This circuit was obtained from an old American documentation, and practically any general-purpose transistor and sensitive relay can be used according to the supply voltage.
Using the 555 this circuit was found in a Popular Electronics from the 90's. The circuit directly...
This circuit was found in the December 1971 issue of Radio Electronics magazine. The circuit is...
This circuit is suggested by Texas Instruments in his Optoelectronics Sourcebook. Any photo...