CDU for driving high current solenoids (blue) using big electrolytic caps, instead of thrashing your power supply with burst currents.
Power from the supply charges the capacitor through the Darlington pair. When the switch is closed, the capacitor discharges through the solenoid, the base of the Darlington pair is pulled down to ground, shutting it off.
Forgot which website I found this on, it was something to do with model railways though.
