Found in a Popular Electronics from the 90's, this circuit detects the approach of objects by the reflection of the radiation emitted by the LED. The relay is a 12 V sensitive type.
This oscillator that generates signals below the audible range (sub audio or infrasound) was found...
This circuit simulates a unijunction transistor trigger with two bipolar transistors. The circuit...
The model aircraft spark plug filaments operate with a low voltage of the order of 1.5 V. This...