The Digi-Log Clock
place. Photo 6 shows the mounting
screws holding the circuit board in
place. The wires from the display
come into the base from the middle
spacer, exit under the circuit board,
and come around its edge. Some of
this can be seen in the photo.
There are several notes concerning Photo 6. The three large black
objects with white stripes are connectors. If you look closely, you can see
the wire-wrap wires connecting to
them. This made it easy to disconnect
the circuit board from the display during de-bugging. The power supply is at
the top with the power jack coming
out the back of the base just below
this. At the bottom, there are two
push-button switches (SW1 and SW2)
that are used for setting the time. Not
seen are the LED current limiting
resistors (R4-R10). These are mounted
on the bottom of the circuit board
because it made things easier for me.
■ PHOTO 6. Final assembly of the clock.
The two small switches at the bottom
are used to set the hours and minutes.
Connectors were used for the display
wires for easier troubleshooting.
setting of hours and minutes.
The accuracy of the clock
depends upon the accuracy of the
watch crystal. These are typically tun-ing-fork types with a basic tolerance
of 20 PPM (Parts Per Million). Since
there are about 2. 6 million seconds
per month, an error of 20 PPM
converts to an error of about eight
seconds per month, worst case.
It has been noted that the display
looks similar to the planetary motions
of the solar system. You could design
a clock that shows the relative movements of the planets by having each
ring represent a planet’s orbit and use
separate timers for each ring/orbit.
You could also get fancy and show the
absolute positions of the planets by
adding a new display for the year,
month, and day. In this way, you could
pick a date and see exactly where the
planets would be in the sky at that
time (in the past or the future).
Operation is simple and straightforward. Apply power to the clock.
After about a second, the clock
should display 12:00 (12:00:00 if seconds are used) and should immediately operate. Pressing the Set Minute
switch (SW1) will increment the minutes display every time it is pressed. At
worst case, you will have to press it 59
times to set it to the proper minute.
Holding it down does NOT continuously increment the display (but that is
a software upgrade you could write!)
If the minutes display passes through
the hour ( 59 minutes to 00 minutes),
the hour display is NOT incremented
(as it would in normal operation).
Setting the hour is accomplished by
pressing the Set Hour switch (SW2) in
a similar manner. Each switch press
increments the hour display. There is
no provision for setting the seconds.
However, since the time-keeping continues during the setting of the clock,
the seconds can be “set” by applying
power exactly on the minute (because
the clock power on reset initializes
itself to exactly 12:00:00). The
seconds are not affected during the
A different type of clock has
been designed that integrates digital
and analog concepts. It’s simple
to build and operate. Additionally,
there are many different ways to
customize the clock to match your
RESISTORS (1/4 watt 5%)
❑ R1-R3 10K ohms
❑ R4-R10 100 to 1000 ohms (see text)
CAPACITORS ( 25 volts minimum)
❑ C1, C2 33 pF
❑ C3 220 µF
❑ C4, C5 0.1 µF
❑ D1-D44 LED (see text)
❑ D45 Center LED bi-color (see text)
❑ D46 Bridge rectifier 1 amp 50 volts
❑ U1 PIC16C73A microcontroller
❑ U2 78L05 low power five volt regulator
❑ SW1, SW2
32,768 Hz watch crystal
Normally open push-button switch (SPST)
DC power jack
February 2007 45