Thursday, July 3, 2014

Quiz Buzzer

About a month ago a friend of mine called and asked if I could make a buzzer system for a quiz game for students. Unfortunately, I only had two days until the competition and I did not get it working in time. Hopefully, I can fix it and send it to them to be used at the next competition in a few months.

The plan was that each of ten students would have a button connected to a buzzer by a few meters of wire and the first to press the button after a question was asked would cause a sound and light to go off. The connectors are RCA style and the buttons are from an automotive store.

These lead to an A42 enclosure I was able to sample from Serpac. The enclosure felt really sturdy but the plastic was surprisingly easy to cut and work with with just a knife. I will definitely use them again when I need an enclosure. The circuit itself was fairly simple with just an MSP430 microcontroller, buzzer, two 595 shift registers, and two 165 shift registers. I did not get around to adding the LEDs, although the firmware does use the shift registers to turn on the correct light when someone buzzes in.

To control things I made a separate detachable board with a 16x2 LCD. It will be easier to transport if the cable is detachable and it will also let me add a longer cable if the judges doing the scoring need to sit farther away from the enclosure. Other than the LCD, the board has a 555 to generate negative voltage for the LCD contrast and another 595 shift register to run the LCD. It also has buttons to change the time given to answer and to reset the counter.

For the cable I used Cat-5 again and this may be part of the reason it does not work correctly. For the connector I wanted to make something that would only plug in one way so that people other than me could use it without fear of plugging it in backwards and shorting something. Each side has a pin that can only be connected one way.

A project like this would take me a week at least but I tried to finish in two days because my friends needed it. The night before I stayed up soldering until 7am and when I finally switched it on, the LCD came on as expected and the buzzer sounded when I pressed one of the contestants buttons, but it made a high pitched screeching noise when it was finished buzzing. When I turned it off and tried again the LCD wouldn't display anything and only worked some of the time when I tried it again, so we weren't able to use it in the competition after all.

Why did it fail? In retrospect, I have had problems with this particular LCD in the past. When I was working on my calculator project I noticed that the LCD would often not work when being driven directly by a microcontroller but worked fine when being driven from a shift register like in this project. I'm not sure why it worked before and not now but the fact that it was flaky before might be a clue to my problem. Also, maybe my assumption that it will always work at voltages less than 5v is not correct or maybe supplying negative voltage to the display somehow has consequences. On the other hand, I observed the same faulty behavior with the contrast left unconnected. Another source of problems could be the Cat-5 Ethernet cable I am using for wiring. Each pair is twisted together and running a separate signal through each side of the pair instead of grounding one side might be leading to signal corruption. Finally, the loud screeching of the piezo buzzer is something I will have to figure out as well. Hopefully, when I understand more about this it will help me straighten things out with my clock project also.

Going from nothing to a nearly complete project in only two days was a lot of work, but in the end there was not enough time to troubleshoot problems. When I move into my new apartment next month I will have time to figure everything out and then I can send the working system back to Kyrgyzstan for my friends to use.

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