Preparation to light up an Apollo DSKY electroluminescent panel

OVERVIEW OF THE DSKY

The Apollo Display-Keyboard (DSKY) is the human interface to the guidance computer within the Command Module and Lunar Module. It consists of an array of pushbuttons that form the keyboard and two side by side panels of lights. The left panel is a matrix of incandescent lamps which indicate various operational, caution and warning conditions for the computer and spacecraft. The right panel is a single electroluminescent panel that displays 21 decimal digits, three signs and a few lighted text blocks.

Apollo block II DSKY electroluminescent panel

The EL panel forms the digits with seven segments but these are not LEDs like modern displays. The segments are formed with phosphor coated metal traces, as are the segments of the + and - signs or the rectangles that light up with text. Over the top of all of this is a transparent conductive coating.

When an appropriate AC field is applied to the segment traces and the top coating, it causes the phospors to emit green light. Typically the AC is up to 275V at 800Hz with the voltage varied a bit by the cabin dimmer potentiometer. The AC is switched to each segment by contacts in tiny latching relays. Signals from the guidance computer cause the relays to be set or unset, thus illuminating or extinguishing each segment.

The DSKY is supplied with only a few relatively low DC voltages, mainly 5, 14 and 28V. It is also fed with an 800Hz clock signal. One module inside the DSKY produces the high AC voltage which is then routed to the relay modules to drive segments.

Decoder modules look at the 15 bit word coming from the guidance computer, evaluating the top four bits as a row number and the remaining 11 bits as the set or unset value for eleven columns of relay. One column is generally used to hold the sign value, which is +, - or blank. Generally the remaining 10 bits are used to represent two digits. A five bit value is used to control the seven segments of a digit using a bit of clever relay logic.

Each relay module contains 42 relays, six columns by seven rows, most of which are latching types with DPDT contacts but a few are non-latching. The DSKY uses four such modules, thus has the capacity to handle 168 relays. They can be thought of as a 2x2 matrix of modules, so that each of the possible fourteen rows controls 12 total columns.

A row most commonly controls two digits, thus 11 rows are needed to control the 21 decimal digits on a DSKY. Controlling the signs for the three registers takes the first column of two rows each. Other relays in that first column are used for functions such as lighting the Uplink incandescent light or flashing the verb and noun fields.

Not every contact of every relay is brought out to an external pin; some are interconnected inside the module and some are not used. There are a number of select pins that pick the row to control, set and unset lines that affect an entire column but only act on the relay whose row is selected.

Other relays have individual or small group select signals as well as set and unset. Finally, the non-latching relays simply expose the coil through two pins. All told, there are 137 contacts on the back of a relay module to handle 42 DPDT relays.

Two main versions of the DSKY (and guidance computer) existed, block I and block II. The later block II versions were the ones used on all the manned missions. I have been loaned a block II EL panel along with one power supply and two relay modules from a block I DSKY.

Apollo block I DSKY power supply

Apollo block I DSKY relay module

Although the two versions are overall incompatible, they are similar enough in using 275V 800Hz AC and relay modules that I can combine those to control about half of the EL panel at any one time. I would need more relay modules to simultaneously power all 21 digits and the other lighted features.

BENCH TESTING THE RELAY MODULE

I set up a lengthy test procedure to verify the operation of the relay modules. I began by verifying that all the coils had continuity, all the steering diodes for set and unset lines were working, and then that every single relay latched and unlatched (or switched under power if it was a non-latching type).

The relays operate with about 13VDC applied to the set or unset line, grounding the select line simultaneously. My bench power supply provided that power and let me watch the current levels as they relays operated.

For every set of relay contacts, I verified that they were closed or open under the appropriate condition, set or unset, of the relay. I entered the codes for displaying a blank and some digit values to check that the segments were properly switched. For the special cases where a segment can be lit with two different patterns of relays, I tested both.

BENCH TESTING THE POWER SUPPLY MODULE

I checked continuity, freedom from shorts and for appropriate values on the few pins on the outside of the power supply, ensuring that they corresponded to what I expected from the schematics. I then prepared to power up the module by feeding it +14V, +28V and a reference 800Hz signal. I used a signal generator and a bench power supply.

I did have to wire up a potentiometer to simulate the cabin dimmer knob and bridge a few contacts that needed external wiring. With everything ready, I applied power while watching for signs of trouble such as excessive current or magic smoke escaping.

BENCH TESTING THE EL PANEL

I created an elaborate testing plan for the EL panel as well. I verified that no segments were shorted to the top plate or two each other, but that any top plate contacts were connected to all the other 'cathodes'.

My initial power-on will illuminate only the three glowing lines that separate the three register values, and the three text blocks highlighting VERB, NOUN and PROG.

The output was hooked to a scope where I could validate both frequency and voltage level. As soon as that was good, I delivered the AC to the initial segments (lines and text blocks) to see whether the panel would glow.

A second phase of testing would statically route AC to a chosen set of segments for digits and signs and other features, to be sure that the crack in the top glass layer has not compromised the function of the panel.

CONNECTOR ISSUES

The modules and the EL panel all use a special connector that was used in the Apollo program and a few other aerospace applications for a short period of time in the 1960s to early 1970s. The company who made them is Malco and the connector is a Mini-Wasp. They are essentially unobtainium, although Marc's company built a run of these for our use while restoring the AGC.

Power supply module with 12 MW male connectors

Relay module with 137 male MW connectors

Apollo EL panel with 168 male MW connectors on rear

Given how precious they are, I won't use them until a final 'production quality' connector is ready for long term use. In the interim I have found some flimsy connectors that I can press into service for testing and temporary connections.

makeshift connector for testing and prototyping

Rectangular cross section similar to shape of male MW pin

EXAMPLE CIRCUITRY FROM RELAY MODULE

Relay contacts to control the sign segments

The relay contact DS1B1 in the top picture is hooked to the B segment of the middle picture. The relay contact BS1A1 in the top picture is hooked to both A and C segments of the middle picture. These relays are controlled by selecting rows 1 or 2, then powering the SET line to latch the relay to power the segment(s) or the UNSET line to switch them off.