The circuit presented is from 1994 and can be developed with more modern operators. We found the configuration in the National Semiconductor Linear Applications Handbook.
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...