Coil negative signal conditioning

Ecu Hardware Modifications
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Location: Castlemaine, Vic

Coil negative signal conditioning

Postby pman92 » Tue May 15, 2018 9:57 pm

Hey guys,
I'm playing around with the idea of building a custom dash cluster for my HZ. I plan to build my own pcbs and use an atmega or another arduino compatible microcontroller to control aircore gauges with a MLX10407 air-core driver IC, an also have an LCD odometer and maybe even digital speed readout etc.
It's kind of a long term project so I'm playing around with individual bits at a time before I try the whole project (Ive already raided some clusters for air cores and ive got 10 mlx10407 ic's on the way so I can accidentally destroy a few).
Getting most of the input signals seems relatively straight forward. The vehicle has a VT fuel pump module so I'll be using the sender on it. Oil pressure and coolant temp should be relatively easy even if I end up getting new senders. Vehicle speed I can tap into the 12v square wave vehicle speed sensor.

However I'm not too sure how to go about rpm. I know I can tap into the hall effect distributor or the signal from the ignition module into my '808. I'm probably just making it hard, but that seems too easy and I want to do it from the coil negative. Then I would have the understanding to make it useable on vehicles without electronic ignition, and I could even directly measure dwell from the square wave.
There doesn't seem to be a lot of good information about how to do this from my google searches.
I understand the coil - has a quite high flyback voltage of several hundred volts when the "points open" (module switches off coil). I also know I dont want to pull much current out of that flyback pulse and tie the voltage down or I will induce a missfire.
I'm not looking to pick up on that flyback voltage, but rather filter the very noisy signal and end up with a 0-5v square wave to feed into the microprocessor if thats possible.
I'm imagining I'll need some sort of opto isolation and a low pass filter to prevent noise getting through. But ive got no idea how to set this up.
I'm also worried about coil negative voltage if the ignition module is current limiting, but I'm thinking this isn't a problem on a VN V8 bosch module?

Thanks in advance

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Re: Coil negative signal conditioning

Postby fl0wl0w » Fri Jun 22, 2018 2:48 am

You could easily just use a high value resistor and a zener diode for whatever voltage you are wanting to clamp to. also a low value capacitor may be good to filter out noise and the flyback voltage. You don't want too high of a value for the capacitor that it start distorting the signal.

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Re: Coil negative signal conditioning

Postby antus » Fri Jun 22, 2018 12:39 pm

I agree with fl0wfl0w - there is a such a design documented here: ... ignal.html
Its probably overkill for you but a clear writeup for the less experienced so ive mirrored the content here in case it disappears.
Vehicular Signal Acquisition and Processing - Tachometer Signal.pdf
(263.99 KiB) Downloaded 29 times
ignitionelectrical.jpg (59.54 KiB) Viewed 487 times
Have you read the FAQ? For lots of information and links to significant threads see here: viewtopic.php?f=7&t=1396

Posts: 286
Joined: Thu May 03, 2012 10:50 pm
Location: Castlemaine, Vic

Re: Coil negative signal conditioning

Postby pman92 » Sun Jun 24, 2018 9:54 am

Thanks for the help guys.
I was thinking something like that would be enough, but pretty well everywhere ive seen online has used an optoisolator. Is that just overkill?

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