The following should be read in conjunction with Andrew Carruthers interesting article on a mill rpm display in MEW 288. Whilst setting me thinking and strong on aesthetics it was lacking in information on the display unit’s type, model number or cost. The wiring diagram only referred to the unit he had (though it would appear to be common to many) and the DC negative is not earth. Since the power supply is 240v mains this incorrect terminology could be dangerous. If a spare power supply is to be reused, check that it is DC and not AC although it is a simple task to rectify the ac with a bridge rectifier and smoothing capacitor. Also check on the polarity of any fitted plug, the centre is not always POS +.
Putting “hall effect tacho” into a search engine will result in numerous suppliers online. The pictures in this and Mr Carruthers articles will identify the right unit and prices seem to vary from £7 upwards. The one I purchased came with all that was needed, including the mounting bracket for the sensor and a magnet for £11. The unit needs a power supply of between 8v and 24v DC, about £6, if needed. Setting up the supplied sensor and magnet is easy. On the rear of the sensor is an LED, photo 1, that glows when powered up and increases in intensity when the magnet triggers the hall effect device. Fitting the magnet to the shaft, pulley or whatever and rotating it will show if the magnet is the right way round and aligned before it is permanently mounted. I had carelessly forgotten that the small and very powerful magnets used are very brittle and will shatter if allowed to smack onto steel. No prizes for guessing how my memory was refreshed!
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