Build an RPM counter for attaching to fixed shafts and such (think for winding coils)

Thread Starter


Joined Oct 10, 2014
I was trying to figure out if there was a way to easily count the number of revolutions/turns I make on a drill or other rotational device - the use is for winding wire around transformers mainly and other similar apps. I will usually use a hand-held drill with a central shaft so the device can fit around it.

I was thinking of using something like a laser or some other detection method - don't know what is usable for this. I pictured the ring around the shaft all black or white covering like 90% and the other 10% the opposite color or some reflective material, depending upon the detection method. As the shaft spins, the 10% is counted for each time it passes the detector.

IDK if there are devices like this on the market currently or what the best method is to make this. Anyone have any suggestions?


Joined Nov 30, 2010
There are mechanical counters. An arm on the shaft hits the lever and it counts how many times the lever has been pressed.


Joined May 4, 2013
Magnet and Hall Effect device with counter?
...Or a reed switch and a magnet.

The cheapest counter you can get is a calculator at the Dollar store.:p Verify that the calculator can do automatic constant calculations. Most do.
(An automatic constant is one of the numbers from the last calculation used over and over again.)

Take the calculator apart and connect a reed switch across the equals key. Now do a calculation such as: "0 +1 =" The result now equals 1. Each press of the equals key or tick from the reed switch will add 1 to the result. Note that if use the diameter of the coil instead of "1" you will measure the length of the wire as it is wound onto the coil.

Be aware that the switch debounce time used by the calculator limits how fast you can count.


Joined Jul 18, 2013
As well as the retro-reflective method above, I have also used a slot sensor.
There is also digital and mechanical counters on ebay for $4-$10.


Joined Sep 17, 2013
To wind coils neatly using a hand-drill you may find you have to rotate the coil bobbin so slowly that you can simply count the turns yourself. But there's no fun in that :).


Joined Sep 24, 2015
The way I wound coils (back in high school) was to measure the wire diameter (0.125) and the diameter and width of the winding bobbin. Knowing how many turns it would take to fill one row (side to side) I knew that when I wound magnet wire from one end of the bobbin to the other, 100 turns would be 1.250 inches. So a bobbin of 1 1/4 inches would take 100 turns of 0.125 diameter wire. Wind across again for 200 turns - and so on.

Knowing the diameter of the bobbin would yield the circumference (Dia x Pi) would give me the approximate length of wire per turn. Multiply that by the number of turns and I knew how much wire (lengthwise) I would need. With two inch leads I could calculate the total length of the wire I needed to achieve a very specific number of turns and just spin the heck out of the bobbin and get a reliable number of turns.

But since we like the idea of a turns counter, Bloccs's counter would work just as nicely. And for a magnet I'd use one of those magnets found on those sonic toothbrushes. They're very small and very powerful. Just stick it to the side of the shaft and spin away. The counter can do the rest.