time the counter is incremented, the next digit is
enabled. The multiplexing works as follows: First,
the microcontroller resets the decade counter to
enable digit DISP1. Then, data is output to the
seven-segment lines to turn on the appropriate
LED segments in the first display. The LED
segments in DISP1 are left on for only two
milliseconds. Next, a clock pulse is generated to
increment U3. As the counter increments, DISP1
is turned OFF and DISP2 is enabled. At the same
time, the seven-segment data is changed to turn
on the appropriate LED segments of the second
display. The sequence continues for each of the
three digits before the entire process is repeated.
This gives the display a 1/3 duty cycle and a refresh rate
of 166 times per second. At that speed, the human eye
cannot perceive the multiplexing and the digits appear to
be lit continuously.
■ FIGURE 4. Assembly.
to your thermometer. Figure 4 shows the final assembly
details for mounting the plastic panel to the PCB using
four standoffs and eight screws.
Operation
Construction Hints
The Parts List shows all of the individual components
needed for this project. The HEX program file for the
microcontroller and the PCB Gerber files are available for
download on the Nuts & Volts website
( www.nutsvolts.com). Alternatively, the pre-programmed
microcontroller, printed circuit board, or an entire kit of
parts is available for purchase (ordering details shown in
the Parts List).
Figure 2 shows the component placement
locations on the printed circuit board (PCB).
Double-check the solder connections as you go
along. Mount the resistors and crystal to the PCB
first. Then, solder in the NPN transistors and the
diode bridge, followed by the three IC sockets.
Next, solder in the resistor pack RP1 and the
mono/ceramic capacitors C1-C5.
Move on to the voltage regulator and the
three seven-segment LED displays. Make sure the
displays are oriented so the decimal points match
what’s shown in Figure 2. Next, mount the
jumper post and the electrolytic capacitors. Note
that capacitor C7 has to be mounted
differently from capacitors C5 and C6 in
order to clear the red front panel. C7 lies
on its side against the PCB. Lastly, connect
the power supply wires from the wall
transformer and the DS1822 sensor.
Use approximately 12 inches of three-conductor wire to remotely connect the
DS1822 to the PCB as shown in Figure 3.
Since the DS1822 sensor will be in water
for aquarium use, seal U2 and the wire
connections using waterproof epoxy or
silicon. A transparent red acrylic front
panel will give an attractive finished look
Operation of the jumbo LED thermometer should be
straightforward. If jumper JP1 is shorted, the display will
be in Fahrenheit. If JP1 is left open, the display will be in
Centigrade. Plug the wall transformer power supply into
an AC outlet and within two seconds, the display should
show the digital temperature. If the display flashes “EEE”
the DS1822 sensor was not detected. Re-check the wire
connections to U2 if you encounter this situation. NV
May 2010 41
