LM-2 w/HEI

[bytor]

Any of you attempting to use an LM-2 AFR meter with a HEI distributor? Here’s a solution I came up with to solve interfacing an LM-2 AFR meter to an HEI distributor tach output. I had all kinds of problems getting the LM-2 to display the correct RPM or pick it up at all. Innovate offers a solution in the user manual to build an attenuator to condition the coil-negative signal. The signal can vary greatly depending on your ignition. I never could get it to work reliably. The coil-negative tach signal has a lot of ringing in the waveform that I think causes the LM-2 not be able to sync up. If you’re running any type of ignition box, your set because they typically provide a nice clean square wave pulse out for a tach.

So I came up with a circuit that provides a clean square wave pulse to the LM-2. It’s worked flawlessly for a few years now. The circuit is nothing more than a ‘one-shot’ that triggers on the tach pulse from the coil front ended by a voltage comparator to get above the ringing in the tach waveform. The baluns (coil looking thingys in the schematic) are not required but I still have them installed.

There's probably other simpler ways to do this but it works for me. Also, here's a cool free circuit simulator I find handy. http://www.falstad.com/circuit/

Here's the circuit:


Here's the input waveform at point (A) in the circuit:


Here's the output waveform to the LM-2:

 

[grumpyvette]

thanks thats a rather interesting bit of info that I,m sure will be most useful at least for those guys who want to know how to test, if things are working correctly, rather than guess at the results they getting..

 

[Indycars]

You must have a background in electronics to come up with that!

How does it connect to the HEI, would that be R9 and what wire on
the HEI goes to R9 ??? Then the output to the LM is "D"?

Maybe a picture of the real thing would be nice if it's possible.

 

[bytor]

You must have a background in electronics to come up with that!

How does it connect to the HEI, would that be R9 and what wire on
the HEI goes to R9 ??? Then the output to the LM is "D"?

Maybe a picture of the real thing would be nice if it's possible.

Yep, R9 connects directly to the 'tach' terminal on the HEI. The tach terminal is basicaly the negative side of the coil. Pin 3 of the 555 (D) is the output and connects directly to the LM2 tach input.

 

[mathd]

Nice little circuit, and its totally safe to use with its zener diode for reverse and overvoltage protection.
Wish i had an oscilloscope, i can see countless use for my hobby
But am wondering why the 555? used as an amplifier/driver? or to have a perfect squarewave output? or to manage the dutycycle of the output?
I used voltage comparator its been a while (usually the lm747)but i have no experience with the 555 timer so i dont really understand theyr purpose like in this application. I see your using it in monostable mode obviously

 

[bytor]

why the 555? used as an amplifier/driver? or to have a perfect squarewave output? or to manage the dutycycle of the output?

I'm just using it as a one-shot to generate a consistent pulse with a constant duty cycle. The 555 is a very versatile old school IC. It's been around since the early 70's, only need's a few components to work and dirt cheap.. The LM-2 likes a pulse in a certain duty cycle, I don't remember it off the top of my head but I'll dig around and find it. I could have probably simplified this circuit by not using the baluns and the 741 voltage comparator but once I got it working, I didn't mess with it.

 

[mathd]

Can you get the math library with logworks3?
trying to use the match formula for torque/pwoer but i always get this error:
"unknown constan/variable RPM"
i tryed everything but i cant get it to work. tryed google and nothing about it.