This special effect doorbell circuit was found in the January 1984 issue of Radio Electronics magazine. The components are quite common and transistors such as the BC548 and BC558 can be used.
This circuit is given in two versions, having been found in a publication from the 90s. The second...
This circuit was found in an ancient American documentation. Used integrated circuits are uncommon...
This Texas Instruments circuit uses a phototransistor to fire a TTL trigger port. Other...