Tachometer questions.

**nricciar** · 09-27-2005, 01:20 PM

I currently have the phidget 8/8/8 interface kit, and was hoping to use it to get engine rpm from the ignition coil and speed from a hall effect sensor.

After looking around on the internet on ways to get a 0-5V signal out of the pulse that comes from the coil I found the frequency to voltage converter LM2917. The problem is i've never realy done anything on the hardware side except building a circuit that pretty much had step by step instructions.

I found a sample circuit for the tachometer setup at http://www.niksula.cs.hut.fi/~mdobru...cho/tacho.html but it is lacking some information, or I am. The questions I have are probably insanly simple to answer hopefully, and if anyone can help me out id be most gratefull.

- What type capacitor should i be using for this setup? I assume the voltage out of the ignition coil is quite a bit and after looking through digi-key there are so many choices I dont even know where to start.

- Same as above but for the zener diode. What would be suited for this application?

- And hopefully someone can confirm that with that circuit all i should have to do is hook the Vout line to the data pin on an analog input for the phidget, same for ground i assume. But do i need to use the 5 volt line from the phidget interface kit? (probably somthing i should ask on the phidget forum i guess)

Sorry for the pretty long post, but I hope i covered all my issues atm.

**warnockm** · 09-27-2005, 01:33 PM

I don't think you want to get the voltage out of the ignition coil. You can sense the pulses through the spark plug wires. You could build an EM sensor that goes around the spark plug wire. Everytime it pulses, read it and you will know your RPMs. It would be have to be very sensitive. I remember reading the pulse on a oscilloscope and you could see it, but you could also pick up the pulses on each spark plug.

**BassBinDevil** · 09-27-2005, 02:23 PM

You want to pick it up on the primary side of the coil, from the points, or whatever gizmo acts like the points in the engine. If you have an MSD ignition, it'll have a tach output wire.

As for the circuit, use a 1/2W or larger zener for the 12V zener, like a 1N4742.
The zener will limit the voltage into the circuit, so the capacitors (unnamed) and C1 can be a 50v monolithic ceramic, or pretty much whatever nonpolar cap you have lying around. C2 could be a nonpolar cap, or more practically a 2.2 uF tantalum electrolytic, - lead to ground.

Use a 7808, 78M08, or 78L08 to get 8V from 12V.

**warnockm** · 09-27-2005, 02:27 PM

Originally Posted by BassBinDevil

You want to pick it up on the primary side of the coil, from the points, or whatever gizmo acts like the points in the engine. If you have an MSD ignition, it'll have a tach output wire.

As for the circuit, use a 1/2W or larger zener for the 12V zener, like a 1N4742.
The zener will limit the voltage into the circuit, so the capacitors (unnamed) and C1 can be a 50v monolithic ceramic, or pretty much whatever nonpolar cap you have lying around. C2 could be a nonpolar cap, or more practically a 2.2 uF tantalum electrolytic, - lead to ground.

Use a 7808, 78M08, or 78L08 to get 8V from 12V.

oh yeah. good idea. So what year/make is this car?

**nricciar** · 09-27-2005, 03:00 PM

The car im putting this in is a 1988 Crown Victoria. power nothing except steering

**warnockm** · 09-27-2005, 03:17 PM

Originally Posted by nricciar

The car im putting this in is a 1988 Crown Victoria. power nothing except steering

Well i have a distributor/ignition coil for shooting a potato gun (don't ask). it's about 10 years old, but its an aftermarket unit and it has a tach-out pin on it. See if this is on your car and work w/ this signal.

**nricciar** · 09-27-2005, 04:17 PM

also could someone help me understand this formula a bit better...

Vout = f in * Vcc * R1 * C1

the max RPM's ill be dealing with is 6000 rpm, 8cyl engine.... so thats 400Hz if im not mistaken. pulse per every two rotations of the crankshaft?

Vcc... after reading the datasheet more on the LM2917 it looks like it might be able to run at 5V which would be nice seeing as then i could just plug it into the phidget analog interface. any confirmation?

R1 and C1 this is where im totaly clueless. the datasheet states that you cant just use any size to finish the formula here and there are some requirements that went way over my head... anyone here worked with this chip before?

Vout needs to be no more than 5V at 6000 RPM. and thats all i know for absolute sure

**BassBinDevil** · 09-29-2005, 11:32 AM

Here's the National Semi app note: http://www.national.com/an/AN/AN-162.pdf
and data sheet: http://www.national.com/pf/LM/LM2907.html

The '2907 is probably more suitable for 5V operation. I did a Google search, figuring there had to be a complete design out there someplace (with board layout etc), but didn't hit anything.

There could be an output wire or terminal already on the ignition module, if Ford offered an optional tach. Does this beast have EFI?

I put together a tach using this chip (either the '2917 or '2907); the calculated component values got me within a few blocks of the ballpark, but had to be tweaked a lot. If I remember when I get home, I'll look for my notes on how I built it.

**nricciar** · 10-02-2005, 09:15 PM

Yea this car is EFI, and there is a tach wire to get the signal from. I think ive figured out what i need to do for 5V operation I just was hoping someone could confirm what i need before i purchase some parts I dont need.

The best ive done is assemble a board where all the parts were spelled out for me ahead of time, so this is a whole new ballgame for me