DIY LTCC or similar system for LT1s

[spfautsch]

After careful consideration, I'd like to retract any statements I might have made in my previous post that suggested your 10k potentiometer idea wasn't an excellent one. A few moments after I finished the last edit of the last post the words "impedance matching" hit me like a brick and a flashback from "electronics principles 2" from 29 years ago lit up my mind like a big green led.

Dug out a 10k 10-turn pot from my parts collection and managed to at least get an inverted signal out of the 7414 IC. Even better, it ran until I turned the pot far enough to lose the EST signal and dwelled enough coils to blow the 15 amp ingnition fuse. BTW, remind me to fix the stall detection because it's not firing the coils that are currently dwelling.

Fine tuning will be needed, but I think this will work. Thanks for the nudge in the right direction!

Don't mistake this for an apology about leaving the ignition control module connected as a "cheat". The word "crude" comes to mind.

[lionelhutz]

If it can't go above 4.6V then try using some diodes in series as the load. You could use up to 5 or 6 in series. They will limit the voltage so it can't go above 4.6V. They will also not load the circuit so much that it reduces the signal level you have to work with. You could use a zener too, but I don't trust them as much as a signal diode.

[spfautsch]

If it can't go above 4.6V then try using some diodes in series as the load.

I think vilefly's impedance matching idea has it handled. I tweaked on it a few minutes last night and again this morning. Ran for several minutes this morning without setting a code. Evidently the ignition module has pullup circuitry in it as well so what was needed was the correct balance of pullup + pulldown. I'll try to monitor the ADC tonight with eehack and fine tune.

[lionelhutz]

I'd think the ignition module more or less has the base of the coil driving transistor connected to that pin.

If you know the resistor in the PCM then it's easy to add an external resistor to create the required voltage divider which lets the pin go to say 4V.

[vilefly]

Didn't mean to offend by leaving the ignition module connected, but I always picture myself on the side of the road without help in the ozarks. I usually leave myself a backdoor to reverting these things back so I can make it home in case of emergency....such as keeping the original plug wires and coil in the trunk.

The resistor trick is commonly used to null out offset voltage in oscilloscopes and such. Ancient chinese secret. heh.
We may have to get you an oscilloscope for your birthday. You are far too skilled to be working without one. Keeps the frustration and guesswork down, for sure.

[spfautsch]

I was mostly being sarcastic - no offense taken. I know all too well what you mean about being stranded on the side of the road in south central Missouri. If this is going on your daily driver you'd be best served by not even removing any of that "legacy" hardware for a few years until we know how this will handle real world conditions. I don't know how much room there is on a b-body but on the y-body I'd rather pull the engine out than to try routing plug wires without removing the left wheel, inner fender and most of the belt driven accessories. And forget about attempting it the side of the road.

It should have occurred to me to try a trim pot Sunday when I was working on the comparator because the project I borrowed the potentiometer from was itself a voltage sense / comparator circuit. Was too busy looking for an "easy" solution and not thinking systematically. I probably spent the better part of an hour digging through parts totes looking for my original (failed) ignition module to hack open in hopes of finding something useful.

I'd think the ignition module more or less has the base of the coil driving transistor connected to that pin.

I initially thought that myself but there's definitely more to it than that because the module only switches on / sources current through the coil when the EST line is turned off and allowed to float. So minimally there's an inverting input stage there.

Fingers crossed I hope to finalize the initial circuit design tonight. Once I'm happy that it's doing more or less what it should be doing I'll try to finish the schematic. After that I probably won't be very active on the project until I get the engine out / back in.

[lionelhutz]

If the module is charging the coil while the line is floating then it could be a NPN transistor? It gets turned on when the PCM pull-up resistor is connected to it?

[spfautsch]

Doubtful it's a bjt transistor because the PCM isn't sinking any current when the EST line is high. Certainly not enough to drive a bjt into saturation. Maybe an n-channel fet, but my guess is it's a little more complex. If I can find my old one I'll hack it open just for fun.

Whatever the case, it's not an issue. vilefly you're a genius for your impedance matching comment. I was able to do away with the external Schmitt trigger. I had no idea what voltage to shoot for so I took a WAG and adjusted the potentiometer until I was reading 4.29 volts (219 raw) on the adc channel. If anyone has a chance sometime soon I'd like to see what yours reads at idle. See kur4o's post on page 17 for how to measure.

I was able to add ignition supply voltage sensing for the dwell compensation and tested successfully. I still need to look over the controller log for any anomalies, but no DTCs were set and everything seemed to work as expected.

It may be my imagination or the ridiculously cold weather, but it seems like it takes another 90 degrees of rotation before the engine fires. I don't normally run this thing when I can't wear shorts but it seems to be just noticeably longer cranking with the exception of hot restarts. I'll have to test more on this but it does start every time and I haven't noticed any odd behavior.

Current firmware source 0.9.2 now on github https://github.com/spfautsch/diy-ltcc/tree/master

I will concentrate on finishing the schematic, but if anyone wants to start working on a test board before that materializes PM me any questions. The code should be relatively well commented as far as what pins are what, but it's not like a "blink" arduino sketch. Very little of the arduino core libraries were utilized due to the time sensitive nature of the main function blocks.