We found this circuit in the October 1996 Electronics Now magazine. The circuit uses LDRs as sensors and drives relays. The relays must have voltages according to the power supply.
Based on FET operational amplifiers, this circuit was found in a manufactirer documentation. The...
This circuit, obtained from a 1965 magazine, can still be assembled using some modern equivalent...
This circuit uses an operational amplifier that is still common today. The indication is made by...