Gear clock using PIC 16f628A

  
The heart of the clock is a PIC 16f628A microcontroller (PDF). This microcontroller has an internal oscillator however an external 20MHz crystal oscillator is being used since it will have to accurately keep track of time for weeks and months. The microcontroller is interfaced to two buttons and one motor. The interface is very simple, it consists of two buttons. When the left button is pressed the clock advances time using the motor. When the right button is pressed the clock decrements time using the motor. The only issue is when you need to correct time by many hours you would have to keep the button pressed for a long time.
Gear clock using PIC 16f628A - schematic

The stepper motor is also always energized to prevent the gears from slipping. To overcome this issue when both buttons are pressed the stepper motor is deenergized and the minute gear can be spun freely. The motor is a unipolar stepper motor that has been harvested from an old 5 1/4 inch floppy drive. This is the motor that used to move the read write heads back and forth, to get one of this size and power you’ll need to find a nice old one. There are many sizes and styles of steppers so it may take a bit of digging to find a usable one. Modern floppy drives don’t have steppers with this level of torque. This motor moves 1.8 degrees per pulse which means that with 200 pulses it will make one full rotation. Since it’s a unipolar motor it is simple for the PIC to drive it with only 4 transistors. You will also need to be aware of motors with different number of pulses per rotation however 200 is the most common. The gears are made out of MDF. They were painted to have a metallic look however the look I was going for was not achieved. Initially I was thinking of making the gears look like they were made of metal and left to rust for a few dozen years. I found some cool products that would give me that rusted effect but they were a bit too expensive. I settled for a can of Krylon Black Metallic Hammered Finish paint. The sample on the lid is a very nice black with subtle bit of gray. I think this might be from a bad batch...



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