Software keyer

    Pi exercise 2 - failure to success: For my second Pi exercise, which dealt with signal-level conversion (hardware) and interrupt processing (software), I used a Pi 3 Model B. While Arduino digital I/O pins are 5V (TTL level), Raspberry Pi pins are 3.3V.  In order to function as a TTL input a GPIO 3.3V signal needs to be raised to 5V.  For this I used a TTL level shifter from MCM Electronics. However, it would also be possible to make the required voltage level change using only general purpose components.

     The application itself was a software keyer. It was supposed to make either dots or dashes on touching one of two capacitive sensors (see photo). In other words the program would be an “event driven” application. Sometimes when I touched one of the sensors a stream of dots or dashes would be produced, but other times nothing happened. Once in a while, either a dot or a dash would stick in the on state. My first thought was that input pins were not reliably firing or that corresponding interrupts were not being caught reliably. Both these guesses were wrong.

    In the back of my mind the program’s structure was suspect, as
the event handlers invoked code to produce dots and dashes directly. As a final try I reorganized the program so that event handlers only set or reset global flags indicating to start or stop a dot or dash stream and nothing else.  The part of the revised program that produces dots, dashes, and characters (the main part) executes a continuous loop. Moreover, the revised program runs ‘nice-ly, relinquishing time when idle.

    Transfer to PCB: Adafruit has a half-size prototyping PCB kit that comes with a socket. I figured it would be just barely possible to fit the Python keyer’s GPIO interface components on the small proto board and although I will never use this keyer I wanted to do this—well, I already had a 26 pin cable that mates with the proto board socket, and thought I could plug the other end into the Pi’s 40-pin socket by sanding a little bit off the plug end. This much worked, but I hadn’t noticed that the ends of the ribbon cable plug are what hold it together!

    When the sanded end came apart I pushed the top of the plug back on and rechecked the continuity of each pin with an ohmmeter. Then I Epoxed the housing and made a mental note not to pull on the cable while disconnecting the plug
—of course, one should not do this even with an unmodified plug, but ...

    There was no room for the touch sensors on the half-size board and, in any case, they were not relevant to the ideas being tested. In place of on-board sensors the small PCB has headers for attaching either a paddle or other accessory.

    The test rig for the Pi software keyer involves so many interconnected pieces as to call into question the sanity of whoever thought it up:

There must be a better way! 

    Demo: python-keyer-revised.mp4

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