Electronic tacho - high and unstable reading

Hi electronic gurus ......

Ive fitted a simple digital tacho to my cb390 today

Three wires for ignition, earth and sensor.

Connected the sensor to points output, ie the blue or yellow wire to the coil.

I’m getting very high and unstable rev readings, which i’m guessing is due to noise ?
I have the tacho set to 0.5 pulses per rev, which should be right, bit even if it set it as high as 6 ( 12 cylinder!) tacho readings still high and unstable.

I did also try wrapping the pickup wire around an ht lead but go no readings this way.

Suggestions please to suppress the noise and get a steady signal to the tacho

Steve

try a 10k resistor inline, if that doesn't do it, try running a 15v zener diode from the output of the 10k resistor to ground

beyond that, you're going to need to do some research, any filtering for noise should be internal to the tachometer, I know the better models are very refined and have no problems picking up signal from a noisy line, cheaper ones might not though, I'm not sure

Thanks, i was thinking along similar lines so will give those ideas a try !

Depending on the tach, it might be relying upon an induction pulse from the high tension side of the coil.

Try taking the signal wire from the tach and wrapping it about three or four times around one of the plug wires, instead of connecting it to the points wires.

Thanks all.

I tried the resitor and zener, and wrapping the sensor wire around the ht lead.

No joy, but.... if i apply a 12v pulse to the sensor wire then it does appear to register correctly. so i’ve ordered some opto-isolators and will ‘experiment’ with a circuit to take the negative pulse from the points and ‘invert’ it to a positive pulse

Watch this space

Steve

I'd make sure the condenser is well connected, grounded, and in good shape. It should eliminate any switch bounce noise.