This circuit uses a crystal phone and can be built with more modern transistors like the BC548. The original circuit is from a 1979 publication and is powered with only two AA or AAA batteries.
This circuit, from 2001 documentation, uses a phototransistor as a sensor and an operational...
This circuit was found in the September 1997 issue of Electronics Now magazine. The circuit uses...
The integrated circuit used in this 80's application is very hard to find today. The circuit is of an...