Apollo 11’s touchdown indicator

Science Technology
Apollo 11’s touchdown indicator

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 –



Remembering Apollo 11 & One small step for open source software…

20 thoughts on “Apollo 11’s touchdown indicator

  1. Anonymous says:

    >It’s function was to light the appropriate indicator lamps

    Oops. It’s its, not it’s.

    1. Collin Cunningham says:

      oop – thanky

  2. Chris N says:

    Belt-and-Suspenders engineering at it’s best!

  3. justDIY says:

    It’s sad to see how we’ve regressed so much in regards to space travel technology. Long ago we possessed the technology to send a group of men to the moon, today we’re left relying on technology from our parents to travel to near earth orbit, and that soon will be gone as well, with no viable replacement as of yet.

  4. jpa says:

    I find it quite disturbing that there are no flyback diodes in the circuit. I wonder if the transistors of that day were strong enough to withstand the spikes, or broke down anyway.

  5. Bob Darlington says:

    Flyback/snubber diodes were sometimes built into the relay. Sometimes they’re left off the schematic for clarity, but I can’t imagine why in something this important. Hey, they only had to work once!

  6. deviceone says:

    I was really surprised to see the 7 segment displays. That had to be one of the most advanced electrical components in the lander

  7. oskay says:

    These were high-voltage electroluminescent seven segment displays– an incremental advance of existing technologies.

    High voltage electroluminescent indicator lighting was already common on the dashboards of cars in the 1960’s, and 7-segment displays can be found in patents as early as 1908.

  8. Michael C says:

    I’m a bit surprised to see that the components aren’t all labeled with values or other identifiers. Were these circuits built by the engineer who designed them, or by someone else? If I was the engineer designing this, I wouldn’t want to leave it to chance that the person building it would select exactly the right components. I can see that the transistors are labeled so their details could be specified elsewhere, but the resistors are all just blank.

    1. 5Volt says:

      This is the handbook, not the construction manual. Probably the construction manual for even this relatively simple part of the LEM would be a volume in itself and parts value would be in there with make/model compliance to codes and specifications for every single part, relays included.

      Same reason why the flyback/snubber diodes ( @Bob Darlington ) aren’t in there.

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