The frequency of this circuit, for the indicated components, is 1 kHz. Other operational amplifiers can be used, and the power supply must 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...