We found this circuit in the July 1994 Radio Electronics magazine. The circuit uses a general-purpose transistor, and the power supply can be up to 12 V depending on the relay.
This circuit is from 1967 General Electric documentation. It uses SUS which are not very common...
This indicator for audio signals uses only 3 components. LEDs are common and the audio signal with...
Using three gates of t4011 this oscillator produces a rectangular signal with frequency given by...