The circuit comes from an old issue of Elektor magazine, shown below:-
The LED leads are left long and the LED itself is glued into an alloy plate that is recessed into the edge of the door so that the circuit board just hangs off the LED leads. A space chiseled out of the door behind the plate houses the circuit and battery, there is just enough room to fit a standard C type torch battery.
The unit works by charging a capacitor very slowly, then releasing the charge momentarily to light a LED. Since the LED is only ’on’ for a small fraction of the time, the power consumption is minimal and the unit will comfortably run on a single 1.5 volt torch battery for a long period of time.
The original unit that I built used a single C type Duracell battery and did not have a switch – its been flashing 24/7 for over 5 years at the time of writing this, and shows no sign of slowing down yet. When the battery starts to die, the LED shines less brightly apparently.
The power consumption can be reduced even more if the circuit is switched on and off by means of a pin switch (like the ones used on car bonnet to trigger the alarm) embedded in the hinge edge of the door so that when the door is closed the flasher is turned off and uses no juice at all. Sounds easy, but most internal doors are reinforced internally with strips of card or similar, so you will need to find a long enough implement to poke or drill a hole across through all of these, and then use a long piece of wire to fish the wires through the hole.
The first one has worked so well, and for so long, that I am building two more for the other two doors.