may help others later, is when replacing the points with electronics, don't connect the electronic's power wire to the coil, find an alternative switched place to connect to. The electronic circuitry is what interferes with the current tachs.

Herb

Herb Adler

I'm sure Herb is right, the noise is still there at higher revs but the mono is being retriggerd before it arrives. would you like to know where the poles are in that Smiths filter?
Stan Best

Thanks for the data.
Herb, what ic is used here? Motorola?
Stan, that would be helpful.
I shall do some more testing this week. I only have a gunsons test meter which offers rpm, pulse width, pulse length and frequency. Limited but still very useful. I can certainly check the monostable output for double triggers.
Richard Coombs

Hi Richard,

I have no idea about the IC. All it has on it are what I have printed beside the schematic. Googling those numbers came up with nothing. Maybe a custom chip.

Herb
Herb Adler

I make them about 150 and 200 Hz, 6000 rpm is 100 hz so they have clamped it down close to the active frequency band. Smiths did commision a custom from Texas Instruments in the early 70s for use in tachos that I definitley know off so its possible this is a proprietary chip as well. This division was based at Witney near Oxford.
Stan Best

Ok Herb.
Stan, the frequency for 6000 rpm is indeed 100 Hz but remember you get 2 pulses per rev in a 4 cylinder so the rev counter input sees 200 Hz. The original chips are no longer available but a simple 555 monostable with a filtered input should easily cope. I suspect it's the filtering that I've got wrong at the moment.
On the plus side I've got a 555 calibration oscillator running at 133.3 Hz (or 4000 rpm) and the meter sits rock steady.
Richard Coombs

This thread was discussed between 19/05/2012 and 20/05/2012

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