Monday, June 30, 2014

Clock Modification

The unfinished clock without the arms
For Christmas 2012 I got a battery powered alarm clock. It felt pretty sturdy but only plays a beep when the alarm goes off and it does not have a backlight. There was plenty of room inside the case so I decided to add a microcontroller to play the unofficial national anthem of Kyrgyzstan Кыргыз Жери (Kyrgyz Jeri) instead of just the beep. Although I have been working on it on and off for the past few months I have not gotten the bugs worked out.

The mechanism inside the clock runs from one AA battery and has a small buzzer that looks like a piezo. The positive side of the battery is connected directly to the buzzer by a wire. The other side of the buzzer is connected to a chip under an epoxy blob that switches the buzzer on and off. The 1.5v of the AA battery is not enough to run the MSP430 I am using so I wired in two AAA batteries. The clock case has just enough room for the extra batteries and a small piece of protoboard for the microcontroller. I opened the mechanism to solder in some wires so that the microcontroller can monitor the line switching the buzzer and drive the buzzer directly instead. One of the small gears inside quickly melted from the heat given off by the soldering iron a few inches away. Luckily, all of the local clocks use a similar mechanism and I replaced the whole thing for about $1. The wires from the mechanism are connected to a switch so that the buzzer can either be run by the microcontroller or by the original clock in case the extra batteries die. Another feature the clock is missing is a light, so I wired in an RGB LED I bought a few years ago. It has a light so that it can be used as a backlight but it also flashes colors in time with the music when the alarm goes off.

At first I tried soldering components directly to a DIP socket but this was surprisingly difficult, so I added everything to a small piece of left of protoboard. The AAA batteries are too big to fit in the clock case side by side so I couldn't use a battery holder. Instead I soldered wires directly to the batteries. This will make them hard to change but hopefully the MSP430 will use very little power waiting for the alarm to go off. Once I got everything soldered, the microcontroller would play the music it should but it would not stop playing even when it was disconnected from the line switching the signal. This is a problem I still have not figured out but I'm sure it has something to do with fluctuating signals in the circuit.

There were a number of other strange problems as well. The P1IES register used to enable edge selects for interrupts on the MSP430 sometimes gives the wrong value when you read from it. It would be useful to read it in an interrupt to know which phase of the cycle the interrupt was last configured to fire for. There was also a problem working with 32-bit values. It seems that doing math with constants and assigning the result to an unsigned long does not automatically promote the constants involved to 32-bit values as well. When I got the wrong value, I tried it out on a PC and got the expected value, which was a little perplexing. It seems that constants default to the bit size of the processor, which seems silly from a portability point of view. To have the constants treated as longs, the L prefix has to be added. Another strange thing that happened was that the buzzer always made a high-pitched whining even when not being driven. The batteries for the microcontroller also drained alarmingly fast. It turns out that both of those problems are related to the buzzer being electromechanical instead of a piezo. These buzzers do not charge and stop current from flowing like a piezo. It seems they only have about 40 ohms of resistance and this is why the batteries drained so quickly.

There are still a lot of bugs to be worked out but I hope to make progress soon and finish this little project quickly.

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