Wednesday, December 5, 2018

Continuing work in preparation for the next round of Apollo Guidance Computer restoration


PCBs arriving back from manufacturer, ready to add components

Ken Shirriff and I designed circuit boards for the Core Rope Simulator driver, and the DSKY Substitute, respectively. is sponsoring the CuriousMarc YouTube channel by providing free PCB manufacturing to us.

My design is a 10 layer board that sits across the top half of the DSKY enclosure, driving the displays which include 15 lights, 21 decimal digits, three plus/minus signs, plus lighted lines and legends. It hosts an Arduino Mega underneath plus interface electronics with the Apollo Guidance Computer.

The interface involves 23 circuits for 28V signals originating from the AGC, 8 driver circuits delivering 28V signals to the AGC, as well as a circuit that senses the switched 14V supply of the AGC and blanks the right side display when the power is off (for standby).

This requires almost 400 parts and 1200 pins/pads in a 6 1/2 by 4 1/2 area, mostly using surface mount parts arrayed on both sides. This includes 9 relays, 9 high current quenching diodes, three 7-segment display driver chips and two LED driving chips, plus almost 70 LEDs and their current limiting resistors.

The boards will be impressive to view and the job of soldering all those components is going to be a long, tedious but important one. I have a scheme in mind to solder on portions of the circuit at a time and test them incrementally, allowing any rework before the board is jammed full.

Ken's board hosts a Beaglebone controller and was a more normal 2 layer board. We expect it to be very nicely made and ready for Ken to begin soldering on the components.

Building the DSKY Substitute

I continued the long fight with Fusion 360, the CAD/CAM package from Autocad, trying to finish up the mechanical design of the keyboard assembly that is the bottom half of the DSKY enclosure. Here I had to create pushbuttons that are internally lighted with keycaps that have their legend cut out, fitting in the 7/8" x 7/8" shape of the enclosure, and acting realistically when pressed.

The DSKY keys travel down 1/4" inch when pressed, quite a long travel compared to most pushbuttons. They require somewhere between 21 and 26 ounces of pressure to depress to the stop and won't turn on until they are 3/16" down.

These buttons are separated by a mere 1/8" on all sides, which required me to design the entire keyboard structure. A printed circuit board will sit below the keys, with .488" x .488" pushbuttons soldered in position. A plastic honeycomb with 1/8" thick walls forms the barrels of all the switches.

The honeycomb wall sides have slots formed in them that control the travel of the pushbutton to 1/4" and retain the keycap just below the surface of the top faceplate when they are not activated. The plunger that moves up and down in the honeycomb is a 3D printed translucent plastic part that sits in the 7/8" x 7/8" cell, with tabs on the side that slide up and down in the slots on the walls. Small LEDs on the printed circuit board will inject light into the translucent plunger.

The inside of the plunger is hollow and has a spring inside that couples the plunger to the pushbutton switch inside and below it. The pushbutton moves only part of the distance and resists with partial force, but the collapsing spring provides the additional force and movement to offer the net effect of 1/4" depression and 21-26 ounce resistance.

I arranged holes in my printed circuit board and in the plastic honeycomb structure to mount them together, to mount down through a .78" standoff to the bottom of the DSKY enclosure, and to permit some shifting to fine tune the alignment of switch, honeycomb and plunger to the faceplate.

A fellow space enthusiast who is building a DSKY replica that is far closer to the real unit than mine is providing the keycaps wth the cutout legends. I will use 3D printing services to manufacture the honeycomb and 19 plunger assemblies. I may need to test different springs until I get the right travel from free to compressed shape and the proper resistance to movement.

Replacement connectors to hook to the AGC

Marc's employer is a high tech connector company and has agreed to sponsor this project by creating some connectors to attach to the Apollo Guidance Computer. Two connector blocks were used to connect the AGC to everything in the spacecraft and to provide a maintenance connection for ground support equipment.

The AGC used Malco mini-WASP connectors, which are complete unobtainium. The company underwent a number of acquisitions and is now part of TE Connectivity but the connectors have not been made or sold for many decades. Fortunately, NASA has a pretty complete specification for them in their archives and those are being used to build modern replacements.

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