Nuts and Volts - May 2010

■ 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.