Thursday, July 11, 2019

Improving the interface board and patch panels for demos, plus LM Model set up to display jet firings


I designed a replacement board to house interface circuits that allow use of modern 5V TTL or 3.3V LVCMOS devices to drive or monitor the AGC signals. The spacecraft itself uses either 28V logic levels or short 10-14V pulses on wire pairs for signalling, neither of which are friendly to an Arduino or FPGA.

Each of these new boards is essentially a rack width (almost 19 inches) and houses one third of the total input-output circuits used by the AGC. I chose that fraction because the PCB fab I used had a minimum order of three boards for their fast turnaround prototype service. Thus one of the PCBs houses 19 type D, 13 type C, 11 type Y and 18 type XT circuits. D and Y circuits are inputs to the AGC of discrete and pulse type respectively. C and XT circuits are outputs from the AGC, discrete and pulse.
Apollo to modern logic interfaces board
I built the board and then subjected the circuits to testing to be sure they worked properly. Using signal generators, oscilloscopes and the like, I produced pulses with my type Y circuit and read them with a type XT on my board. In the same way, I toggled logic levels with my type D circuit which were read correctly on the type C. I used a signal generator first but later an Arduino produced both discrete and pulse test signals and detected the results from the receiving circuits

This board will be mounted just above one of the two rack wide patch panel boards we use to route all the AGC signals from the dense A51 connector to banana jack receptacles that are easily accessible. Banana plug cables hook any chosen circuit from the AGC to the interface board and from there to switches, Arduinos etc to work with the signals.


I began the task of soldering small 30 gauge wire to tiny LEDs that I will place inside the nozzles of the Reaction Control System (RCS) quadrants on the Lego Lunar Lander kit. I had to figure out a way to route the wires that is not distracting to the viewer. The LEDs I will mount inside the Ascent and Descent engine bells are larger.

The Lego kit also arrived today. Only 1087 parts to assemble; somehow it took five hours to build. I skipped the moon surface portion as well as the astronaut figures, concentrating on the portion I will display, but that was a small fraction of the kit.

LEGO model to install LEDs in RCS and engine bells
I needed to work on an anchor point to hold and orient the LM. I can't use the top of the LM because the ascent stage pops off the descent half, the weight would be enough to have it fall aprt if supported only by the top. I needed  something that holds the descent stage and rotates, which will allow me to orient it engine bell forward for part of the descent and then engine bell down for the last portion of the landing.

The solution was some clear acrylic with a stand and pivot. A trip to Tap Plastics nearby will give me the materials I needed. While there I will buy materials for the rack stand to hold the two patch panels and my interface circuit boards. They have to disassemble to travel in my luggage for the demonstration trips, otherwise I would just build permanent stands or use a half height rack enclosure.

Installing the LEDs for the RCS engines was quite challenging. My first plan was to use 0603 surface mount LEDs with wires tacked onto them, super-glued into the model. The problem is the delicateness of the LED and the small size. I had wires break off ruining the LED as I tried to place them into a thruster cone.

Preparing 3mm LEDs

Inserting 30 ga wire through drilled holes in LEGO pieces

I then bought some 3mm thru hole LEDs, bright red when illuminated but transparent white otherwise,  and prepared them for mounting. Using my 30 gauge black wire to tack onto the base of the LED leads, I could cut the rest of the leg off and use heat shrink tubing to insulate. Two wires per LED until downstream where I can combine all the common wires.

I put a 1/8" plastic drill in a small press, placed the lego pieces in a soft vise and drilled out openings inside the four thruster cones. I also drilled out an opening near where the quadrant attaches to an arm on the LM body. This gave barely enough room to fit eight wires through the final hole.

Using my stereo microscope, various tweezers and lots of patience, I threaded the wires through the LEGO parts and pushed in the LEDs. After a bit I had two of the four quadrants done and found that it was not too frustrating as long as I took my time on each.

By Thursday morning I had all four quadrants, 16 thrusters in all, with LEDs wired out to a terminator breadboard which also housed the 16 resistors. The wires ran into the descent stage and out along the bottom, where the black wires blended pretty well against the black LEGO bricks.

Light in RCS quadrants in place, wiring exiting at bottom

It was a simpler job to wire up an RGB serial addressable LED into the Descent Engine Bell. That completed all the lighting I need to use the LM model to show in real-time the thruster and engine firings that occur during the lunar landing we will perform for our demonstrations.


I wired up every signal from the AGC to its circuit pin on the patch panels. This gives us the flexibility to interact however we want, making use of the interface board to do so using regular TTL and LVCMOS devices and controller boards. There are more than 300 wires involved in completing this task, spread over two days to give my back a rest between sections.

Next up was building the stand for the two patch panels and the interface circuits board. The main principle was to use screws and nuts to assemble the stands but allow them to fit compactly in my luggage for transport. It will take a few days for Tap Plastic to cut all the pieces I need but by Saturday I should be assembling the stands.


I put an Arduino Mega 2560 into duty as a controller for my LEGO Lunar Module display. It hooks to 18 of my type C interfaces and 2 of my type XT circuits. Since my one interface board implements only 13 type C interfaces, I had to partially build a second board to provide the patch positions for the remaining 5 C interfaces.

That build was done quickly, although I was short three red banana plug receptacles which are needed for type C circuits. I had to make do with three yellow jacks which normally would be used for XT circuits.

From the Arduino viewpoint, a type C circuit coming from my interface PCB is simply an INPUT_PULLUP, an input with a weak pull-up resistor inside. The type XT circuits produce pulses that must be detected by the Arduino so I used interrupt service routines for those two pins triggered on rising edges.

Using the MIT documentation of the equations used in their hybrid digital/analog simulator of the LM, I collected the information I needed to properly simulate the throttle of the descent engine. Our AGC will increment or decrement a counter in the engine controller to vary thrust. That engine has a minimum thrust level of 10%, achieved even when the counter is at zero. The counter goes up to a maximum equivalent of 3200 which represents full thrust of the engine.

I also have the lag characteristics they used for the engine. It takes 2.3 seconds from when the Engine On command is received until it is producing the minimum 10% thrust, and when switched off it takes 380 ms. Every thrust change command requires about 300ms to take effect.

I am attempting to model the engine LED in the model based on the MIT defined characteristics. This adds some complexity to the Arduino program but the results should be worth it. More on this soon.

1 comment:

  1. Thanks for documenting all of this -- it makes for fascinating reading for fans of the restoration project. Keep up the amazing work!