Door buzzer or alarm (CB20492E)

This solar powered circuit was found in a 1991 documentation. The transistor is a general purpose NPN.

Automotive Open Fuse Indicator (CB7124E)
This circuit indicates through LEDs when an automotive fuse is open. ...
Turns Counter (CB18037E)
This is a project that can easily be adapted for other purposes such as counting people, objects,...
Frequency-Corrected Astable (CB25651E)
This frequency-corrected 1 kHz circuit has a nearly perfect rectangular signal. The circuit is...