Creating square wave signal from ingition coil
Turbo Mini Forums

I've seen somewhere on the internet somewhere (I'm sure is was some add on for megajolt) a device which you could connect to the negative terminal of your ignition coil, and it gave you a digital square wave signal of your rpm.

Does any of you clever bods know how I can do this, or get one of this gizmo's from?

Cheers

Jimbo

Team Racing

You could probable do it with a 555 or similar, If I get bored over Easter I'll have a play.

the mega squirt has a circuit for this, its a zero crossing detector.

basically its a comparitor.



http://www.megamanual.com/ms2/pcb.htm

I know seet fa about electronics.


But would a simple npn transistor do it ? It just switches an earth off or on ? I guess that would be a square wave.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

I don't kow stevie, could it be that simple?

Team Racing

You will have to watch the transient voltages of the LT ignition as they could easily reach 400v at full chat

http://www.bgsoflex.com/v22/megasquirt_ShemV2.2.pdf

bottom left of page 2, the zeners and opto isolator protect your device from teh high voltage spikes.

"coil" goes to the -ve on the coil
"IRQ-1" is your 0-5v square wave output
"Vcc" obviously +5v
and the ignition return jumper should be shorted (this is present to allow the selection of various sources)

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

Cheers for that Denis,

quick thick questions about the capacitors, are they the ones shown here:
http://www.maplin.co.uk/Module.aspx?ModuleNo=371


Mylar 0.001 WW15R
Mylar 0.01 WW18U

Team Racing

Not sure of what you are looking for. To me square wave means duration of 0 and 1 is the same.
If you are looking for a chip that gives you an analog RPM signal from the ignition 12v base there is an old National Semiconductor part that does everything, just can not find the reference right now.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

If you're looking at making something yourself, run the coil -ve through a voltage divider and put some Over Voltage Protection on it (Zeners). Then run the signal through 2 or maybe 3 Schimtt Triggers, in series. These will clean it up and provide a definite ON / OFF state output in sync with your coil trigger (points or electronic points).The Over Voltage Protection is required as when the points (or what ever you are using) Open, there will be a fairly large back EMF (voltage) produced from the coil that will instantly destroy any transistor or chip connected to it.

This may help with the Schmitt Trigger;-

www.fairchildsemi.com/an/AN/AN-140.pdf

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

http://www.jbperf.com/

I apologize to have spread dirty gossips about LM1815 and haven't revised my dusty school notes. I'm certainly getting too old and confused by this fast evolution of the electronics. Won't do that again, sorry.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

I need to do this too did you get a solution Jimster?

http://www.autosportlabs.com/product/tach-adapter/

Worked on mine.

did you just use one input and output to gauge, looks like is to merge 4 perhaps coil pack signals into one and clean it up, looking at the circuit its just a resistor and diode??

Will this help you do what you want? It is for use with megajolt, and perhaps pre-dates autosports labs making a pretty circuit board to do the same thing.

So I only need one diode 1N4004 and a 15-18V zener in series in the signal to gauge looks simple like that..