converting signal from ignition coil to a varying voltage - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: March 17, 2020, 01:48:42 pm
content: I want to make a shift light that triggers from as close to 0rpm (0hz) to at least 7k rpm (~233Hz)I have an old tachometer circuit that I salvaged from a Vaz tachometer, I thought I could use it somehow but it seems it will be harder than to think of something else, I want to use as much of the original circuit as possible if possible.here's the circuit:[ Attachment Invalid Or Does Not Exist ]the signal that goes into the input is really noisy. it comes from the negative terminal from the distributor that gets switched 4 times each revolution of the engine. there's a capacitor to ground in that line that I don't know the value ofbut that line gets switched from 12v (minus the voltage drop in the coil's resistance and other stuff) and gets shorted to ground when a cylinder supposed to fire.and since it's a coil it will produce a lot of negative voltage when it switched off afaik.anyway. how could I do this?I also want to switch a relay with it not just a led, not any big one tho.

author: langwadt
author_rank: Super Contributor
timestamp: March 17, 2020, 01:57:48 pm
content: revolutions per minute, not second. So your frequency is off by 60

author: AVGresponding
author_rank: Super Contributor
timestamp: March 17, 2020, 02:17:44 pm
content: If it's a 4 cylinder 4 stroke engine, it'll be firing 2x per rpm, not 4, as each complete cylinder cycle takes 2 revolutions.

author: langwadt
author_rank: Super Contributor
timestamp: March 17, 2020, 02:27:18 pm
content: Quote from: ThickPhilM on March 17, 2020, 02:17:44 pmthat depends, many engines run wasted spark, firing every evolution because it is simpler to run every thing off the crank, sometimes with a coil shared between pairs of cylinders with thespark plugs in series. The weak spark in the exhaust cycles doesn't do anything

author: langwadt
author_rank: Super Contributor
timestamp: March 17, 2020, 02:30:24 pm
content: Quote from: ELS122 on March 17, 2020, 02:23:20 pmthey might all spark every revolution but there is only 2 powerstrokes

author: AVGresponding
author_rank: Super Contributor
timestamp: March 17, 2020, 02:45:19 pm
content: How accurate do you need it to be?If it only needs to be approximate, a tuned RC circuit and a volt meter would likely do it.You could probably get within a few percent without going to extreme lengths.EDIT: Just reread the first postStill, a variation on it: tuned RC circuit with an op-amp to sense the level and swing the output at your desired threshold?

author: AVGresponding
author_rank: Super Contributor
timestamp: March 17, 2020, 04:00:34 pm
content: As long as you get the RC correct so that you get the switch on voltage you want at the rpm you want, get polarities right and don't exceed VBE/VCE etc, yes I see no reason why not.You'll need a pot in there to fine tune it I should think, and bear in mind that as you load and unload the transistor its switch on voltage will fluctuate, so it might be better to use one transistor to sense the level and use this to switch on a load transistor.Those KP315's are unijunction transistors, I don't think they'll work well in a level sensing function, due to the negative on-resistance characteristic. A bipolar transistor is definitely better here. Should be ok for the load switching though.You could use pnp transistors, and add an extra stage to invert the output from the level sensing one (it would be on until your threshold is reached then switch off).It's also worth noting that the circuit you posted provides an oscillating output signal by switching on transistors to pull the output low, so the default output without an input signal will be high.I suppose you could regard this as a lamp test function, since I think it'll light the LED/power the relay until the engine starts running.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 17, 2020, 06:36:01 pm
content: If you mean at the input on that circuit you posted, I'd say experiment to find what works best.You aren't going to damage it by using say a 12V pk-pk signal, the diode will block any current flow from the input to the ujt emitter.Essentially the input signal is switching the state of the transistor by pulling it low and turning it off, and with such a small value cap as 6.8 nF there's not going to be much current flow in any case.If you mean what signal level from this circuit to your extra circuit, I'd say start low and raise it until it starts to work.I'm unclear here as to whether you have KP315 or KT315, as although KT315 is a bipolar junction transistor, your drawing with KP315's on it shows them as unijunction transistors. Unijunction transistors work differently to bipolar, once on, their forward voltage drops, so they have quite high hysteresis and won't turn off as quickly.By and large they have a much higher saturation voltage than bipolar transistors, 1V pk-pk probably isn't high enough.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 17, 2020, 07:03:24 pm
content: The frequency isn't a problem, it's just that if they are unijunction and not bipolar, they'll not want to turn off easily once they're on.It might mean they don't switch off after you shift gear, if the rpm drop isn't sufficient.Ultimately the only way to know for sure is to build it and see, even simulating it in LTSpice or whatever your choice is for such a tool doesn't guarantee success or failure.Half the fun is building it anyway, and if it doesn't work, you get to fiddle with it until it does!

author: den
author_rank: Contributor
timestamp: March 18, 2020, 06:26:21 pm
content: Oh man. The spark is firing not when the coil is connected to ground, but when it is disconnected - and at this point collapsing magnetic field can generate up to 400 V in the primary. Be careful connecting electronics to that point.Concerning shift light - if that has a purpose (not just an interesting thing to build), then you first need to define when it should light up. Is it a guide for achieving max. performance (drag race) or minimize fuel consumption? In both cases matter is more complicated than shifting up at constant RPM, so microcontroller usage is probably unavoidable.

author: Circlotron
author_rank: Super Contributor
timestamp: March 19, 2020, 04:36:58 am
content: Quote from: ELS122 on March 19, 2020, 02:53:23 amI have used SPW47N60C3 mosfets with great success. Always switch the low side of the coil. If you put a 100nF cap across source to drain the coil primary voltage that appears across the mosfet when it switches off to make the spark will be a half sine of typically 400V, (depending on how many amps you are switching) and about 20uS duration. The spark will begin about 10uS after the coil is switched off and continue for approx 1.5mS. There will be no negative voltage because the mosfet body diode stops the coil negative terminal from swinging below ground. You could put a voltage divider on the mosfet drain and have a comparator that trips at say +200V. This will give signal pulse maybe 10uS wide that is practically immune to noise and false triggering.

author: james_s
author_rank: Super Contributor
timestamp: March 19, 2020, 05:03:04 am
content: Quote from: CJay on March 17, 2020, 04:09:58 pmAwkward or not, that was the standard way of doing it for decades in many millions of cars. Even my 1990 has the tach driven from one side of the coil primary. It's true that there are challenges but it's a problem that has been solved for a very long time, awkward or not it works reliably and doesn't require adding any additional sensors to an engine that may not have them.

author: krayvonk
author_rank: Regular Contributor
timestamp: March 19, 2020, 05:58:03 am
content: Your schem is beautiful, but I thought u were right when you didnt want to use IC's for it.I was wondering if you could give a beginner some help.With the first one, is that a register? I cant make out what it is, and I dont know why the caps are there.I love the unlisted size cap which has the leaky loop on it, why do u have that, I do those myself. (excuse me if u dont know what im on about... I fluked an oscillator once, but I never got it working after that, because I had a horrible brain attack, and im still recovering from it.)- and whats that potentiometre for, your not controlling frequency with it of course, cause ur getting timed by the spark plug - its near the outputs, thats why i say that. and i dont even know what a shift light is.I thought u said it was a tachometre, what has that got to do with changing gears? (or is it to do with some kind of electronic fuel injection?)

author: floobydust
author_rank: Super Contributor
timestamp: March 19, 2020, 06:33:40 am
content: It's difficult to pick off an ignition coil primary. +400V, ringing and -ve voltage too.I use a BJT with >50V to turn on, then one-shot. Most circuits on the web wrongly load the coil to get a weaker spark or trash the input transistor.OP's circuit resembles VDO, input with hysteresis and one-shot and integrator.But the schematic is wrong, no way it's 43R and nothing to protect the input transistor from -ve spikes and ringing.

author: Circlotron
author_rank: Super Contributor
timestamp: March 19, 2020, 11:42:33 pm
content: Maybe you could have some sort of optical sensor to detect the position of the tacho needle and shut off the ignition if it passes the set position. That’s pretty high tech for a Lada, going from what I’ve seen on Garage 54.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 20, 2020, 12:04:59 am
content: Quote from: Circlotron on March 19, 2020, 11:42:33 pmI love that channel, those guys are hilarious...In the UK we don't have Lada's any more, emission standards being the cause, as far as I know.I remember seeing my first Niva and thinking "ewww, a Lada!" and then "hey, it's pretty cool looking...".Optical position sensing of the tacho needle would work up to a point, but the trick would be getting the shift light to stay on if the needle went past it.Given there's some sort of signal from the tacho, looks like two 12V DC square waves in counterphase, though with several hundred ohms output impedance so not much in the way of current, there's plenty of scope to use this to generate a variable DC output, or a simpler threshold switch.I don't see a need to complicate it further by adding IR diode/phototransistor pairs reclaimed from a VCR for example...Garage 54 would probably wire the battery positive to the tacho needle and screw a self tapping screw into the dial to make a contact, and connect it to an air-horn or something equally nuts...

author: den
author_rank: Contributor
timestamp: March 20, 2020, 09:42:22 am
content: Quote from: ELS122 on March 19, 2020, 02:53:23 amI'm sorry, did I misread something? - "the signal that goes into the input is really noisy. it comes from the negative terminal from the distributor that gets switched 4 times each revolution of the engine. there's a capacitor to ground in that line that I don't know the value of but that line gets switched from 12v (minus the voltage drop in the coil's resistance and other stuff)and gets shorted to ground when a cylinder supposed to fire."Quote from: ELS122 on March 19, 2020, 02:53:23 amWell, because of the reason I mentioned in my post... Maybe before thinking about the circuit implementation you should think at which RPM you want your shift light to trigger. And then calculate RPM and vehicle speed after shifting up (or down).