Controlling an analog automotive tachometer

Well, this might be a way to do it - see enclosed…

a square or pulse input would do at variable frequency…

note : a small transistor won’t do as the drive device - it’ll need a several amp rating MOSFET

regards

Allan

revdrv.pdf (20.1 KB)

allanhurst: Well, this might be a way to do it - see enclosed....

a square or pulse input would do at variable frequency..

note : a small transistor won't do as the drive device - it'll need a several amp rating MOSFET

That seems promising. I think I've got some MOSFET's kicking around, I'll try it and report back.

If it doesn't work first time, don't give up - try reversing the meter wires...

Allan

Just figured out how to make any civic 92-00 tach work in the bench.

It has 3 wires. + 12v, - ground, signal (12v)

To generate the wave you can use tone function or TimerOne.

The tach needs 12v to drive it. Arduino can do only 5v. So it needs a NPN -> PNP to feed 12v.

http://www.w9xt.com/img_1224968365_15450_1308499014_mod_380_283.jpg

But I couldn´t pass 3000rpm.

Put the osciloscope probe and saw it wasn´t getting 0-12v wave, instead was 3-12v above 3000rpm, and almost 7v-12v at 8000rpm.

I solved this using a 20k pulldown resistor.

Hope it helps.

Just for future reference if anyone is working with an old Faria tach like this thread shows, there is an internal PCB with transformer, diodes, caps, etc. so you can't expect to drive it via the external connection points provided to hook up to an engine. It works based on the current flow through the positive power supplied to the ignition coil on the engine. You can read more about the design by searching Google Patents for US3005155. You have to pry open the case and disconnect the meter movement from the original electronic package and then work directly with the meter movement itself.

The meter movement is a "taut band" design movement that works as an ammeter, and will read full scale with somewhere between 0.650 mA and 1.000 mA.

I was able to drive it directly by the Arduino to full scale using a PWM pin with a 10k / 10K voltage divider. The duty cycle number has to be limited to 250 or so in order to ensure you still have pulses - at 100% duty cycle (255) the meter will drop to zero.