Nuts and Volts - June 2014

wrapped tightly around the Teflon tubing with four turns as illustrated in Figure 4. Even though the Teflon is slippery, the tight wire wrap mechanically holds the Teflon tubing securely. Several drops of Crazy Glue were placed on the wire wrap, and the Teflon tubing was pushed back inside the brass support tube so the glue would anchor the wire, and thereby the tubing.

After the glue dried, both ends of the Teflon tubing were cut off with a razor blade. This tends to flatten the end of the tubing, so its circular shape should be restored with a small wire or tapered tool.

The Case

A LMB Heeger case #502 was purchased from Mouser Electronics; it has a compartment for a nine volt battery. A 1/8” diameter hole was drilled in the top of the case at one corner, being careful to miss any plastic obstruction inside the case top. The brass support tube is fitted into this hole and the support can be attached with five minute epoxy adhesive, or alternately drilled and fastened with self-tapping screws so the support arm extends out over one corner of the case.

Four rubber feet are attached to the under side of the case, so it will be stable and less likely to rattle around.

Pendulum String

The string is 0.010” diameter Spiderwire EZfluoro fish line purchased from Walmart or a similar sporting goods store. This is sold as 100% fluorocarbon, but is a related material known as PVDF (polyvinylidene fluoride or polyvinylidene difluoride) with a low melting point of about 177° C.

A knot following the instructions on the package was tied to the additional armature wire of the relay and then fed into the Teflon tubing from the bottom. The knot can be more securely fixed with a drop of Crazy Glue. The glue doesn’t stick to the Spiderwire but will mechanically lock the knot and help prevent it from unwrapping.

Expose one adhesive face of a piece of double-sided foam mounting tape and attach it to the PCB fiberglass bottom of the relay. Expose the second side of the foam tape and position the relay firmly in place so the fish line is directly over the Teflon tubing as shown in Figure 5.

Pendulum Ball

The pendulum ball is a 3/4” diameter stained wooden ball that was purchased from Hobby Lobby (#211201). A 1/16” diameter hole was drilled about 3/32” deep into the ball and a doubled-over 22 AWG copper wire was inserted as a loop and secured with five minute epoxy.

With the case on a table, secure the free end of the fishing line to this hook with another knot so the ball will hang free about 1/4” above the table. This knot can also be mechanically locked with a drop of super glue.

The length of the model’s string from the top end of the brass support tube to the top of the ball measured 4”. Add to this the radius of the ball and it gives the total pendulum length to the center of mass. The frequency of a simple pendulum is given as: where g is equal to 32. 2 feet/second2 (the acceleration of gravity). Using these values, the frequency of the model is about 1.5 Hz.