88IntegraLS

I built the Dual lm1815 circuit as per the schematic shown in the faq, but when I hook it up to the megasquirt and bench test it, only the left (24 tooth) circuit outputs a sharp oscillation in voltage as I spin the CAS by hand. The 2 tooth circuit outputs a constant ~5v.

I'm trying to bench test the conditioner circuit somehow, because I've had no luck trying to get spark with the unit in the car.

Any tips / suggestions? I've checked and double checked my grounds and resistor / capacitor values, correct pin nodes, etc. I have the .01UF capacitor with two 82k resistors in parallel between the two chips instead of hte single 39k.

Mainly I'm looking for a way to verify that I have an operational conditioner circuit. I was thinking when it's fully operational with the megasquirt, I would be able to spin the CAS and see LED17 blink.

renns

Here's a few ideas:

1.) Swap LM1815 chips, and re-test. That'll verify both LM1815 are OK.
2.) Swap VR input signals, and check if the 2-tooth VR provides an output using the left circuit.

If you have the board connected to MS, and the MS on the bench, then you can use MegaTune to watch for an rpm jump when you spin the CAS. The function of the LED's is software configurable, so you may want to set one up as 'IRQ Trigger' for testing. It should then flash every time a tooth passes the VR.

muythaibxr

with the 82 k resistor on the vr sensor conditioner for the 24 tooth wheel... you're going to be severely limited in max rpm.... the maximum recommended rpm with the way you have it set up is going to be 4530 rpms.

I'm not sure but did you remember to flip the right lm1815 so that pin1 is on the bottom instead of the top? Also, I guess the right lm1815 could be broken... in which case the good test would be what renns said to do.
The 82k and 32k resistors also go to +5v, they don't directly connect the 2 lm1815's together....


If you hook the lm1815 circuit to the MS and spin the CAS, if you have the ms set up for rotary with the FC outputs, you'll be able to see the middle LED blink.... the other 2 leds might blink too, but if you hold the cas at a steady RPM below maybe 1000 or so... you should be able to see the middle led blink. This is because the middle led is the coil-select signal... and has the longest on/off time since it's always on/off for 180 degrees of engine rotation.

88IntegraLS

Thanks for the help guys, I was thinking the next step would be a chip swap to verify that the LM1815 on the bad circuit was operational or not.

I did get both chips in oriented as shown in the schematic though. I acutally didn't at first and had to split my perf board and rotate one!

Off to Digi-Key's wesite I go, once again. I'm going to stock up on parts this time, at the rate I go through them I'd like to have a complete set of spares and then some.

Rex4Life

Ken,

Is there any way to hook up the stim to the second LM1815 circuit and test it out? I was thinking maybe take the stim tach output and run it into the VR2 connecction--what do ya think.

Scott

renns

Scott,

That may not provide the desired results, as I believe the stim provides a 0-12v signal, whereas the VR provides a signal that cycles above and below the 0v reference. The LM1815 is a zero-crossing detector, providing an output when the input signal drops from a positive voltage to a voltage below 0v. If the input signal doesn't drop below zero, it'll likely not give an output, or will be erratic at best. I've noticed on the scope if the VR sensor is connected reversed (approaching pole causes negative voltage, then swings up above zero, dropping back down to zero), the LM1815 does eventually provide an output, but it's after the signal has settled to approx. 0v, which would throw timing wayyyy off. VR sensor polarity is certainly something worth confirming up front!

A while back I built a simple VR simulator using a small microprocessor separately powered from the MS. The micro used a potentiometer input, and provided a 36-1 square wave 2.5v to -2.5v output, by biasing the micro ground with respect to MS ground. With a few components and a small microswitch you could do this by hand as well. Just use the 24-tooth VR circuit, set MS to MSnS mode, rather than wheel decoder. That way it'll calculate rpm solely based on the pulses received from the tach input, and ignore the 2nd trigger.

Personally, if I was pursuing the stock FC ignition setup, I'd use the V3.0 board, which has one built-in VR conditioning circuit. Knock two opposing teeth off the 24-tooth wheel, and configure it in software as 12-1 missing tooth. That simplifies the wiring, as only the VR sensor for the modified wheel is needed, and it also eliminates the need for the daughterboard. The daughterboard may still be desired for other optional stuff, but at least the install can be completed, run, and tuned in a simpler form first.

Rex4Life

Roger,

Thanks for the info, either way it looks like alot of work. Probably just easier to hook up the MS and see if it works.

Reason I ask is cause I was up last nite testing a few circuits and that was one I could not figure out how to test.

And yea, I took yours and Ken's suggestion on using the 3.0 board with a 2nd LM1815 circuit (see my update in Stock ECU thread). Maybe next month I'll be able to test out the ignition, just want to get fuel working good first.

Thanks again for your help,

Scott

muythaibxr

When I test I just hook a spare CAS up to the VR inputs to the ms/daughterboard, and spin the CAS with a drill.

I actually posted several videos of me testing this in another thread.

Rex4Life

Yea but I don't have another CAS. I'll just hook it up and see if it runs.

I was just hoping there was a simple way to hook up the stim and check it out, but Roger's explaination was very good--basically it won't work like I thought it might.

No big deal.

Thanks,

Scott

muythaibxr

well, I would strongly recommend trying outside the car first... it's a lot easier to debug these things sitting on a test bench than it is in the car....

I'd go so far as to pull the CAS out of the car. It's not hard to stab it back in and re-time the car once it's running.