Quick Arduino Hack Lets Tach-less Car Display Shift Points

A tachometer used to be an accessory added to the dash of only the sportiest of cars, but now they’re pretty much standard equipment on everything from sleek coupes to the family truckster. If your daily driver was born without a tach, fear not – a simple Arduino tachometer is well within your reach.

The tach-less vehicle in question is [deepsyx]’s Opel Astra, which from the video below seems to have the pep and manual transmission that would make a tach especially useful. Eschewing the traditional analog meter display or even a digital readout, [deepsyx] opted to indicate shift points with four LEDs mounted to a scrap of old credit card. The first LED lights at 4000 RPM, with subsequent LEDs coming on at each 500 RPM increase beyond that. At 5800 RPM, all the LEDs blink as a redline warning.  [Deepsyx] even provides a serial output of the smoothed RPM value, so logging of RPM data is a possible future enhancement.

The project is sensing engine speed using the coil trigger signal – a signal sent from the Engine Control Unit (ECU) which tells one of the ignition coilpacks to fire. The high voltage signal from the coilpack passes on to the spark plug, which ignites the air-fuel mixture in that cylinder. This is a good way to determine engine RPM without mechanical modifications to the car. Just make sure you modify the code for the correct number of cylinders in your vehicle.

Simple, cheap, effective – even if it is more of a shift point indicator than true tachometer, it gets the job done. But if you’re looking for a more traditional display and have a more recent vintage car, this sweeping LED tachometer might suit you more.

[via r/Arduino]

42 thoughts on “Quick Arduino Hack Lets Tach-less Car Display Shift Points

  1. One usually stops looking at the tachometer to know when to shift gears a few months after getting one’s driver’s license (assuming you drive a car with manual transmission during that time).

  2. Wait… does the word “Tachometer” mean soemthing entirely different in other places of the world than the one I live in? There is a huge tachometer right in the middle of the dash. It tells you the speed of the car. The instrument that is missing is the rev counter. There is no way a company could sell a car without a tachometer.

      1. speedo == tachometer with a different scale. A separate rev counter is simply superfluous if you know what gear you’re in.

        In some older cars they simply put the shift points on the speedometer with tiny numbers.

        1. That might be true for older vehicles, although I doubt it because 2500 RPM in 1st gear isn’t the same speed as 2500 in 4th. Most passenger vehicles since the 90s get the speed from the vehicle speed sensor, which measures wheel speed and not engine speed.

          1. Of course it isn’t. It’s a different speed for each gear. You simply memorize what those speeds are, and knowing what gear you’re in you aim for that speed before changing gear.

        2. For some, the distinction between the two is what they’re measuring, not how they’re measuring. If you were using GPS to determine your speed then you couldn’t call it a tachometer.

    1. That is usually refered to as a speedometer I believe. Tachometer (or tach gauge) is the correct name for an instrument indicating the number of revolutions per time period. Strictly speaking a rev counter would just count the number of revolutions made

      1. Hurray Ancient Greeks: takhús = fast

        All tachometers measure the speed of something. A rev counter measures the engine speed, and the speedometer measures the wheel speed. They’re both tachometers.

        1. And there I thought it was simply a typo, missing a “y” before the “o”. I thought – cool, finally someone came up with a means to measure beyond the speed of light … and then … MEH … it was just another “Schätzeisen” gimmick.

  3. wow, I wasn’t aware we germans use the wrong word :D

    and also: I wonder when ppl will stop using blindingly bright lights for dashboards. The unholy “blue LED hype” should well be over by now, shouldn’t it?

      1. on at least one of my cars the speedometer was connected to a non drive wheel for true rate of movement measurement. wheelspin in the drive wheels showed up as a tach surge while the speedometer never budged. also seen with hydroplaning, which is a remarkable disconnect of wheelspin from rate of travel.

    1. They aren’t, actually: Tachometer stems from greek tachýs ‘fast’ and métron ‘scale’ – so tachometer in german is the correct word for what the english speaking world calls a ‘speedometer’.

    2. You can easily use blue LEDs on the dashboard. Just do not stick to the old rule of “20mA for an LED” :-) Probably 100µA could be sufficient, at least at night. One time I tested an LED and wondered, why it was only slightly glowing – the connection went through my finger. Further tests showed, that even with 100nA a faint glow was visible (normal daylight, no sun, made shadow with hand).

      1. The human eye doesn’t have superachromatic lenses, so it gets into big trouble trying to focus both blue and red-green light at low levels when the pupil aperture is large. You’re simply messing with your eyesight having blue lights in the dashboard, plus, even relatively small intensities of blue light kill your night vision.

  4. I did this with 30 LEDs and 3 LM3914’s on my motorbike about 30 years ago. Because it did frequency to voltage conversion (diode resistor capacitor) it had to calibrated with a pot. The vibrations of the bike destroyed the pot. So it was a FAIL lol, but interesting while it worked because it was quite unique at that time.

  5. Nice work! But … for the Astra (and any other car of the last 20 years) a simple plug&play – OBD display would work, too. OK, it’s more expensive, but comes with a lot more things to display (temperatures, true speed, ignition, voltage etc.).

    1. Small displacement gas engines really need to be revved up that high to get some oomph out. Mine for example gets max power at 5.5k with redline at 6.25k. If you want to reach the claimed bhp, you need to stay within that narrow margin.
      I routinely drive around 2k, but when you need the power, why not hit the sweet spot at 5k? That’s what the engine was made for, even if it starts to scream :)

      1. The peak Volumetric Efficency sometimes occurs much higher than crushing speed with some engine/gearing combinations, especially if the engine is choked for air but still required to have minimal emissions. The shift points on cars of old took this into account for maximizing fuel economy.

  6. Or, Velleman kit K2625 digital tachometer, I have these fitted to a few tractors, because they’re cheaper than repairing existing ones, it’s a simple fit on petrol engines but on old mechanical diesels you need some form of sensor.

  7. I had a similar project a few years back with my Honda 1990 CRX. My tach broke so I wrote an arduino serial driver to read the working memory registers from the ecu. The LCD is written using a PCF8574 I2C bus so that I could down the road program in some rally style shift lights using another PCF8574 and just chain it with the LCD. I also half wrote some PID boost control but I kinda lost interest after I solved the main problem of not having a tach and boost gauge.


    1. Well you had it easy lol. Last time I ‘replaced’ a tacho the original had a mechanical cable to the engine cam shaft that spun a magnet in the tacho. The indicator was driven by an aluminium casing that surrounded the magnet and was itself offset or moved by the force of Eddie currents and it’s movement was opposed by a spring. So my starting point was several reed switches surrounding the magnet to get the switching frequency up high enough for a digital timer.

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