Sunday, July 14, 2019

Final preparations for 50th anniversary demonstration tour with AGC


I glued together an acrylic stand that would allow the LM to sit up in the air about a foot. It has a pivot to set the LM with the descent engine facing forward during the initial part of our landing demonstration and then to pivot to place the engine bell downward for the last portion.

It required a bit more bracing than I expected but a quick trip to Tap Plastic got me the extra parts I needed. It can be disassembled and carried in my suitcase for setup at each demonstration event.

I needed a small enclosure to hold the resistor boards, controller and wiring out of site. I rummaged through my parts bins and found something useful, even if not visually appealing.


The stand to hold the electronics panels up is more challenging due to the combined weight. I stated with two vertical strips of 1/4" acrylic but, just as with the LM stand, I had to add bracing before it was ready for use on the trip.

Starting from the bottom, we have the right hand patch panel placed. This panel will have the 35 banana plug leads from the DSKY replica pulling forward, which is why I chose it for the bottom spot. Next is the main interfaces board which will convert spacecraft signals to modern TTL levels.

Above that is the left hand patch panel. I placed the interfaces board between them to shorten the runs of banana plug cables that splay from the interfaces to the two patch panel boards. Finally, the top position is held by the secondary interface board which provides the last five AGC C level output signals needed for my LM model lighting. These needed aluminum holders to connect them to the acrylic vertical rails.

I picked up some better bolts and nuts for assembling everything and stowed them in my luggage with the stand pieces. I had to reinforce the acrylic with thicker rectangular braces just to keep every thing standing vertically. It was quite solid and dependable once completed.

Our 'rack' of patch panel and interface boards


I first set up a temporary version of the controller firmware without the input pullup resistors, thus making each input stay at ground unless I explicitly inject a high level. This let me test the key inputs - the 16 RCS jet signals that cause LEDs to light on the model's RCS quadrants. Along with that, I tried out the engine on and engine off commands, viewing the descent engine bell light at minimum thrust brightness and watching it go out again.

To perform a more comprehensive test, and to help with checkout, I set up a self-test capability at power up of the controller board. It will light each logical group of RCS thrusters in turn; up, down, forward/back and side.

It then will turn on the engine, throttle it up to full thrust, take it back to half thrust, and turn it off. This allows me to check quite a bit without needing a tester to drive the controller.

An external tester (another Arduino) produced pulse patterns to further test the throttle control portion of this model. I had it all wired together ready for testing Sunday morning. By afternoon I want to start packing everything for my departure Monday morning.


Our lab and my home testbench makes use of triple section power supplies as well as more than one supply box.  That would be painful to have to haul around on the demonstration tour next week. Mike is bringing the big 28V supply that will feed the AGC and supply my interface circuits.

I need to bring a beefy 5V supply to power the DSKY and the controller for the LEGO LM model. There is another voltage necessary if we will be sending pulses into the AGC (the Y type circuits) - 14V - and I don't want to drag around a separate supply. Fortunately, the AGC delivers 14V on another pin of the patch panel.


  1. Many thanks for keeping us all interested and entertained with your posts. It has been a fascinating journey, even for us passive observers.

  2. The demo was awesome, thanks for all the time you (all) spent on this!