Found in an Electronics Now from the 90s this mixer has several inputs. The integrated circuit can still be found easy and the power supply need not be symmetrical.
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...