We found this circuit in an old RCA publication. The components are of the time, admitting equivalents. The circuit is triggered in two ways using LDRs.
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...