A source of repeating contact closures in the range of 1/10 pulse per second to 10 pulses per second was required for life testing of arc suppressors for relays. It was desirable to vary the on-to-off ratio, and to manually actuate a single shot. An electro-mechanical multivibrator was built, using RC timing and unijunction transistors (UJTs) to operate the relay. A magnetic latching relay was the bi-stable element.
In operation, the relay applies 28 V to the UJT and the RC charging circuit. When voltage builds up, as determined by the capacitor and the setting of the 100 K potentiometer, the unijunction transistor fires, transferring the relay to its other state. The same action takes place in the second unijunction circuit, causing transfer back to the original state.
For single shot operation, one unijunction circuit is replaced by a push-button, used to discharge a capacitor into the proper relay coil. This switches the relay and initiates the timing action of the remaining unijunction circuit. (Electronic Design, March 16, 1960, p. 271)
Here's another UJT-based Idea for Design, this one submitted by Ernest F. Wilson, Senior EE, Edgerton, Germes-hausen & Grier Inc., Boston, Mass. The UJT was a versatile device that found many applications.
