This circuit is intended for detecting 3 levels power grid voltage, and it can be adapted to operate with the most diverse voltages. The circuit uses neon indicator lamps.
An interesting timer is shown in the figure, using one of the 4 NAND gates of the integrated 4001....
General Electric suggests this circuit in a 1980s documentation. The operational of the time is...
This simple adjustable source uses a PNP transistor that can be the BD136 for even slightly higher...