As part of my car management system I want to measure the engine's rpm.
Since my car doesn't have OBD or anything I've built the circuit found here: GitHub - designer2k2/multidisplay: The MultiDisplay Project, An opensource datalogger, boost controller and display for cars
Now before I connect it I want to make sure I do it right.
Where should I take my signal from, coil negative or positive?
My engine doesn't have injection, so that's not an option.
I think it could work with the high side of the points in old-fashioned points ignition system, but am a little worried about the voltages involved - the input section uses a diode to protect against high input voltages, and there is 22k is series - however I'm not sure how high the voltage gets on the coil primary when the points open, its been a long time since I've seen non-electronic ignition!
There's probably a simple precaution you can take - add a high voltage rectifier diode at the input, anode to the unit, cathode leads out to the coil points.
I'm sorry, should have mentioned this:
I have replaced the points with an electronic ignition kit (this one to be exact: http://www.simonbbc.com/electronic-ignition-kits/ford-fiesta-powerspark-electronic-ignition-kit-4-cylinder)
So I should be able to just connect it to the coil negative then?
Or do I still need a high voltage rectifier diode?
So I went ahead and connected it, but I'm not getting a reading.
I've checked the circuit over and over again, everything seems to be correct.
The only thing I can think of is that I've used the 14-pin version of the LM2907 instead of the 8-pin version.
Looking at the datasheet (http://www.emesystems.com/pdfs/parts/LM2907.pdf) I assumed that I just had to use pin 3 & 4 and pin 11 & 12 as 1 pin.
Is that correct or did I overlook something there?