I would like to read a GM engine's (chevy 350/5.7L) RPMs with arduino analog port. The signal I am getting originates from the ignition coil (the blue wire in pic below) and is a choppy square wave that alternates from 12.5V to ground. The frequency of the signal varies with the RPM.
Currently I am using a Faria Tachometer to read the RPMs and on the back of the Tach you can see that the gauge is set to interpret 3 pulses per rpm (this is because V6 engine or 6 cylinders/2=3). The Tach spans 0-4000rpms and at 3 pulses per RPM this would mean I need to detect 0-200Hz. I have found a breakout board that converts 0-200Hz frequency to analog 0-10V, which I plan to step down to 0-5V and send to arduino analog pin. The breakout board is attached below and although the frequency I specified is different from the picture that is just because the company uses the same picture for all their modules.
My questions are: will this frequency module work for my application? Will a voltage spike from the ignition coil fry my boards, and if so how can I protect the Arduino and frequency module?
Can You dig up some data about that module?
I aimed at converting the 12 volt pulses into something readable by an Arduino and then measure and calculate the frequency directly, not converting to analog and then, by analogRead, make it a digital value.
Sure thing. Based on what I've read this module is based on LM331 IC configured in F-V mode. Datasheet see page 8. Honestly I could work with either, that is converting 12V to 5V and sending to arduino or using this board to do it for me.
A four stroke engine ignities once every second revolution. You can say 0.5 pulses per rev. Multiply this by 8, for an 8 cylinder motor, by 12 for a 12 cylinder one. 4 pulses per rev for the 8 cylinder one, 6 for the 12 cylunder, and so on.
Thanks for the comments guys, and it looks like there could be a tach config issue. I went out there to verify, and yes this is a chevy 350 V8 engine not a V6. So I am curious as well as to why it is set for 3 pulses per rev and not the 4. As a side note, there are several more engines just like this to say the least, and so if they were configured wrong that would be quite disappointing.
tach manual Looks like the dial on the back of the gauge may refer to something different than I first expected. See page 3, but it is related to pulses someway or another.
I had a quick look at the data sheet for the H11L1MS opto coupler. It is an open collector Schmitt trigger device. You’d probably need a pull-up resistor or define the receiving GPIO pin as input pull-up.
This appears to be the relevant sentence from page 3 under "4 cycle engines".
The correct tachometer will have a white label on the side indicating which switch position is for each engine type. This label will include 4, 6, and 8 cylinder engines for positions 1, 2 and 3.
Alright so it appears this tach is set up properly. Although a bit counter-intuitive that the V8, which has 4 pulses off the ignition coil per rev, uses the tach dial setting of 3 on this Faria gauge.
6v6gt:
I had a quick look at the data sheet for the H11L1MS opto coupler. It is an open collector Schmitt trigger device. You’d probably need a pull-up resistor or define the receiving GPIO pin as input pull-up.
Thanks for this comment, I think I'll have to do some reading to understand the reason for this, but I haven't had the chance to wire this thing up. Parts should be here early next week. Will a H11L1M optocoupler work? Because it seems the H11L1MS is harder to acquire.
