Thursday, December 13, 2018

Almost completely done with display board for DSKY substitute


Installing components and testing by stages

My first task was rethinking the circuitry to switch 5V on to the illuminated lines and legends of the right side of the display when 14V is present (meaning the AGC is not in standby). The current method doesn't work because I messed up the relay driver. The driver fires when its input is at ground but the input will sit at 5V while the AGC is active.

Fortunately, I have the 5V produced by my AGC BPLUSSW sensing circuit routed to the Arduino, thus it would be a simple matter to invert the state and generate an output on an unused pin. I don't have any routing from the unassigned pins, but I can tack a wire onto the back of the connector and hook that to the input resistor of my relay driver.

Of course, I would have to cut the trace to the input resistor to isolate it from the sensing circuit to which it was originally wired. Since this trace is easily accessible on an outer face of the board, I was able to accomplish the cut easily.

In the midst of checking out what was needed, I noticed that my switched 5V line gets some power when the 7 segment displays are active - not through the switching relay and not a full 5V, indicating that I have a short somewhere between the switched 5V net and one of the display segment lines.

I have to find and correct the short before I power up the switched 5V line, or it will also jam on one of the segments for a register, wherever the short currently is. I used the DesignSpark tool to highlight the 5V net and checked each spot where it had a via near a display pin. That didn't help, so I then did a continuity test of the switched 5V net to each pin of the 7-segment displays.

I quickly discovered that the short is to pin 4, segment e (which adds to a 9 to form an 8) of chip 2 covering Register 2 and Noun. I then highlighted where this net passed in the hope that I could find a correctable short.

I discovered it, with a untented via that was extremely close to the pad of a 0603 LED. After I cleared the bridge of solder, quite invisible to the naked eye if you didn't expect a tiny via under it, the short was gone.

The white and green LEDs arrived this morning, which allowed me to add them all to the board. That leaves only two categories of component to add to the board - headers and limiting resistors. I had to add the headers and install the Arduino Mega 2560 now, which I will then use for the remainder of the section tests.

With the Arduino in place and a modified version of the DSKY software installed (mainly to generate some display activity without requiring AGC input) I fired up the board. It worked properly as far as I could tell right now.

Keeping the display active, it will be possible to select limit resistors for the LEDs forming the lines and legends that are placed around the digit displays. To energize them, I had to fudge a connection to the switched 5V power plane because my sensing circuit is not working properly.

Once I felt that these LEDs glow at the same perceptible brightness to the digits and signs. I will solder down the resistors if I have them in stock. I chose 680 ohms for both the lines and the legend fields.

The next set of limit resistors to select were for the Comp Acty light that sits above the Noun digits. I also chose 680 ohm for these and soldered them down.

The caution lights shine white and the warning glow yellow. These are adjusted relative to each other and to the digit displays. Once I set the values for a white and a yellow (actually a pair of LEDs for each position, meaning two resistors per position),

The white were much brighter than the other LEDs, so I initially used 3K resistors for the white while the yellow worked fine with 680 ohm. However, I am still not satisfied so I will have to try a higher resistance until the brightness is similar to the other LEDs.

I did run out and buy the 680 and 3K resistors to complete the board, knowing I will replace the white LED resistors later. I still have to do the rework to fix the 14V sense circuit that lights the lines and legends.

Front of the board, almost ready to install

Read of the board with Arduino installed

No comments:

Post a Comment