This Schmitt optical trigger with LDR was found in old documentation. The relay must be sensitive, or an additional step must be used to trigger it.
This alarm circuit was found in a 1982 publication. It uses components that can still be found...
With this circuit, signals in the range of 10 kHz to 150 kHz can be received at close to 4 MHz...
Found in American documentation from the 1970s, this circuit is intended for amplifying analog TV...