Tachometer Circuits For Vintage Ignition Systems

Added: 2026-05-11

[00:00:00]Okay, in this video we are going to have a look at an automotive project. Now, we are going to design and build a tachometer.

[00:00:08]Now, that's a device that will display the RPM of a four-stroke engine. Now, to do that, I first had to build an ignition simulator, which you can see here. Now, this simulates the ignition system of a car that's running. So, here's the coil as connected to my spark plug. So, if I energize it, you can watch the spark plug sparking.

[00:00:28]So, the frequency of that spark is equivalent to a four-cylinder engine revving at 10,000 RPM. Now, of course, my four-cylinder engine is not going to rev that high, but building the tachometer, if it's picking up 10,000 RPM reliably, it'll work all the way down to idle.

[00:00:45]Now, this tachometer is for the vintage cars because the new cars have coil over over plug. They have a coil for each spark plug controlled by the computer.

[00:00:57]So, this tachometer is for the vintage cars that have points and condenser, and a distributor, and a coil. Okay, here's your typical ignition coil in a vintage car.

[00:01:08]And you can see the two terminals.

[00:01:09]That's your primary side. So, this side marked plus is connected to the 12-V battery.

[00:01:15]And the minus is connected to the points.

[00:01:18]So, the points would ground this for the dwell time.

[00:01:23]And then, the when the points open, we'll get a spark, and that comes out of the secondary, which is in the middle of the of the coil. Now, this is fed to the distributor.

[00:01:32]Now, the distributor has a center connector where the coil is connected.

[00:01:39]Then, inside is a rotor that distributes the spark to all the spark plugs. Now, this is for a V8, but the car that I'm going to design my tachometer for is for a four-cylinder.

[00:01:50]So, that's your vintage ignition system.

[00:01:53]So, we'll build a tachometer that will read the RPM of a four cylinder engine.

[00:01:58]Okay, there are two ways in which we could hook up our tachometer circuit to our ignition system. Now, the first way is inductive. You can see I built an inductive sensor.

[00:02:08]I got some hookup wire, four turns around the cable coming out of the coil, and we could detect the pulses coming out of the coil. Now, if we measure the time between the two pulses, we could actually calculate the RPM of the engine. Now, the second way is directly connecting to the coil, to the negative side. That's the side that goes to the points, and we could pick up those pulses also, same as coming out of here. And when we get the time between pulses, we could calculate the RPM of the engine.

[00:02:39]Okay, next, we're going to have a look at the pulses coming out of my inductive pickup, which is some hookup wire, four turns around the spark plug cable coming out of the coil.

[00:02:52]So, I'll energize my ignition simulator, and there's our pulses.

[00:02:58]So, if we measure the distance between those spikes, it's very short. That's when it's firing.

[00:03:04]We could calculate the RPM of the engine.

[00:03:08]Okay, here's my breadboard with my tachometer circuit.

[00:03:12]And the reason why I'm making this video, I want to clean up this breadboard. I want to take off all the components, so I thought I'd do a video first to document it before I stripped everything off. Now, when I work on my car outside, I don't want to bring out my bench scope or my bench power supply, so I have a portable scope. And this is my portable power supply. It runs off off a lithium ion battery, an 18650. You can see here.

[00:03:38]It has a charging port, USB-C, and it'll give me 3.3 volts and 5 volts.

[00:03:43]So, it's pretty handy when I'm outside.

[00:03:46]So, this is my circuitry here. So, I have an optocoupler, which is connected up to my inductive pickup, so it's totally isolated. Then I have a Schmitt trigger NAND gate where I'm cleaning up all the signals. Then I have a flip-flop for syncing the pulses into the microcontroller. And the microcontroller I'm using is an RP2040. So, that's going to measure the time between two pulses, which will give me the RPM.

[00:04:09]Now, this is a platform I'm using cuz what I could do I hook up a Bluetooth module like this.

[00:04:16]Now, that will send the RPM data to my smartphone.

[00:04:20]So, this is a platform I'm using so I can measure battery voltage, coolant temperature, oil pressure. So, all those sensors are underneath the hood. Then it's sent by Bluetooth to my smartphone.

[00:04:30]So, that saves me all the wiring from the underneath the hood into the cab of the car.

[00:04:35]So, next we're going to have a look at the schematic diagram of my tachometer circuit. Okay, here is the schematic diagram of my tachometer interface.

[00:04:44]And the heart of the circuit is an optocoupler. You can see I'm using a 4N33.

[00:04:49]So, when picking an optocoupler, find a sensitive one with a two-transistor Darlington output.

[00:04:56]Also, look for a high CTR, that's current transfer ratio. In this case, the 4N33 has a CTR of 500%.

[00:05:05]So, find one with 500% or higher.

[00:05:07]Now, you can see my induction pickup on the very left. That's four turns around the coil wire.

[00:05:14]And that's fed into the optocoupler with a 2.2k current limiting resistor.

[00:05:20]And I have a switching diode, a 1N4148, that's clamping the reverse voltage. In case there's a reverse voltage across the emitter, it will clamp it to 0.6 volts. Then I have a speed-up capacitor 0.01 across the resistor.

[00:05:35]So, the output of the optocoupler has a little bit of filtering with a 10k pull-up and a 0.1 microfarad capacitor.

[00:05:43]And that's fed into a Schmitt trigger NAND gate, which is a clean it clean up the signal. So, at this point here, you're going to have a very clean uh signal pulse train.

[00:05:53]And then you could use it uh for whatever you want to do. If you want to go into a microcontroller, or if you want to put it into any type of circuitry.

[00:06:01]In my case, I have two more buffers, which is driving an LED for indication.

[00:06:06]Okay, so now that you have a nice clean pulse train, you can take that pulse train and feed it into your favorite microcontroller, and you could do a frequency to RPM conversion.

[00:06:17]So, that's the signal that we got from the Schmidt trigger NAND gate. So, the next part of this video, I'm going to answer a question that a friend had. He wanted to measure the distance between two spikes, and he fed that into a the GPIO of his microcontroller, and he was missing some of the spikes. It wouldn't detect some of them, cuz they were so narrow.

[00:06:37]So, on a fast microcontroller like RP2040, you'd probably work. You could use an interrupt or the PIO, but I think he was using the Arduino.

[00:06:45]So, this was the solution, and you could use this on any microcontroller.

[00:06:50]So, here we have a D flip-flop, a 4013.

[00:06:54]It's edge-triggered, positive edge-triggered, the clock on pin three.

[00:06:58]So, it's sitting there waiting for the spikes, right here.

[00:07:01]So, as soon as it sees a spike from zero to one, whatever is on the D will be latched over to the Q.

[00:07:08]So, now the microcontroller has all kinds of time to sense it, because it's latched. As soon as it senses the the the spike, it's going to send a pulse to reset the the flip-flop, so the Q will go will go back down to zero.

[00:07:23]So, that way, the D flip-flop is always looking for the spike, and it'll it'll detect it every time, and give the microcontroller enough time to sense it, and then give back an ack acknowledge back to reset the flip-flop for the next pulse.

[00:07:38]So, this is the circuitry I'm going to use uh just to show that it works.

[00:07:43]So, we could uh we could obtain the RPM from the pulse train.

[00:07:48]Okay, I have Tera Term up and running on my computer. And it's connected to my RP2040 microcontroller.

[00:07:55]So, I'm going to run a program that'll measure the distance between two pulses in microseconds. I have a microsecond tick timer running.

[00:08:03]So, the program is called TAC. And I'll run that many times.

[00:08:09]So, I'll run it. And it'll energize the coil.

[00:08:14]And you see that's in microseconds.

[00:08:16]3,200 microseconds, so that's 3.2 milliseconds.

[00:08:20]So, next we'll look at RPM. So, clear the screen.

[00:08:24]And the program is called TAC 3. And do that many times.

[00:08:30]So, I'll run that. And that'll give us RPM.

[00:08:40]So, it's about uh 9,300 RPM. So, it's close to 10,000.

[00:08:45]So, that's how we measure the time between two pulses in microseconds. And then we could calculate the RPM.

[00:08:52]Okay, I have my HC06 Bluetooth module plugged into my breadboard.

[00:08:58]And I have a program running where I'm going to pair my program to the module. And watch the LEDs will go solid when it's paired.

[00:09:13]So, it says connected.

[00:09:15]So, there's my sensors. My first one is TAC.

[00:09:18]So, I'll run my simulator.

[00:09:25]And there's my TAC values.

[00:09:29]Sent over by Bluetooth.

[00:09:33]Okay, here's a quick look at the TAC circuit that's connected directly to the coil, to the negative side of the primary. It it a clamp circuit, an opto-isolator, and a Schmitt trigger NAND gate to filter out all the noise to give us a clean signal. And if I turn on my simulator, it's going to activate it, and the final pulse train will activate this LED. You see it come on.

[00:09:55]So, the pulse train that's feeding this LED is clean, [music] same as the first circuit.

[00:10:01]Okay, here's the schematic diagram of the tach circuit that's connected directly to the coil to the negative side of the primary, which is connected to the points. So, here's our coil, and you can see the plus terminal is connected to the battery.

[00:10:14]Minus terminal goes to the points, and we're going to tap off that negative terminal into a 1N4007 diode. Then, we're going to go through a clamping circuit, a 5.1-V Zener.

[00:10:27]It's going to clamp this point at 5.1 V, and I'm going to feed that into the opto-isolator, which is an HCPL3700.

[00:10:35]Now, the output of the opto-isolator, pin five, has a little bit of filtering, 10K and.047.

[00:10:42]And then, that output is fed into the 4093 Schmitt trigger NAND gate, which will clean it up. And we'll get a pulse train, which is feeding our indicator LED, which will be very clean.

[00:10:53]Now, the voltage that we see on the on the negative terminal looks something like this.

[00:11:00]So, when the points are open, we're going to see the battery say set 14 V.

[00:11:04]Then, the points close, it's going to ground the negative terminal, and it's going to build up magnetic field in the coil, and this is our dwell time. Then, when the points open, we're going to get a big EMF spike. It's going to go up. We're going to get ringing, then it's going to go back to 14 V.

[00:11:20]So, this is what we got to filter filter out all the noise in the high voltage, and just look for this pulse here, and that's what this circuit does. So, then we could calculate RPM.

[00:11:32]Okay, here's the inductive pickup on my classic Mini. So, here's my coil, ignition coil, and here's the wire coming out of the ignition coil.

[00:11:42]And here's my inductive pickup, four turns around the wire, and I got it hooked up to a connector so I could disconnect it.

[00:11:50]So, I have another connector that fits in here.

[00:11:54]So, you just plug it in like that.

[00:11:57]Then you could run that into the tach circuit.