The Propeller Clock is a mechanically scanned LED clock. Seven light emitting diodes spin, giving the illusion of numbers in the air.
This is the original open-source persistence-of-vision display. I was in the middle of a tough job and I made the Propeller Clock to keep myself from working during a holiday. I was teaching myself PIC assembly language and this was my second program.
How this clock works:
A motor spins the “propeller”, and a small microprocessor keeps track of time and changes the pattern on seven LEDs with exact timing to simulate a 7 by 30 array of LEDs. It is an illusion, but it works nicely.
If you want to build this clock, you will need a few things, including:
Skill with motors and mechanical things.
Prior electronic experience.
A dead VCR or floppy drive or other source of a suitable motor and miscellaneous parts.
A programmer that will program a PIC16C84 or 16F84 microprocessor.
The Propeller Clock Parts List
C1, C2 – 33pf ceramic
C3, C6 – 0.1uf ceramic
C4 – 47uf electrolytic
C5 – 47,000uf supercap(memory cap)
D1-D7 – light emitting diodes
D8-16 – 1N4001 general purpose 1 amp rectifiers
R1 – 120 ohm DIP array or seven 120 ohm resistors
R2-R6 – 10k ohm
J1 – three terminal Berg connector
SW1-SW3 – normally open pushbutton switches
U1 – PIC16C84 programmed with mclock code
XTAL1 – 4MHz crystal
MOTOR – Sharp RMOTV1007GEZZ
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 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) ON 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:
mclock.txt A full description how to build it.
mclkpart.txt The parts list.
mclock8.asm The source code in Microchip MPASM format.
mclock8.hex The hex code ready to load into a PIC16C84 or 16F84 chip.
mclksch2.gif A large and very readable schematic diagram.
mclkmoto.gif A drawing of the modifications to the motor.
If you can’t get that kind of motor, you can use the motor from an old disk drive. If you use a disk drive motor or any other DC motor with brushes 180 degrees apart, you should use the slightly revised code or the numbers will be a very wide:
mclockt3.asm The source code.
mclockt3.hex The hex code ready to put in a chip.
If you don’t have any way to put the program into a PIC 16C84 or 16F84 chip, you can build your own programmer.
Printed Circuit Board Layout for a version of the clock.
The hardware design and software are covered under the GNU General Public License.
Frequently Asked Questions:
Q: What’s this .047 FARAD capacitor, don’t you mean microfarad?
A: You can get supercapacitors at many places, including Jameco Electronics.
Q: What’s a Berg connector?
A: It’s a name for a generic connector that fits onto 0.025″ square posts on 0.1″ centers, like the connectors found in computer cases for the lights and switches.
Q: I saw a big version of this at a store but it was different. Have you seen it?
A: Yes, I’ve seen a lot of “persistence of vision” clocks in recent years. I made this one in 1996, so I think they all copied me :)
Q: My VCR head motor has lots of wires. Which ones do I use?
A: I didn’t use a head motor. I used a reel motor. It only has 2 wires and runs on DC.
Originally published January 25, 1997