This simple circuit is based around an astable multivibrator which is used to indicate two states for use as a power indicator. In the first (default) state, it will flash at around 1Hz, which could be used as a fault indicator; such as on the front panel of an amplifier when it's in DC protect mode, for instance. The second state, used via the "SW" (middle) connection on J1 to display solid ON (indicating power-on, normal operation), by bring the line to ground (GND). This connection steals the base current of Q1 so the LED can remain on solid.

Construction is quite straight forward, with all components (except LED1) are mounted on the top side of the PCB. The LED is mounted on the bottom side of the PCB, so it can be mounted to the front panel with the LED passing through a hole in the panel.