This tiny unit conditions the DC power from any generic 9V-13V adapter to provide a clean-sounding effects pedal power source. Hum is greatly reduced by a very simple one-resistor, three-capacitor filter circuit.
I use it with a very noisy, low-quality, ancient, hacked radio shack wall-wart adapter. The adapter plugs into the right DC jack, and a daisy chain cable powers my pedals from the left DC jack. The LED indicates that the DC filter is engaged. If no power adapter plug is connected to the right DC jack the internal 9V battery is utilized instead-- Great for emergencies!
The top of the enclosure has a photoresistor (LDR) connected to a stereo 1/4" jack. The LDR works like a variable resistor between the jack's tip and ring. This makes it compatible with stereo cables for 'expression pedal' connections or CV send/returns. This configuration is also compatible with mono cables-- in that case the LDR will work as a variable resistor between tip and ground.
Connecting the LDR to another pedal's expression pedal input, expressive filter sweeps, manual phase/chorus modulations, etc are created by blocking the LDR's exposure to light. The unit also includes a reverse polarity protection diode.
The biggest challenge was fitting a switch, two LED bezels, a 1/4" jack, two DC jacks, a tiny circuit board (including two radial electrolytic capacitors) and a 9V battery into the little case. Shrink tube came in very handy... :)