Building a driver for old tachometer gauges

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Hi;

I am working my way through the school of hard knocks. My self imposed project for today is to build a driver board for a tachometer gauge that has a coil and three inputs.

Pin 1 to pin 2 = 440 ohms
pin 1 to pin 3 = 220 ohms
pin 2 to pin 3 = 220 ohms

Voltage to the existing board is 12V for this set of data. maximum current drawn is 90mA at a gauge reading of 0 and falls to 60mA at a reading of 6000 RPM

I have made a chart showing pin 1 and pin 2 voltages from 0 to 6000 rpm in 500 rpm increments. (attached)

The main question is what do you think the Motorola 7777 chip does and is there a replacement chip that could do the job. My initial thought is to use an AVR and either PWM the voltages or use two Frequency to voltage chips generating the frequencies with an AVR based on the coil tac input pulse rate.

If anyone has links on information I might read on how the actual coil works in this configuration or something on the 7777 chip it may make things easier.

Thoughts on generating the what seems to be a sin wave out of phase with some existing chip that is made to do this type of thing. I don't know perhaps the 7777 is a micro controller but I don't think so.

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I think you'll find the meter is a single pole stepper. You apply a sine and cos value to the two phases - the resultant angle is what the meter reads. If you use pwm, then the inductance of the coils needs to be considered otherwise you'll get some weird readings. You don't need to generate a sine wave as such, just the sine and cos values: eg for 45degrees on the meter, apply 10*sin(45) Volts to one coil and 10*cos(45) Volts to the other - much like your graph. A magnetic vector is created and pulls the magnetic rotor in line with it. I did do something using a L293 and pwm some time ago. I never figured out the non-linearity.