■ FIGURE 3. Connections.
Multiplexing
Multiplexing involves lighting each of the three LED
digits one at a time. Even though only one digit is on at
any single moment, the scanning is done so quickly that
all three digits appear to be lit simultaneously. The seven
segments of each digit are controlled by seven data lines
from the ATMEL AT89C2051 microcontroller U5. During
multiplexing, it is necessary to drive the LED’s with more
current than the pins of the AT89C1051 can provide, so
the PNP darlington drivers in U4 are used to provide the
additional LED source current capacity. U4 also does
voltage level shifting, allowing the displays to be driven
from the unregulated input voltage. The CMOS decade
Thanks to the amazing DS1822 digital sensor from
Maxim Integrated Products, no temperature calibration
is required for this thermometer. The DS1822 digital
sensor provides 9- to 12-bit Centigrade temperature
measurements. It has an operating temperature range
of – 55°C to +125°C, and is accurate to ± 2.0°C over the
range of – 10°C to +85°C.
The DS1822 communicates over a 1-Wire™ serial
data bus. The bus was originally designed by Dallas
Semiconductor to provide low-speed data, signaling,
and power over a single signal wire. Of course in real
applications, the bus requires at least one more wire for
a ground connection. Even though most 1-Wire devices
can take their power directly from the same wire used
for data communications (parasitic supply), projects can
be more robust if an additional wire is used for
powering the device.
So in this application, the DS1822 actually uses
three wires. Perhaps naming it the 3-Wire bus would not
have sounded as interesting.
1-Wire is currently a registered trademark of
Maxim Integrated Products, Inc.
counter U3 provides a way to turn on each of the three
digits in sequence. U3 cannot drive the digits directly, so
six TIP122 NPN darlington transistors (Q1-3) are used for
LED current sinking.
Two output lines go from the microcontroller to U3.
One is used to reset the counter, so the data presented to
the seven-segment outputs can be synchronized with the
proper digit. The other is connected to the clock input of
the CD4017 and is used to increment the counter. Each
PARTS LIST
REF DESCRIPTION SUPPLIER PART
Digi-Key Corp;. www.digikey.com
Jameco Electronics; www.jameco.com
C1,C2 Capacitor, 22 pF, 50V Monolythic
C3,C4 Capacitor, 0.1 mfd, 50V Ceramic
C5 Capacitor, 10 mfd, 16V Electrolytic
C6 Capacitor, 100 mfd, 16V Electrolytic
C7 Capacitor, 1,000 mfd, 25V Electrolytic
DB1 Diode Bridge, 1.5A, 100V, W01
R1-7 Resistor, 100 ohm, 1/4W, 5%
R8 Resistor, 330 ohm, 1/4W, 5%
R9 Resistor, 4.7K ohm, 1/4W, 5%
R10 Resistor, 10K ohm, 1/4W, 5%
U1 IC, LM78L05, 5V Regulator, 100 mA
U2 IC, DS1822 one-Wire Temperature Sensor
U3 IC, CD4017 Decade Counter
U5 IC, AT89C2051 Programmed with software
U4 IC, TD62783AP, 8 CH Source Driver
Transistor, NPN, TIP122
CY1 Crystal, 11.0592 MHz, HC49US
TXFR Transformer, Wal, 12 VDC, 700 mA
DISP1-3 Display, LED seven-Seg CC, 2. 3 in, LSD23255
A complete kit, a printed circuit board, or a
pre-programmed microcontroller can be
purchased from the Nuts & Volts Webstore
( www.nutsvolts.com) or call our order desk
at 800 783-4624.
Digi-Key Electronics (DS1822-ND)
Digi-Key Electronics (TD62783APG-ND)Q1-3
Jameco Electronics (325227)
Jameco Electronics (97201)
MISCELLANEOUS
Jumper post and shorting block; IC sockets; Hex spacers - 3/4 inch length; Acrylic plastic front panel;
#4-40 screws; three-conductor wire.
40
May 2010