26-09-2013, 03:50 PM
Propeller Clock" Mechanically Scanned LED Clock
Clock%22 Mechanically.docx (Size: 220.28 KB / Downloads: 20)
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.
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.