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.
This circuit uses two light-activated SCRs or LASCRs, uncommon components today. The power depends on...
I found this circuit in a Popular Electronics from November 1999. The magazine is no longer...
Other series amplifiers such as the MC3301, MC3401 and LM2900 can be used on this circuit. We...