older relays that are larger with snap-on covers. Higher or
lower voltage versions can also be employed with a DC
power supply such as a wall wart. If you use a wall wart,
make sure that the unloaded voltage does not exceed
the 18 voltage rating of the CMOS 7555 pump oscillator.
Figure 2A illustrates the cover locks located on each
side of the relay. Two cuts were made on each side of
the relay with a sharp Xacto knife so the cover could
be carefully removed.
Figure 2B illustrates the internal parts of the relay.
The black plastic switch actuator can be carefully
removed with a pair of tweezers or small long nose
pliers. Then, the relay armature can be removed by
pulling it forward from under the golden colored spring
clip. The armature is modified by adding a length of wire
to increase its motion and provide a
place to tie the pendulum string.
A stiff 0.020” diameter steel wire
was used, but almost any wire such as
22 AWG copper wire will work. The wire
can be attached with five minute epoxy
adhesive or soldered in place so that the
wire loop will add 1/2” to the armature
as illustrated in the photo. This will allow
the pendulum mass to be pulled about
A 1/2” wide by 1” long piece of
scrape PCB (printed circuit board)
fiberglass was glued to the relay as a
base with five minute epoxy (illustrated in Figure 5).
Alternately, scrap pieces of plastic about 1/16” thick can
Pendulum Support Arm
The pendulum support arm is made from brass
tubing as shown in Figure 3. Similar tubing can be
bought from a hardware or online store. It consists of a
3/32” OD x 0.014” wall brass tube inserted inside a 1/8”
OD x 0.014” wall brass tube for rigidity. The smaller
tubing extends out about 1/8” from the larger tubing at
the bottom, and they are secured together by soldering
the outer ends together. Other tubing such as stainless
steel can be used.
The support should be rigid because one model
employing a single 1/16” OD x 0.014” wall brass tube
failed to work because the support vibrated and damped
any oscillation. This could be demonstrated by holding
the end of the support tube rigidly with a third-hand
alligator clip to observe the pendulum oscillation.
The tubing was bent by hand around a 1-3/4”
diameter glass spice bottle to form a smooth 180° bend.
Any excess tubing can be cut off with a
Dremel cutoff wheel. A 1/2” wide flat
strip of brass with a 1/8” hole was
soldered 1/4” up from the bottom as a
support to the case. Alternately, a metal
washer can be soldered at this point to
act as the support.
The inside of the 3/32” brass tube
was lined by inserting a 0.040” OD x
0.010” wall Teflon tubing inside to
reduce friction and wear. Similar Teflon
tubing can be purchased from online
stores. Alternately, the solid copper
wire can be pulled out from a piece of
Teflon insulated hook-up wire for the
At the upper end of the Teflon, a
piece of 30 AWG copper wire was
38 June 2014
■ FIGURE 3.
■ FIGURE 2B. Actuator conversion.
■ FIGURE 2A.
Relay case removal.
■ FIGURE 4.