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Trying to count motorcycle ignition pulses.

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potoole64

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I'm trying to use a Basic Stamp (BS2) to count pulses from a harley sportster ignition. I'm taking the pulses from the same same point that feeds the tachometer, which is the low side of the ignition coil (the output from the electronic ignition). The pulses are first fed into a voltage divider which cuts the volts down to about 4 volts. Resulting in a pulsating dc voltage in the range of 4 volts. The pulsating dc is fed into a Schmitt trigger which cleans up the pulses, then these pulses are sent to the BS2. The BS2 is programmed with a 'COUNT' command to count the pulses. It counts them, but with numbers that are three, sometimes four, times what they should be. I rev the engine and use the tachometer to set the engine speed to certain RPMs.
I've also used, inplace of a Schmitt trigger, a monostable multivibrator and get no counts at all.

However, I can do this operation in house using either the Schmitt or the monostable MV and get proper results.
I use a sound recorder to 'capture' the pulses from the sportster ignition, bring that ckt. to the BS2 in house and using either the Schmitt or the monostable MV and get proper counts.
The Schmitt and the monostable MV are made using 555 counter chips.

Any ideas as to why counts are off when pulses are taken directly from the sportster ignition, but ok when using a sound recorder to 'capture' the pulses first?

I suppose it would be best to include schematics, if needed, I'll try to do that later. Right now its quite late at night.

P.O'T
 
I would guess that the problem is caused by spikes on the power lines. How is the BS powered and could you try powering it from a separate power supply to ascertain if this is the problem.

Mike.
 
If it uses dead sparks (sparks on the exhaust stroke) you'd get twice the predicted rate.
If you have an AM radio and freq. counter fed from the radio earphone jack you can also pick up the spark rep rate.
 
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Counting pulses from motorcycle ignition.

My BS2 is powered by a 9v battery that is attached to a "Parallax Homework board", which comes as a microcontroller kit. The main part of the kit is the PIC16C57C microcontroller.
I use a sound card oscilloscope (software downloaded from net) to display ignition pulses that I have 'captured' with a sound recorder. I've included an image of the pulses and some ckt. diagrams.

Thank you
PO'T
The pulses displayed are over a period of ~100 milliseconds. Each pulse half (1-neg, 1-pos) is about 6 MS in length (12-14MS tot.). Time between pulses vary as RPM varies.
 

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I'm aware of the extra pulses during exhaust, but I'm getting, using the Schmitt trigger to clean the pulses, three to four times the pulses. In house, counting my sound card 'captured' pulses the Schmitt or the monostable are able to eliminate the extras, and give me the proper count.
 
Willbe:
Thanks
How could I use the AM radio, freq. counter and earphone setup to pickup the pulses. That sounds very interesting. Where could I find the ckt. setup for that operation?

PO'T
 
Where could I find the ckt. setup for that operation?

Tune the handheld battery-operated AM radio to a dead spot, pick up the periodic popping noise on the radio loudspeaker by holding the radio only a few inches away from whatever emits the RF spark energy then hook a scope or freq. counter to the radio using the earphone jack.
Do your engine RPM/period calculation (scope) or your engine RPM/popping noise freq. calculation (DMM freq. counter).

An FM radio, of course, will not work for this.
 
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