Hall Effect Rotation Sensor:
Mark Dresser sent me this design. Send any questions or comments to him.
Here is a simple non-contact rotation sensor that can be built on perf board and fitted in a 2 x 4 brick. It functions exactly like the official part except that it produces 48 counts per revolution instead of 16. Notice that the sensor end of the brick is filed slightly to clear the magnet wheel. The heart of it is a quadrature hall effect sensor P/N A3425LK from Allegro (www.allegro.com). This part is rugged and relatively insensitive to distance and alignment to the magnet. It requires a ring magnet with 4mm between pole pairs (more on this later).
The diodes are BAT85 schottky diodes-- do not use standard silicon diodes such as 1N4148, their voltage drop is much higher and will not provide acceptable output voltage levels. Any schottky diode will work but the BAT85 is nice because it is small enough to fit flat on the board. The resistor values are also critical to reliable operation.
The magnet is made from a 48mm length of rubberized fridge magnet material. The encoder is constructed from a wheel turned or filed down to just under 15mm diameter so that the ends of the magnet strip butt together neatly at the joint. After sticking the magnet to the wheel it is protected with a piece of heat shrink tubing. Most of the fridge magnets that I have checked have a pole spacing of very close to 4mm. I got an educational kit from The Magnet Source (www.magnetsource.com) with two 50 x 50 mm self-adhesive squares of fridge magnet and a viewer card that shows pole locations. The fridge magnet should be cut at pole locations so that the count continues smoothly at the joint. Other sizes of wheel could be made as long as the circumference of the wheel and the magnetic strip are a multiple of 4mm long. Note that the sensor can also be used as a linear position sensor by moving it along a long flat strip of fridge magnet.
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