Quasar 3073, Manual

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

the displays. This again divided by 500 to obtain a 2Hz 
signal which is used to flash the colon on/off. The 
seconds display is updated on every second 2Hz signal, 
that is, 1Hz. 
 
The RA4 pin on IC1 is set as an output and is used to 
discharge the 1n5 capacitor via the 470R resistor. 
When RA4 is taken high, its output is open-circuit and 
the capacitor charges via the 2K2 resistor and the LDR. 
As mentioned above the capacitor charges faster when 
the LDR is in bright light (low resistance) and slower 
when the LDR is in dull light.The charge time is 
monitored by RA4 and used to control the display 
dimming. 
 
The RB0-RB7 outputs of IC1 drive transistors Q1-Q8 
via a 470R base resistor. When the outputs are low, the 
transistors are switched on to drive the segments in the 
six displays. The segmenta are driven via 82R resistors 

while the decimal points are driven via a 180R resistor. 
The 0.56” display segments are driven by 220R 
resistors. 
 
Different feed resistors are used because the 2.3” 
displays have 4 series LEDs per segment and two 
series  LEDs in the decimal points. The 0.56” displays 
only have one LED per segment. 
 

Upside-down Displays.

 Normally with multiplexed 

displays such as this the same segments for each digit 
are connected in parallel. This circuit, however, is 
different. Two displays are mounted upside down. This 
has been done to obtain a colon between the hours and 
minutes and to obtain the am indicator. 
 
The common cathode connections to each display are 
driven by IC3, a ULN2003A 7-transistor array. IC3 is 
driven via IC2, a 4051 8-channel analog switch or 
demultiplexer. In this circuit it has two roles. Firstly, it 
acts as a decoder which converts the binary signals on 
its three input lines (A, B, C) to drive six outputs, one 
for each common cathode LED display. Secondly, it 
provides logic level (voltage) translation, changing the 
5V signals on its inputs to 12Vsignals to drive IC3. IC2 
can do this because it has three supply connections: the 
Vdd pin (16) connects to +12V, the Vss pin (8) 

connects to the  –5V from REG1 (ie, 5V below  +12V 
supply) and the Vee pin (7) connects to 0V. 
 
As well as acting as the B & C outputs to IC2, pins 17 
& 18 of IC1 are monitored via diodes D5 & D6 which 
connect to the Minutes and Hours switches resp.The 
other side of the switches both connect to the RA3 
input (pin 2) of IC1. Normally, pin 2 is held low via the 
10K resistor to pin 5. However, if a switch is pressed 
and the B or C line driving the switch is high, the RA3 
input will also be pulled high. This signals to IC1 that 
the switch is pressed. IC1 can determine which switch 
is pressed because it ‘knows’ which line (B or C) is 
high at the time. 

Construction 

There are two PCBs. Assemble the components onto 
each circuit board according to the overlay. There are 
several points to note: 
 
- Links. On 3073B use the four zero ohm resistors 
provided. There are 4 on 3073B, not three as I wrote on 
the overlay! On 3073A use the tinned copper wire and 
cutoff’s from the resistor legs to make the links.  
 
- mount the two 0.56” displays using half of the 20 pin 
IC socket.  
 
- two of the 2.3” displays are mounted upside down. 
 
- mount the LDR so its top face is level with the 
displays. 
 
If you are going to mount the Clock in a case decide if 
you want the two switches S1 and S2 to be higher than 
they are. 
 

Connecting the Boards.

 X1, X2 and X3 on 3073B take 

the three 

female

  8-pin connectors. Solder them into 

place.  

 
 
The three, 

male

 8-pin connectors X4, X5 and X6 are to 

be soldered on the underside of 3073A so that, after 
soldering the two boards will fit together both 
mechanically and electrically. Note that the connectors