This circuit was found in a 1967 US documentation. The circuit applies to modern general purpose transistors. The circuit has no offset. The power supply must be symmetrical.
This circuit uses a photocell as sensor and a sensitive relay according the power supply voltage....
This 2005 circuit from a Linear Technology manual makes use of the LTC610HV and an optical coupler that...
The 4093 or any other NAND gate can be used for the very sensitive configuration of a R-S...