Motor rpm counting circuit? - HomeBrew Forum at eHam.net

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eHam.net Forum : HomeBrew : Motor rpm counting circuit?

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Motor rpm counting circuit?	Reply
by W6MQI on June 14, 2009	Mail this to a friend!
I’m looking for a simple circuit to count the revolutions of a motor. The motor is 12vdc runs at 9 rpm and will be mounted up on a tower. I thought about attaching a rotary encoder on the shaft of the motor and from there to the counting circuit. The circuit needs to be able to count up and down via 7 segment displays. I’m not to good at digital circuits, but I figured it can’t be all that difficult to do. I’ve looked at a few hobby circuit sites for ideas but seem to be coming just short of what I need. So if someone knows of a good site with circuits that can be built home brew style and might fit what I’m looking it would be much appreciated. 73 Dave

RE: Motor rpm counting circuit?	Reply
by K8AC on June 18, 2009	Mail this to a friend!
Are you really looking to count RPMs, or determine the position of a variable capacitor or inductor driven by the motor? I'll assume the latter. An approach I've used that has been very reliable is a digital voltmeter (surplus from All Electronics). A multi-turn pot is mechanically coupled to the motor output shaft using a cogged belt and matching pulleys from Small Parts Inc. You can buy belts of any length and choose the pulley diameters to fit your application. For example, my vacuum variable capacitor took 20 turns to get from stop to stop and I matched that to a 10 turn pot using a 2:1 gear (pulley) ratio. Here's a link to my site containing a photo of the control unit I built using the digital meters: www.k8ac.net/BalancedTuner.html . It's the one described as the second version. The circuit was quite simple - the regulated voltage (I used 8VDC) was applied across the pot and the wiper provided the current position indicator voltage. That took only 3 wires to accomplish. The latest version of my controller uses a PIC processor, feeding the position sensing pot voltage to an analog to digital converter on the PIC chip. That allows program control of the positioning and sensing. Let me know if you have any questions about any of this.

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