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Mecanical scan led
The Propeller, a mechanically scanned LED clock by Bob Blick.

The clock is on a spinning piece of perfboard, but it must get power. I thought of many ways to do this, including using two motors(motor one has its shaft fixed to a base, and motor two spins the body of motor one, generating electricity), making a rotary transformer, or using slip rings.

I decided to do it another way, taking power from the spinning armature of a plain DC motor. In order to run the wires out of the motor, I removed the bearing from one end of the motor, leaving a big hole.

There are three terminals inside most small DC motors, and it acts a lot like three-phase alternating current, so it must be rectified back to DC. A nice side effect of this is that the position of the motor can be detected by taking one of the phases straight into the microprocessor.

Step One: Mangle a Motor.

Find a VCR, perhaps a Sharp or a Samsung, with a flat reel motor. The motor I have is marked JPA1B01, but Sharp knows it by the number RMOTV1007GEZZ. Take it apart without mangling the brushes(there are little holes to slip a paperclip into to move the brushes out of the way), and notice that it has one ball bearing and one sleeve bearing. Knock the sleeve bearing out of the case and glue or solder it to the other end of the motor, as an extension of the ball bearing. The shaft of the motor will have to be repositioned slightly to get the right height, press it in a vise with a hollow spacer on one end. Take a Berg connector with three wires and solder them to the three terminals on the motor's armature. Glue a short threaded spacer to the shaft at the end that will stick out the hole, and reassemble the motor(be careful with the brushes). You can glue the motor to a VCR head as a weighted base.

Step Two: Build the circuit.

I used perfboard(Vectorboard) and handwired the circuit together. Use an 18-pin socket for the 16C84 because it needs to be programmed before putting it in the circuit. For the 7 current-limit resistors I used a DIP resistor array, because it made it easy to experiment with LED brightness. I settled on 120 ohms. You can use seven regular resistors, because 120 ohms works fine, though it puts the peak current right at the limit for the 16C84. Think about balance while you build this circuit, and reference my pictures, so you don't have to add a lot of balancing weight later. Substitute for any part values you like. Note that I used a 47000uf supercap, it is to keep the clock running after turning it off, so you can set the time. The LEDs get power separate from this. Don't substitute a ceramic resonator for the 4MHz crystal, this is a clock and should be accurate.

Step Three: Program the 16C84.

You'll need a programmer that will program a PIC16C84. If you found this file/web page, you can find plans to build a 16C84 programmer. Program it using the hex file accompanying this document. I have included the source code(.asm) just for your amusement. When programming the chip, set the chip options to: watchdog timer(WDT) OFF and oscillator to normal XT crystal.

Step Four: Throw It Together and Keep Time.

Screw the circuit board to the motor, and plug the three wire connector in. Apply power to the motor. The preferred voltage is 6.2 volts, but it will run from 5 volts to about 7.5 volts. Note that 5 volts gets to the circuit when 6.2 volts is applied to the motor, because of diode losses. The clock may be working at this point, displaying 12:00. If it isn't. there was probably some voltage on the supercap when you plugged in the chip. Turn off the power and momentarily short pins 5 and 4 together(ground and /mclr) to reset the chip. Now when you apply power the clock should work, and you can set it by turning off the power and pushing the buttons(hours, 10 minutes, minutes) the right number of times. If the numbers appear backwards, reverse the polarity to the motor to make it spin the other way. You might experiment with balancing the clock, and the use of foam under the base to reduce vibration.

Step Five: Modifications.

If you look closely at the source code, you'll see that the "dot rate" is adjusted to the speed of the motor to make the display a consistent width regardless of the motor's speed. The motor I used has brushes set 90 degrees apart, and gives two indexes each revolution. The clock displays on two sides, 180 degrees apart. If you use a motor with the brushes 180 apart, the clock will only display on one side, and the numbers will be too wide. You'll want to modify the program, in the section marked D_lookup_3. The value in the W register when Delay gets called effects the width of the digits. You might try sending half of the period_calc value to Delay, perhaps by rotating period_calc right into W(remember to clear the carry flag first). Like this:
rrf period_calc,w
call Delay