I have an old bus with some electrical problems that I am trying to solve.

One problem is the dashboard. For many years, the dashboard was leaking rain water because of a broken windshield. The instruments don't work properly and I can't trust the information (air pressure / coolant temp and fuel level).

For the pressure and other sensors it's easy to do with an Arduino, but the Hall effect sensor for the motor RPM is another story.

I've tried to make a "copy" of the schematic for the sensor circuit. I'm not sure if it is correct or not (especially with capacitors and transistor). My sensor has only two wires, one from the dashboard and another one to ground. I can't find any schematics to explain how it works.

How can I use an Arduino to read the value?

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1 Answer 1


Why do you say it is a 'Hall Effect' sensor? I suspect it is just an inductive sensor that gives a pulse every revolution (or half-revolution).

A Hall-effect sensor would require at least three wires as they require excitation.

As drawn Q4 looks like it conditions the signal from the sensor to provide pulses into U2 (I suspect C12 is not really 1uF, it is much lower).

The pulses at pin 10 of U2 could be counted by an Arduino to display the RPM. The pulses will be at a few tens of Hz to a few hundred Hz. Just measure the frequency and scale to RPM.

The voltage of the pulses may need conditioning - you don't show what the voltage is at U2 pin 16.

  • \$\begingroup\$ Hum, I don't know why I've said it's a Hall Effect sensor, like you say it's maybe an inductive sensor. It's like an ABS sensor I think. I've searched for more information an I've found ABS use a reluctor (that read a reluctance) sensor, that is an inductive sensor as I understand. Am I right? It's Good to know that the Hall effect sensor needs at least 3wires! I need to look how inductive sensor works to understand how Q4 work. Yes C12 is not a 1uF, I don't his value, it's too small an it hasn't any specification on it. \$\endgroup\$ Commented Aug 14, 2022 at 7:56
  • \$\begingroup\$ As I understand how reluctor sensor works, each time the signal is positive, the Q4 trigger, "connect" Q4 pin 1 to ground and make a square signal for the U2 pin 10 which always supplied by a voltage when Q4 is blocking? (I don't how to explain in English, sorry). R22 resistor act like a current limiter right ? For Arduino it's 5V max on an Input (or 3.3V), so I need a voltage divider or something else to get the good voltage if i Supply the sensor with 24V ?Or maybe a mosfet as a fast relay? \$\endgroup\$ Commented Aug 14, 2022 at 7:56
  • \$\begingroup\$ My bus has an 24 Volts (really 22 to 28 Volts) system. I need to test it if you want this value on U2 pin 16. \$\endgroup\$ Commented Aug 14, 2022 at 7:56
  • \$\begingroup\$ @MauriceChocoswiss - You added an "answer" (because you wrote it in the box labeled "Your Answer" below), but it was not the answer to your question, and was clearly an attempt to reply to this answer. Therefore (within the limits of comment space etc.) your (non-)answer has been moved to be the comments above. Please note that Stack Exchange is different from typical "linear" forums. Questions, answers & comments are different here and are written in different places. Please see the tour, the help center & the commenting policy for more relevant rules. \$\endgroup\$
    – SamGibson
    Commented Aug 14, 2022 at 13:12

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