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.
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
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.
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
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.
June 2014 39
f=2π l √
■ FIGURE 5. Actuator mounted in case.
■ FIGURE 6. Pump oscillator schematic.