Tachometer: Sensing Magneto Points Contact

Has anyone used a PIC to sense RPM using the contact points (a.k.a. p-lead) on a mageto? I have built and coded a tach but now have an issue sensing the transition (Yeah, I know I should have looked into this first ...). I have tried a zero-crossing detector, level comparators and a few filters. Still can't get reliable triggering.

I get about 200V positive and negative pulses on the line. Fed the input through diodes, a resistor network and a zener for clamping, but the LC oscillation as part of the process is making it really difficult to sense the spark point (point where the points open).

I'd appreciate any suggestions ... thanks,

Shane

what about if you use opto isolator?

Opto's will not work here ... at least on the front end. The current requirement is sufficient to alter the induced waveform in the primary coil. This in turn alters the secondary and changes the spark firing parameters ... not a good thing.

If this application was with a regular coil (like in a car) then an opto would work ...

Thanks,
Shane

humhum i see... i'm i wrong to say you monitor the spark plug wire ? what about using hall effect sensor??? can you provide the schematic you have to see what can it be modified to improve it?

Hi!

I thinK that mister_e is pointing you in the right direction about the hall efect sensor. I've once used a hall device to sense transistions in the 230V power line (europe) and the PIC had no problems.Make sure that you use a proper filter in order to eliminate the unwanted signals.

best regards

NOMADA

mister_e ... and others.

I am monitoring the p-lead of the magneto. It's a wire which allows the primary coil to be grounded and thus the spark shuts off. The secondary of the coil feeds the plug leads ... through the distributor

I can't get to the plug leads and the p-lead is shielded all the way from the magneto to the kill switch. My only source of pickup is the point where the p-lead connects to the kill switch.

After experimenting today with a dual stage comparator/filter circuit I think I am pretty close. Tomorrow I will replace my diodes with Schottky's as I am getting too much of an overshoot on the negative pulses ... (about 0.8v) which is sufficient to give intermittent false triggers on the TTL inputs. Don't have any Schottky's in my Box 'o Trix. This may just do it; but I am still open to better ideas .... many have done this so it's no rocket science. Just needs to be resolved.

Thanks,
Shane

Hello Sgoed,


Are you playing with a Ultralight? and making a digital LCD monitor for the RPM, CHT and other things?

Dwayne