I dont Know if they are still available, But My German Made Ciclomaster has worked whenever I Put a Battery in it ,
for decades , It just has Not been that important to Power It Up When I'm Not on Bicycle tours somewhere..

It has 2 plugs for Wires, 1 to a wheel sensor the other one to a sensor you mount near your crank arm
that you attached a Magnet onto.

I got it in the Mid 80's.

If You are clever for, RFI, you can put a metal foil or braided wire shield around the 2 wires to the sensors, and ground that separately..

That is done for Microphone cables (but that cable is a Bit oversize to use on a Bike computer)





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