The LM3909 tone generator integrated circuit of this application is uncommon nowadays. The circuit can operate on a single battery and is very sensitive.
This circuit was found in Linear Technology documentation from the 90s. The circuit uses the...
The circuit is from the 1990 Motorola Linear Databook. The circuit can be implemented with other...
This circuit uses a photo-transistor to trigger a circuit sourcing a square signal for the output. The...