You know what they say that constraints drive creativity, I think this post is just another little example of that.

Usually I do not put time constraints to my hobby projects, but I make sure to have some other kind of constraints, like keeping the cost low and trying to reuse stuff that I already have.

In this case I needed a way to measure the angle and speed of a rotating object for some of my projects, and I wanted a rotary encoder that was inexpensive, but precise enough to work at very low speeds.

I ruled out optical encoders (I could have reused a mouse) because there could have been noise issues in the final project, so I decided to go magnetic.

I purchased a couple of Hall effect sensors but I had no idea of how I was going to use them.

Later I realized that the US1881 sensors I bought were bipolar latching sensors (check out the Hallbook for a nice reference of magnetic sensing), meaning that their state would persist when the magnet was out of reach.

These kind of sensors are not designed to be used with the magnetic pole facing the sensor, in a reed-switch fashion, they require alternating magnetic poles to switch states; most of the time these Hall effect sensors are used with ring magnets.

I didn’t want to buy a different sensor (unipolar ones may work better with sparse magnets) or a dedicated ring magnet, so I started to think about how to build a ring magnet from what I had lying around.

One way to build a ring magnet is to place small magnets with alternating poles in a circular arrangement, like proposed in Custom 3D Printed Magnetic Encoder Disks for Robotics Projects by Erich Styger.

However I don’t have a 3D printer, nor such small magnets, but I had around a Tetramag: a toy made of magnetic spheres.

The Tetramag spheres have a diameter of 5mm, that means that a pole covers about 2.5mm (pole pitch) and that the “element pitch” (the distance between the two sensing elements) should be of about 1.25mm.

Check out the section about quadrature in the Hall-Effect IC Applications Guide from Allegro Microsystems for more info.”

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