Selsyn-Powered Intercom (Jul, 1947)
Spells Out Messages
SOME of those mysterious little gadgets that made certain war equipment seem almost like magic are finding their way to the sales counters as surplus goods. One of them is the selsyn, that onetime highly secret device used in antiaircraft weapons, bomb sights, and radar. Selsyns in small sizes can be picked up in dozens of stores now for between $3 and $5, and many of them will operate on ordinary 115-volt A.C.
These selsyns are really directional motors. If two or more are hooked up properly and the shaft of one is revolved, the shafts of the others in the circuit will revolve in exactly the same way, through even the identical arc or fraction of a revolution.
This makes selsyns ideal for operating an intercom system between the house and basement workshop or garage and even between neighboring houses. For each station in the circuit set up a box having a dial face containing the letters of the alphabet and a line or other indicating mark for a space. Mount a selsyn in each box with the shaft through a hole in the center of the dial. One way to secure it is with metal clips or brackets bolted to the back of the box face and to the collar of the selsyn. Then file a pointer from 1/8″ sheet brass or plastic and attach it to the shaft, either between two nuts on the threads or, if the shaft is not threaded, by means of a collar and setscrew.
Take pains with the wiring for, though it is simple, the leads must be connected properly or the circuit will not work. A good method is to attach the leads from each selsyn to a small terminal board and number them in order from 1 to 5, counting as 1 the same first lead from each selsyn. Then connect all 1’s, all 2’s, and so on. Use rubber-covered hookup wire inside the box and any good wire approved for house current between the stations.
Connect one of the stations to the house current across the outside terminals, through a toggle switch and in parallel with a pilot light that will show when current is on. As a convenience in signaling the other stations, wire in a separate buzzer circuit powered by dry cells. If a water pipe or radiator is used as the ground return, this will require but two more wires between stations. Unlike the selsyn circuit, they can be bell wire.
Messages are transmitted by moving the pointer from letter to letter in alternate directions. Buzz once at the start of a message and twice at the end. Use the space between A and Z to separate the words.