This circuit was found in the component's own manual, which is not very common nowadays. Note that the circuit requires the use of a symmetrical power supply.
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...