This circuit is from Motorola's Linear Interface ICs of 1993. The integrated circuit is not common and the Triac is chosen according to the power of the controlled load.
This 1967 Raytheon circuit uses an optical coupler of the era and has a range of 800 to 200 Hz....
This interesting digital counter with relays with complementary outputs was found in the Radio...
This circuit uses a diode as a radiation sensor. The operational ones are old because the...