I've wanted a Reservoir Watch from the minute I first saw one. I absolutely loved the way that the minute hand spun back from the sixty minute position at the top of the hour. As a humble electronics engineer, I still can't quite afford one yet. So I got around to thinking, could I make one myself? Initially I thought I could make one with a 270 degree servo, looking at them, it seemed as though they were modified 180 degree units made to travel further - not always the full 270 degrees. I then wondered about using a DC motor with a position sensor on it but these seemed expensive, were larger and heavier than I would have liked. I decided to use an inexpensive geared servo motor that I could drive at 5V. I built a prototype and left it running for a day or two. This showed that although the principle was good, the "steps" on the stepper motor were not always perfect. I then placed some pins at the extreme travels to detect when the hand has reached either end. That means that every hour, when the hand resets to zero minutes, the motor is automatically synchronized to the hand movement. It also means that should the hand be accidentally nudged away from the correct time (forwards or backwards) the hand will automatically be corrected either once the hand reaches the sixty minute end early, or when the minutes move back to zero. The three PCBs that make up the clock were then designed. One PCB is used as the minute hand. This has solder points on it to allow a loop of wire to be fitted - this forms one half of the two "limit" switch contacts. One PCB carries all the electronics. This has the LED display for the hours fitted and is mounted behind the clock face, using some spacers These spacers, along with the feet at the bottom of the clock face form a stand to allow the clock to stand upright. The clock face carries the limit switch "pins" at the zero and sixty minute points. It also has a grid section of 0.5mm holes that form a display filter for the hours LED display, this disguises the unlit LED display elements. I didn't want to use a traditional red perspex display filter as these are difficult to machine manually when trying to make them fit flush with the front of the PCB. To my eye, the "grid filter" looks neater and could be made for me by the PCB fabrication house. I have to say, I was very happy with how the clock looked and it now has pride of place next to the television in the living room, I never stop enjoying watching the hand spin back to zero and the hour changing at the start of the new hour. The video below should demonstrate the last few seconds of 1pm, the hand rotates anticlockwise back to zero minutes - ending at zero just as 2pm starts.
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