Nuts and Volts - June 2014

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 1/16”.

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 be used.

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 spaghetti.