Quasar 3073, Manual

QUASAR PROJECT KIT # 3073 - 12/24 HOUR GIANT CLOCK

This project was originally published in the electronics 
magazine, 

Silicon Chip

, a few years ago. It is issued 

here as a kit with permission. Some modifications to 
the original published circuit and software have been 
made and are are mentioned at the end of the 
documentation. 
 
The Big PIC Clock is based on a PIC16F84 
microcontroller (uC.) The software in the uC allows for 
very accurate timekeeping. For visibility at a distance it 
uses 2.3”/57mm super large, hi-efficiency red LED 
displays for the hours and minutes and smaller 0.56” 
LED displays for the seconds. The circuit incorporates 
a dimming feature so that at night the reduced ambient 
light is sensed and the display is dimmed. Before we 
construct the clock please read through the following 
theory about clock adjustment and setting and the 
general circuit description. 
 

Accuracy Theory and Adjustment.

 The clock may be 

adjusted for very good long-term accuracy. All crystal-
based clocks have a tendency to run fast or slow. Some 
use a trimmer on the crystal for adjustment but they 
will still drift due to temperature changes over time. 
However, in this design an accurate adjustment feature 
has been built into the software. Carefully adjusted you 
should be able to get the clock to keep time to within a 
few seconds a year. The adjustment technique requires 
you to correctly set the clock and wait a few days to 
see how accurate it is. Then an adjustment mode is 
selected and the number of seconds the clock differs 
from the correct time calculated over a period of 60 
days is entered in.  
 
However, it is not necessary to wait 60 days. In the 
beginning a day or so is long enough to get a good idea 
how fast or slow the clock is running. The only 
requirement is that you calculate the number of 
seconds it would gain or lose in 60 days. Of course the 
more days you wait the greater the accuracy of the 
adjustment 
 
After entering the adjustment figure the clock then 
maintains time by slightly adjusting the length of a 
second every so often. If the crystal was running slow 
then there will be an occasional shorter second 
(999msec) to speed up the clock. Conversely, if the 
clock was running fast there will be an occasional 
longer second (1.001 sec) to slow it down.  
 
In software the adjustment figure of seconds per 60 
days is divided by 10,368 to obtain a reference counter 
value. For example, if the adjustment figure is 60 (1 
second per day) then the reference value will be 
10,368/60 or 172. This value is compared with a 
second counter which is increased once every 
500msec. When the second counter value reaches the 
value of the reference counter, the second counter is 
altered by 1msec. The second counter is then reset to 
count up again.  
 
For our example value, the second counter will reach 
172 after 500 x 172msec, or 86,400msec. Therefore, a 

correction of 1msec is made every 86,400 msec which 
is equal to 1 second per day. Thus there will be 1000 
corrections per day. (One day has 86,400 seconds.) 
The number of seconds per 60 days adjustment figure 
requires a positive or negative sign to indicate whether 
the clock needs to use slow seconds or long seconds. A 
minus means the clock is slow and needs speeding up. 
A plus (no sign) means the clock is fast and will need 
to be slowed. The adjustment range is from 0 to  –255 
and from 0 to 255 seconds per 60 days with 1 second 
per 60 days resolution. This corresponds to 0ppm 
through to +/-50ppm adjustment with just under 
0.2ppm steps. 
 
The time adjustment is initiated by pressing both the 
hatkey switches (one is for hour adjustment and the 
other for minutes) simultaneously. The seconds display 
will show an ‘Ad’. Upon release the current adjustment 
figure will be shown. You can increase it by pressing 
the hour switch and decrease it by pressing the minute 
switch. If the number goes below zero the minus sign 
will appear and these 

negative

 numbers are used when 

the clock is running 

slow

. The 

positive

 numbers are for 

fast

 clocks. 

 
Press both switches simultaneously to return to the 
clock. The time will need to be reset correctly. The 
adjustment number  is stored in memory and will be 
retained unless changed by entering the ‘Ad’ mode 
again and changing.  
 
Example: if you find the clock is 1 second fast every 
60 days you need to add +1 to the current adjustment 
figure. Thus if the current adjustment figure is  –
35sec/60 day correction then it must be changed to  –
34. Or if the current value was 38 then it must be 
increased to 39. 
 

Clock Setting.

 Use the minutes switch to go to the 

correct hour and minute with the seconds at ‘00’  You 
just wait until a reference clock (TV, radio) begins the 
next minute. Pressing the minute switch will go to the 
next minute, reset the seconds to ‘00’ and thus enable 
the clock to be set accurately to about 1/10

th

 second. 

Daylight Saving.

 This is easy to do and non-

disruptive. To advance the clock just press the hour 
switch once. To retard the clock one hour just press it 
23 times. That is, keep it depressed until the previous 
hour comes up. In both cases the minutes and seconds 
are unaffected. 
 

12 or 24 Hour Display.

 You may toggle between each 

display mode by depressing the hour switch when you 
apply power. In 12 hour mode am or pm (your 
decision)  is indicated by the decimal point LED of the 
leftmost display coming on. On initial power-up the 
clock is in 24 hour mode. 
 

Dimming.

  This is under software control. The duty 

cycle for the multiplexed signals to all 6 displays is 
varied. In a multiplexed display only one digit is on at 
any one time but the displays are cycled on/off at such