Found in a 1990s US publication, this alarm circuit detects when a light beam is interrupted. Optical resources may be used for the sensor and instead of the alarm a relay may be used.
This simple metal detector was found in a 1960 Sylvania transistor manual. The phone must be high...
This is one of the many versions of this type of indicator that the reader can find on our...
Found in an English publication from the 1980s, this circuit can be built with equivalent...