Door buzzer or alarm (CB20492E)

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

Car Battery Monitor (CB4374E)
This circuit was found in an English magazine Radio & Electronics Constructor of February 1977....
VU Meter Drive With The 741 (CB1438E )
This circuit is from a 1976 documentation. The diodes can be 1N34 or equivalent, and the power...
3V to 5V Converter (CB4823E)
This circuit is from a Maxim manual and can be used to power shields from batteries. The circuit...