From the MAKE Flickr pool
Flickr member 5Volt points out an especially historic schematic from NASA’s publicly available Apollo Lunar Module documentation. Its function was to light the appropriate indicator lamps once the module made contact with the surface of the moon –
The lights are two (top of drawing) on two different panels, namely panel 1 and panel 3. Both lamps light up when at least one of the probes touches the ground. The circuit is powered from two different sources and only when the descent engine is on – relay 3K7 controlled by switch K16B inside dotted square at left. Should the descent relay fail, switches 1K5B and 2K5B (at left) can be used to manually override switches to both supply lines. The two lamps are independently powered from the two power sources.
The circuit is redundant wherein the bulb besides being powered from two separate power sources, are controlled by two circuits.
Read more on the 5Volt blog.
A quick search turns up this rather gorgeous hi-res photo of Apollo’s control panel –
The actual lamps in question are fairly easy to spot in the large source file –