diode series or array?

"All mechanical" is an interesting term. The one I took apart as a kid (many years ago) had a magnet rotating on the end of the cable. That was near a conducting disk on a spring loaded needle. Eddy currents and such.

The one I played with wasn't very strong. I don't think a scheme like that would work. Maybe newer ones are different.

If there's enough torque behind the needle to move the wiper that could work. but, most mechanical speedos work bt a rotating magnet driving an aluminium disc (or cup) by induction, opposed by the force of a very weak spring.

a carefully placed hall effect sensor could detect the passes of the magnet

the rate of these pulses would need to be translated into the speed... the sort of thing a small microcontroller excells at.

Bye. Jasen

So, you tear apart the speedo, and mount a coil or Hall thing next to the rotating magnet, and voila! PFM!

Cheers! Rich

The analogue needles are definately electronic as I've had the dash out a few times. The car is a 1994 Toyota Supra. I was imagining wiring this unit in parallel with the pos and neg of the drive to the needle, if you see what I mean. I realise it might screw the calibration a little as there is less voltage driving the speedo but would this still not be possible?

it depends on how the needle is driven... if pulses fron the transmission are use to clock a stepper motor like setup that drives conventional speedo mechanism (or drives the needle directly) there will be no analogue voltage available.

moving coil meters aren't suited to much over 90 degreees needle movement. if you're seeing significantly more than that don't expect to find a convenient voltage source behind the meter.

Bye. Jasen