This circuit was found in an Electronic Design. It makes use of a relaxation oscillator to trigger an SCR and two relays. The circuit operates at frequencies of 12 to 180 cycles per minute.
The transistors are 2N918, 2N3464 and other RF equivalents of the time for Q1. For Q2, 2N3399, 2N2894...
This circuit, which can be used as a shield, was found in a 90s Electronic Design. The circuit...
This interesting intercom circuit was obtained from an Argentine publication from the 1970s....