Monday, July 27, 2009

Reed Switch Motor and Parallel Port Tachometer

Hello readers! I'm posting a project here after quite some time. After moving to Bangalore, I had slowed down. Well, now I am building stuff again so I'll be posting much more often!

A few days ago, my brother Amrit Derhgawen, cousin Anand Karpatne, and I decided to make a simple reed switch motor. My brother is a character animator at DreamWorks, but he also happens to be good with electronic things. So, after a few hours of construction and troubleshooting, he finally got the motor working. JOY!

A reed switch motor consists of a rotor made up of 2 (or 4) permanent magnets. An electromagnet and reed switch are placed close to the rotor on opposite sides. I got my electromagnet by stripping a relay. When a magnet on the rotor gets close to the reed switch, the reed switch gets magnetized and allows current to pass through and turn on the electromagnet. The electromagnet pushes the magnet closest to it, and makes the rotor turn. The electromagnet is switched off when there is no magnet close to the reed switch, and this allows the rotor to spin freely from inertia. When the other magnet gets in working range of the reed switch, the electromagnet pushes again.

After making the motor, I decided to measure it's RPM (Revolutions Per Minute) using my computer for the fun of it. So, I connected another reed switch to my computer's parallel port as shown here:

By default, status port 7 (S7) is high on my computer. When a magnet comes close to the reed switch, S7 becomes low. I placed this reed switch very close to the motor's rotor. The software is a C# Console Application which uses Inpout32.dll to read the parallel port. For about every 60 revolutions of the motor, it estimates its RPM.

The motor usually spins at around 3000 to 4000 RPM depending on voltage. Here is a graph I generated using values from the program:

I turned the motor off and on 3 times quickly, and also played with the voltage. You can see all this in the graph!

Do watch the video to see it in action!

For more information on how to make reed switch motors, check out this wonderful tutorial -

Ashish :)