How to use ECU tachometer square pulse signal to control LED brightness by RPM

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#1 21663400 13 Aug 2012 01:27

stanton Henkle stanton Henkle

Anonymous

Post #1
21663400 13 Aug 2012 01:27

I was looking for something that would control the brightness of LED based on the rpm of the engine in my car. I looked all over but no seems to make such a device. Basically I know that the tachometer signal in most newer cars comes from the ecu (older car get the signal from the coil) and is basically a square pulse signal. How would I use this signal to control the brightness of some LEDs for exterior accent lighting? any ideas? Obviously I have very little experience with this sort of thing. How difficult would it be?
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#2 21663401 13 Aug 2012 04:52

Adam Taylor Adam Taylor

Anonymous

Post #2
21663401 13 Aug 2012 04:52

If the ecu outputs a number of pulses per revolution then you can use a micro / FPGA to count the pulses and hence determine the RPM. You can then define a simple transfer function that converts the RPM to a pulse width modulated signal which you can use to drive the LED hence adjusting the brightness.
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#3 21663402 13 Aug 2012 06:26

Calinoaia Valentin Calinoaia Valentin

Anonymous

Post #3
21663402 13 Aug 2012 06:26

Take into account that most modern cars use a CAN bus to transmit the RPM, speed and other informations simultaneosly over the serial bus. And if it's the case, you must decode the CAN bus to get the information you want first. What car do you have ?
#4 21663403 13 Aug 2012 11:54

stanton Henkle stanton Henkle

Anonymous

Post #4
21663403 13 Aug 2012 11:54

It's a 2012 Subaru WRX. I can get to the tachometer input wire. Not sure if the isolates the RPM info.
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#5 21663404 13 Aug 2012 12:06

Calinoaia Valentin Calinoaia Valentin

Anonymous

Post #5
21663404 13 Aug 2012 12:06

The wire that goes to the dashboard or the wire that goes to the stepper motor of the needle of the tachometer ?

A 2012 Subaru has for certain all the data communication through a CAN bus. At the dashboard all the information comes encoded (speed, RPM, coolant temp, fuel tank, all the lights on the dashboard).
#6 21663405 13 Aug 2012 14:46

stanton Henkle stanton Henkle

Anonymous

Post #6
21663405 13 Aug 2012 14:46

It's the single blue wire that goes to the tachometer. Same wire I would use to hook up an after market tachometer. It may be easier to get the signal from the neg terminal of the ignition terminal.
#7 21663406 13 Aug 2012 15:09

Calinoaia Valentin Calinoaia Valentin

Anonymous

Post #7
21663406 13 Aug 2012 15:09

I see... Then it means that blue wire must have on it a signal whose frequency varies with RPM. You need a microcontroller to count the frequnecy of the signal, determine the actual rpm and then use an output port to generate PWM to command the led brightness.
#8 21663407 13 Aug 2012 15:17

stanton Henkle stanton Henkle

Anonymous

Post #8
21663407 13 Aug 2012 15:17

If I use the neg terminal on the coil it sends a square wave signal. For a four cylinder it's 4 pulses per rotation or something like that . I have a chart around here some where that tells me.
#9 21663408 14 Aug 2012 15:22

stanton Henkle stanton Henkle

Anonymous

Post #9
21663408 14 Aug 2012 15:22

So I need a microcontroller and a way to program it? I'm an Army commo guy. I know basic electronics and I'm no stranger to soldering components to a pc board. I know nothing about programing microcontrollers. I don't mind spending
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#10 21663409 14 Aug 2012 15:27

Adam Taylor Adam Taylor

Anonymous

Post #10
21663409 14 Aug 2012 15:27

Once you know how many pulses per rotation it gives out you will need a device to count them and then generate a PWM output. The arduino is very good as are the PIC devices have you ever programmed anything ? you can probably source a cheap development board which will do what you want something like this "http://www.atmel.com/tools/AVRBUTTERFLY.aspx(http://www.atmel.com/tools/AVRBUTTERFLY.aspx)":http://www.atmel.com/tools/AVRBUTTERFLY.aspx
#11 21663410 14 Aug 2012 17:35

Calinoaia Valentin Calinoaia Valentin

Anonymous

Post #11
21663410 14 Aug 2012 17:35

Go for the Arduino board. It doesn't really matter which one of them. They have many example codes and many libraries made by people... Which is really useful
#12 21663411 15 Aug 2012 10:33

stanton Henkle stanton Henkle

Anonymous

Post #12
21663411 15 Aug 2012 10:33

I've not done a little programing. Some Java, Basic, and Cobol (man that was back in day lol). I've also had to write a few simple Macros for some of the less user friendly radios I've worked with.
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Topic summary

The discussion addresses controlling LED brightness based on engine RPM using the ECU tachometer square pulse signal. Modern vehicles, such as the 2012 Subaru WRX, typically output RPM data via a CAN bus, but a direct tachometer signal wire (e.g., a blue wire to the tachometer or ignition coil negative terminal) provides a frequency-varying square wave proportional to engine speed. To utilize this signal, a microcontroller or FPGA can count pulses per revolution to calculate RPM, then generate a corresponding PWM (Pulse Width Modulation) output to adjust LED brightness. The Arduino platform is recommended for its ease of programming and available libraries, suitable for users with basic electronics knowledge but limited microcontroller programming experience. Identifying the exact pulses per revolution is necessary to calibrate the system accurately.
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How to use ECU tachometer square pulse signal to control LED brightness by RPM

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Topic summary