five wires are needed for the rings
(three if seconds are not used) and 12
wires are needed for the spokes. Only
17 lines are needed for 44 LEDs. (It is
possible to save two wires by connecting the “ 10” and “ 11” o’clock hour
LEDs to unused positions of the tens of
minutes ring. But it would require spe-cial-case software and would be more
difficult to implement. Since the I/O
lines are available, simplicity is chosen.)
The electronics are really very easy.
I used a Microchip PIC16C73A for the
µP because I had it on hand. The software should run on virtually any other
PIC with little or no modification
(source code available at the Nuts &
Volts website). The software is trivial,
consisting of only about 200 lines of
code. It’s a bare-bones clock. Feel free
to add more features like an alarm,
blinking display, or whatever you want.
Figure 1 shows the schematic diagram. As you can see, it’s basically just
LEDS and a µP. Most any AC adapter
ranging from 7 to 12 volts AC or DC
of either polarity can be used. (I hate
searching for a specific wall-wart.) The
bridge rectifier (D45) and the large filter capacitor (C3) provide a positive
DC voltage to the low-power five-volt
regulator IC (U2). This powers the
circuit which only needs 10-20 mA for
normal operation. The 32,768 Hz
watch-crystal and two associated
BY GERARD FONTE
capacitors (C1 and C2) provide the
precise timing necessary for a clock.
Switch SW1 is used to increment the
minutes when setting the clock.
Switch SW2 is used to increment the
hours when setting the clock.
The resistor values for the LEDs are
not specified because they depend on
the brightness of LEDs you use. I used
really old and inefficient diffused LEDs
so R4 through R8 were 100 ohms. If
you use high-brightness LEDs, you may
choose to increase these resistors to
510 or even 1,000 ohms. This is especially true if you use high-brightness,
point-source LEDs. Note that the
clock-display LEDs have a 20% duty
cycle and the center LED has a 100%
duty cycle (50% red and 50% green).
■ FIGURE 1. The clock design is
very simple consisting mostly of
a microcontroller and LEDs. The
tens of seconds ring and the ones
of seconds rings (D29-D44) are
optional and can be omitted if
you only want to display hours
and minutes (like this project).
February 2007 43