BUILDING DSKY SUBSTITUTE DISPLAY PCB
Finalizing tests and debugging
I have the modification in place that should be firing the relay to switch on 5V to the line and legend LEDs, but nothing is happening. I verified that the sense circuit is delivering 5V or 0 depending on whether the 14V is connected. However, the relay isn't switching. This requires some debugging.
Looking closely at the driver circuit showed an unconnected transistor lead. With it installed and the code in my Arduino software to echo the inverted state of the 14V sense to the driver, it does indeed blank and unblank the lines and legends.
A second problem I detected was that the Opr Err LEDs are not lighting during the code where I tried to light every lamp. This also requires some debugging. The signal comes from IC5, pin P6, having been set by the startup logic, so I have to suspect a wiring error or soldering problems. I reflowed the solder on IC5 and the problem is solved.
Fine tuning the display position and operation
I set the display board inside the aluminum case to verify that it fits well. I did discover that the right side display, the registers, verb, noun, program and related lighting, fit well vertically inside the opening but seems to be offset to the left a bit too much. That puts the sign (plus/minus) very near to the left edge of the opening.
We looked at methods to even out the lighting behind the legends and caution/warning sections, as well as the horizontal lines and the signs. We determined that the spacing between the LEDs is a bit too wide to fully even out with diffusers, so I will have to live with multiple perceptible light sources.
A sheet of paper at the level of the front panel does a decent job of evening out the light for the caution/warning lights. It will probably suffice for the legends (PROG, VERB and NOUN).
However, the horizontal lines will appear to be dotted lines and the four segments of a plus sign will be noticeably distinct. Correcting this would be far more effort that is warranted for the goals of this project, which is to provide a substitute for a DSKY, not an exact replica.