This Mullard circuit was found in a 1969 documentation. It uses complementary transistors on the output and does not need a transformer. The bandwidth goes from 100 to 20 000 Hz.
The phototransistor of this alarm can be of any type. The relay must be sensitive type according...
This circuit was adapted from a 1995 Electronics Now magazine. He shows how to implement a type of...
This alarm circuit is from a 1987 publication, however the components used are still modern. The...