We're working on a car where we've replaced most of the electrical system. We've also replaced the transmission, and the new gear box doesn't have a speed sensor that we can use for the instrument.

We're replacing that with signals coming from the ABS-sensors but passed through an ECU.

The signal generated by the ECU is a 0-12V square wave with frequency linear to rotation speed (0-800Hz). The instrument takes a sine wave signal with varying frequency. We need help designing a circuit to achieve this conversion.


We've previously tried simple solutions

Old solution

But for whatever reason (not sine-like enough?) it's not working.

Super thankful for any help! At work right now so my only tool for drawing graphs is paint. Sorry.

  • 1
    \$\begingroup\$ Is it really 0-800 Hz? Do you mean at 0 Hz that the signal is some constant value? 0V? 12V? In between? The solutions may be different whether it's 0-800 Hz or 0.1-800 Hz. \$\endgroup\$
    – scorpdaddy
    Apr 8, 2019 at 12:21
  • \$\begingroup\$ The signal from the ECU is speed dependent so that at 0.1 km/h it's a square wave close to 0 Hz, and at a high speed it's a square wave with frequency in the range of 800Hz. The square wave v_on is always 12V, and v_off 0V. The duty cycle is 50% regardless of frequency. The solution doesn't need to work at arbitrarily low frequencies (say 20Hz) so I guess I'm really asking for 20-800. \$\endgroup\$ Apr 8, 2019 at 12:29
  • \$\begingroup\$ Do you have access to battery voltage or any other power source? Or do you have to derive power from the 0-800 Hz signal? \$\endgroup\$
    – scorpdaddy
    Apr 8, 2019 at 12:32
  • \$\begingroup\$ We have access to both 12V and 5V power outputs. (edited my last comment 20-800Hz is absolutely enough). \$\endgroup\$ Apr 8, 2019 at 12:35
  • \$\begingroup\$ Does it really need to be a sine wave, or just centered around 0V? Reverse the R and C in your drawing and that will center it around zero. It needs to pass 20 Hz, so the impedance needs to be low compared to the R and whatever input impedance the instrument has. Zc = 1/(2*pifc), or c = 1/(2*pifZc). You want Zc << Rin. If Rin = 10k, then c = 8 uF for Zc = 100. This implies a polarized cap, put the + side on the left. \$\endgroup\$
    – Mattman944
    Apr 8, 2019 at 12:58

1 Answer 1


When it ”dies” means the speedometer drops dead to 0. The instrument reads a sine in stock form from a VR-transmitter in the stock gearbox. Depending on the speed the amplitude varies between tenths of volts and around 5V at top speed. 9920 pulses/cycles per kilometer. The instrument cluster also has a circuit that converts this sine into a 0-5V square wave and passes this on to the now non existing stock ECU for speed calculations. The big unknown here is whether the instrument itself works frontside of the sine input or backside of the 0-5V square wave. Perhaps it is the built in circuit that needs a more sine like wave to function correctly.

  • 1
    \$\begingroup\$ Was this intended to be the answer to the posted question, or comment on the question? \$\endgroup\$
    – scorpdaddy
    Apr 8, 2019 at 15:57
  • \$\begingroup\$ Sorry... my bad... but apparently im not allowed to post comments on the main question..... should i remove it and repost perhaps? It was ment as additional info to the case. (My car in question) \$\endgroup\$ Apr 9, 2019 at 17:45
  • \$\begingroup\$ I think it would be good to provide a diagram, showing the VR-transmitter, the instrument cluster sine-to-square converter, the speedometer, and any other components; then label the signals between each component, and describe the signal at each point. I can't tell from the post whether there is or is not an ECU between the instrument cluster sine-to-square converter and the speedometer. \$\endgroup\$
    – scorpdaddy
    Apr 10, 2019 at 15:45
  • \$\begingroup\$ It does sound like a problem with the instrument cluster sine-to-square converter - based on the info provided so far. \$\endgroup\$
    – scorpdaddy
    Apr 10, 2019 at 15:47

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