Dirt cheap way to detect change in resistance over tiny changes in wire length?


Joined Jan 23, 2018
If the contact between the wheel and the wire is poor, AND IF there is current flowing hrough that contact, then probably any motion will cause a noise voltage to be developed between the two wires. That noise voltage, thus amplified, can easily trigger a detection that depends on amplified noise. THAT sort of amplifier can be very cheap because noise, frequency response, as well as distortion do not matter.That will be the very cheapest scheme.


Joined Aug 7, 2020
If we assume a rectangular rail of 1mm x 2mm, then it will have a resistance of about 0.35Ω/m for stainless steel.
It could be supplied with a 1A constant current source, using a small switched-mode supply, such as one intended for driving LEDs, which would give 0.35V across the length of the rail if using a metre of rail arranged as in post #30.
A precision op-amp with a gain of 10 could drive the input of an A/D on a 3.3V microcontroller (after a bit of filtering to remove any hf ripple from the switcher). A 10 bit A/D would give a theoretical precision of 1mm, but it would depend a lot on how the noise behaves.