are all hall type sensors the same? reason I ask is I've got a vdo speedo which I want to use with my classic mini, i'm planning to glue magnets to a pot joint and make a bracket for the sensor to read them. so can I use any sensor? it needs a 3 wire one but can I use a bike or car or aftermarket one? being a locost(tightwad) person i'd prefer to get something 2nd hand

It is really a switch with three connections Power, Switched and Earth. A lot of automotive ones such as 1999 on BMW and Rover 75 ABS sensors earth through the bracket. With Hall Effect ABS sensors the output voltage will switch between 0.4v and 1.4v and back to 0.4v as a magnet passes the business end of the sensor.

One thing to watch is getting the polarity correct before powering up , even testing the resistance with a DMM can blow the sensor if the polarity is wrong.

so that throws another question into the ring, does the sensor have to be specifically for a speedo or could I use an abs sensor?

Maybe. The outputs from sensors are not all the same. Some have an "open collector" output, so the high level voltage is defined by the device it's attached to, some output a 5v logic level, some are inverted etc. Most ABS sensors are variable reluctance types with two wires which definitely won't work.

If your speedo uses an open collector type sensor, you could get one very cheaply if you are prepared to wait for delivery:

5-8mm Hall Effect Proximity Sensor Magnetic Switch DC 6-36V NPN NO 300mA | eBay

This thread may help ?

thanks for the replies, looks like I need to read up on the technical details of the speedo to get the correct specification sender