This circuit was found in the Electronics Experimenter Handbook of 1961. The circuit uses door sensors and others. It is powered by 12 V from the car battery.
This conventional series reducer circuit reduces the voltage of a 12 V battery to the selected value...
In the figure we have a converter that generates a negative voltage from a positive voltage...
This circuit covers the bands from 1.8 to 150 MHz, with plug-in coils whose characteristics are...