This alarm with sensors in series and parallel was found in the 1995 Encyclopedia of Electronic Circuits. The number of sensors can be changed at will.
General Electric suggests this circuit in a 1980s documentation. The operational of the time is...
This circuit, from a 1973 publication, locks the relay when the light that falls on the LDR is cut...
This circuit can be adapted to an FM radio or other receiver that does not have a tuning indicator LED....