Mosfet-driven square wave signal converter problem.

Thread Starter

Static222

Joined Jun 20, 2017
5
Hello All.

I have been struggling with the construction of a signal converter circuit, and I was wondering if anyone could give me some pointers. The intention of the circuit is to convert the pulse of a CDI ignition on a motorcycle into a good square-wave signal that an analog tachometer can pickup.

I followed the advice of someone who analyzed the system and built a passive adapter to stretch out the 40 microsecond pulse generated by the ignition coil. Below is the schematic they made:

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I built this circuit with all of the correct components based on what I am seeing here, however I feel that maybe I have the orientation of the mosfet's incorrect or I am missing something about how this adapter is supposed to function. The tachometer should work with a square wave signal. Any advice or analysis on how I could correct this is greatly appreciated.
 

AlbertHall

Joined Jun 4, 2014
8,330
Where are you connecting the input signal for this circuit?
Do you have (access to) an oscilloscope?
 

#12

Joined Nov 30, 2010
18,076
That's basically a pass-through with saturation (to make the wave square) and nothing to change the pulse width. I mean, this circuit is so fast it could follow switch bouncing with no trouble. I'm so old that the last time I checked, a tachometer measured 12V activity at the primary side of the coil.:D This circuit creates 12V square waves, but the dwell time is so small!

One cylinder at 12 grand is 200 sparks per second so that suggests a maximum dwell time of 2.5 milliseconds, not 40 usec.
You got wasted spark design?

Excuse me. I'm getting distracted by the real world. You need a pulse stretcher. They exist but I don't have one on my desktop right now.
 

Thread Starter

Static222

Joined Jun 20, 2017
5
Where are you connecting the input signal for this circuit?
Do you have (access to) an oscilloscope?
The input signal would come from the left side (from coil) which would be the positive signal wire coming from the CDI into the coil itself. Unfortunately no oscilloscope :(

That's basically a pass-through with saturation (to make the wave square) and nothing to change the pulse width. I mean, this circuit is so fast it could follow switch bouncing with no trouble. I'm so old that the last time I checked, a tachometer measured 12V activity at the primary side of the coil.:D This circuit creates 12V square waves, but the dwell time is so small!

One cylinder at 12 grand is 200 sparks per second so that suggests a maximum dwell time of 2.5 milliseconds, not 40 usec.
You got wasted spark design?

Excuse me. I'm getting distracted by the real world. You need a pulse stretcher. They exist but I don't have one on my desktop right now.
Correct! this ignition fires twice per revolution (thus I would set the tach to 2 cylinder mode). I think I may have gotten my mosfet orientation incorrect as far as the Source and Drain go. This is my first mosfet project, and I have seen two different opposing orientations of the 2n7000 as far as the source gate and drain pins. I may try to recreate using the reverse.
 

crutschow

Joined Mar 14, 2008
23,312
If the polarity of the short input pulse is positive, then one problem is that the polarity is wrong for that circuit, as it would filter the short pulse instead of stretching it.
Try removing the second (right) MOSFET, connect the capacitor to the drain of the first transistor, and take the output from there.
That will invert the signal but the tach shouldn't care.
 

AlbertHall

Joined Jun 4, 2014
8,330
If the polarity of the short input pulse is positive, then one problem is that the polarity is wrong for that circuit, as it would filter the short pulse instead of stretching it.
Try removing the second (right) MOSFET, connect the capacitor to the drain of the first transistor, and take the output from there.
That will invert the signal but the tach shouldn't care.
Yes, that sounds right to me.
 

AlbertHall

Joined Jun 4, 2014
8,330
If the polarity of the short input pulse is positive
I would expect the coil voltage to be a damped sinusoid with a lot of mess when the plug sparks. It would therefore go both positive and negative but, as you say, you want the MOSFET to short the capacitor when it is briefly turned on, whereas the original circuit will briefly remove the short from the capacitor and your suggested mod fixes that.
 

yevgeny

Joined May 20, 2017
9
Even input capacitance 10pF - time constant over 50uS. Output time constant 5.6mS. The circuit can't create good square-wave on output.
Replace capacitor for100pF. Also you may replace input resistors for 100k. It should work

Thanks
 

#12

Joined Nov 30, 2010
18,076
Capacitor discharge ignition connects a capacitor charged to about 300V across the coil (200 times a second).
Make that 400 pulses per second because this is a, "wasted spark" design.
(thus I would set the tach to 2 cylinder mode)
Irrelevant to the Tach Driver. Your Tach can ignore half the pulses, but the Tach Driver has to deliver all of them.
If the polarity of the short input pulse is positive,
I think we're looking at a circuit like this:
 

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Thread Starter

Static222

Joined Jun 20, 2017
5
Thank you guys for the input. Here are some pics of the circuit I had built before (before soldering +12v wire) if it helps.

Green = Ground.
White = Signal in from coil.
Blue = Signal out to tach.

Unfortunately I did not get any pics of the bottom of the board before I potted it in waterproofing rubber. In retrospect I should have tested it on the bike before sealing it up.

20170122_191806.jpg Screenshot_2017-06-21-11-35-41.jpg Screenshot_2017-06-21-11-35-55.jpg Screenshot_2017-06-21-11-36-32.jpg
 

#12

Joined Nov 30, 2010
18,076
If crutschow didn't object, I probably got the circuit right.:)
You might try connecting the Tach where the gate of the second transistor is. If the Tach doesn't care whether the signal is inverted and the rise time is slow, you can eliminate one transistor and one resistor.;)
 

Thread Starter

Static222

Joined Jun 20, 2017
5
I will go with this and grab a breadboard to see if I can get it to function. I will post back the setup/results.

Thanks again for the info!
 
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