Timing: Making a High-Frequency Clock
Most small fan or cooling motors will turn at nearly 60 rotations per second under no load (that is, with the fan blades removed). This makes them ideal for the high-frequency clock described above. With a cardboard disc mounted on the motor axle in place of the fan blades, the frequency is typically 50 - 55 rps. Here's are the steps to making one of these clocks (called a clip clock since it uses a clip fan).
- Start with a clip fan that you can buy at a hardware store. (These are small fans that clip to a tabletop.) Completely remove the cage around the fan blades. This may require cutting away the soft plastic with cutters. When the cage is removed, pull the blades off the axle. The stripped-down fan may look something like the photo to the right. Duct tape has been wrapped around the motor housing to cover sharp edges left from cutting the plastic.
- Cut a 6" diameter disc out of cardboard. For the clock face, download the file, clock.gif. Print the file, cut out the clock face, and glue it onto the cardboard.
- One way that works fairly well to hold the cardboard on the axle is to fashion a washer out of a piece of soft plastic. The lid of a film can works well for this. Punch a hole in the center of the lid and force it on the axle to test the fit. Then simply hot glue the lid in place on the back of the clock disc. When the glue is dry, punch a hole through the cardboard and force the disc onto the axle. Photos of the back of the disc and the completed assembly are shown below.
Film can washer glued onto the back of the clock disc
Clock disc in place on the motor axle
(The unit shown in the photo includes a variable speed control (blue electrical box) that was wired onto fan's AC cord. This was added in order to be able to turn the frequency down to small values. It's not needed for using the fan motor as a high-frequency clock.)
- If you want to adjust the clock to have a particular frequency, say 50 rotations per second, you can do this by trimming the diameter of disc. You'll need an electronic stroboscope to measure the frequency. There's no formula to use in deciding how much to trim. Just use trial and error.
- A Caution: Although the fan blades have been removed from the clock, the high-speed paper disc can give paper cuts. Keep body parts away!