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Topic: Need ideas on tachometer using ignition (Read 7383 times) previous topic - next topic


Feb 20, 2013, 05:41 pm Last Edit: Feb 20, 2013, 05:53 pm by M4vrick Reason: 1
I'll open a new thread to do not pollute this one

Some information for the initial question:

Even if it's an old engine you have spark plug, which mean coil. On car engine the tachometer signal is simply from the coil driver, on the negative pole of the coil (a coil is driven by negative).
If you have one coil per cylinder you just need to pick the negative pole on one of the coil.

I'm doing mainly the same, I'm using a darlington network to adapt 12V signal from car to 5V signal in Arduino.


You can wrap a piece of wire around the wire to the coil and every time the coil fires the current flow will induce voltage in your wrapped wire. How much voltage depends on how many wraps you make, it is not automatically 12V and it may be -V depending on which end you call ground.

Get the number of wraps right and you can feed it directly to a led, though I'd go with more wraps and add a resistor.

Darlington's are for high-gain. Why use one to take 12V down to 5V?

Nick Gammon on multitasking Arduinos:
1) http://gammon.com.au/blink
2) http://gammon.com.au/serial
3) http://gammon.com.au/interrupts


Darlington's are for high-gain. Why use one to take 12V down to 5V?

You still have the noise issue. Any transistor will just amplify the noise as well.

I think Grumpy Mike's idea sounds like the best.


If you get a miniature glass wire ended neon this will fire when coupled to an ignition wire in this way. I did this way back in the 60's.
Now if you put a photo detector transistor against this and wrap it in black tape you will have made an optical isolator so the flashes are safely read by your arduino.
Hi!  I have done as Grumpy Mike suggested, I have a neon taped (temp) to a photo transistor/LM393 ebay thingo board.  This is connected to an Uno interrupt.  I get an rpm reading which is awesome, thank you for the idea.

Now I am wondering before installing a more permanent setup, should I worry about voltage or current induced in the photo diode or to the Uno etc???  Should I put a Zener between any points?  Curious for thoughts is all if anyone is still on this old thread. 

Cheers very much

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