The Electronics Hobbyist

This week we’ll look at the circuit for the 2-digit 7-segment display counter using the Arduino.

There are a few options to control multiple displays:

• employing multiple controllers;
• using a 7-segment driver chip like the 7447;
• using a multi-display controller such as the MAXIM MAX7219;
• sequencing through the displays, which is what we have done in our example, as it requires no added hardware.

When we were using a single-digit display, we connected the common anode pin to our Vdd supply, but with two digits we have to drive them independently if we want them to display different digits!

A natural reaction would be to try to use two Arduino I/O pins, each driving a digit of the display. The problem with this scenario is that it is not possible to drive the common anode or cathode pin using Arduino I/O pins, as they cannot source or sink enough current to light all seven segments.
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This month’s Arduino project is to build two 2-digit 7-segment LED display circuits and sketches, one that counts up and one that counts up using mini push buttons. The next posts will explain the circuits and the Arduino sketches.

Materials:

• Arduino Duemilanove
• 1 2-digit 7-segment display (I got a 50-piece LED display grab bag for better value; the one I used was configured as shown)
• 2 Mini push button switches
• 9 Resistors, 100 Ohm
• 2 Resistors, 10K Ohm



• 2 2N3906 transistors (PNP)
• 1 Solderless breadboard
• Jumper Wires in assorted lengths

Sketch for counting up without buttons:
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1. Pregnancy: a pregnancy belt that notifies the father every time the unborn baby kicks? Oh, and it tweets it: http://www.nydailynews.com/lifestyle/2008/12/17/2008-12-17_twittering_from_the_womb_how_soon_is_too.html
2. UAV: Sean Reynolds achieves autonomous flight control with this Arduino-based unmanned aerial vehicle: http://seanreynoldscs.com/WashUAV.aspx
3. Bluetooth: can’t afford a Bluetooth Arduino project? Check out this inexpensive Bluetooth module for the Arduino Mini: http://interactive.usc.edu/members/jbleecker/archives/007361.html
4. Stribe: have you heard of stribes? A stribe is a lighted touch strip music controller: http://stribe-org.blogspot.com/2008/01/description.html

Buy the Arduino Stamp Mini

The Mini at the official Arduino website: http://www.arduino.cc/en/Main/ArduinoBoardMini

1. Light: how do you feel today? Find out with the interactive, multi-function mood light: http://www.instructables.com/id/Interactive-Open-Source-Mood-Light/
2. Sound: have you ever played with water filled crystal glasses to make sound? How about using an Arduino Nano to control the music? Check out this cool video: http://www.vimeo.com/1261369
3. Game: it’s no Wii, but this simple version of Super Mario Bros using an 8×8 LED matrix rocks: http://vimeo.com/9928343
4. iPod: an Arduino-based iPod remote with a single button that toggles between Play and Pause on the iPod: http://davidfindlay.org/weblog/files/2009_09_07_ipod_remote.php

Buy the Arduino Nano Ver 3.0

The Nano at the official Arduino website: http://www.arduino.cc/en/Main/ArduinoBoardNano

1. Jacket 1: a jacket with turn signals that lets people know where you’re headed when you’re on your bike: http://web.media.mit.edu/~leah/LilyPad/build/turn_signal_jacket.html
2. Jacket 2: a jacket that both heats and cools based on one’s body temperature: http://www.instructables.com/id/How-to-make-a-Heating-and-Cooling-Jacket/
3. Shirt: this cool shirt features an RGB LED that changes color in response to motion and tilt: http://web.media.mit.edu/~leah/LilyPad/build/accelero_shirt.html
4. Scarf: this is no ordinary scarf! The interactive passion sensing scarf detects the presence of another scarf wearer nearby and sets the color of an RGB LED to either blue (lonely) or red (passion): http://www.instructables.com/id/Arduino-Lilypad-Interactive-Passion-Sensing-Scarf/

Buy the LilyPad Arduino

The Lilypad at the Official Arduino website: http://www.arduino.cc/en/Main/ArduinoBoardLilyPad

1. CNC Machine: this CNC machine cost aventgps $300 to build, but it took him over 200 hours to do so. Check out pictures of the machine and some sample prints: http://letsmakerobots.com/node/13175
2. Robot: a cool obstacle avoiding, drumming robot with laser printed chassis: http://letsmakerobots.com/node/15803
3. Beer: beer lover or not, one must appreciate the coolness factor behind this interactive keg that sends wireless status updates to a PC: http://www.vachementcool.com/MooSpace/Blog/Entries/2009/11/1_I_built_a_Kegbot.html
4. Skiing: Dwight undertakes a huge project when he sets out to build an interactive trail map for a ski resort: http://crossingwiresarduino.blogspot.com/

Buy the Arduino Mega

The Mega on the official Arduino website: http://arduino.cc/en/Main/ArduinoBoardMega

The Arduino is based on the microcontrollers from Atmel. Here are current Arduinos available (the architecture is open source, and anyone can build their own “Arduino” based on the technical specs that are available for download from the official Arduino website).

• Duemilanove: “Duemilanove” means 2009 in Italian and is named after the year of its release. The Duemilanove is the latest in a series of USB Arduino boards. It is based on the ATmega328 and has 14 digital input/output pins (of which 6 can be used as PWM outputs) and 6 analog inputs.
• Mega: The Arduino Mega is based on the ATmega1280. It has 54 digital input/output pins (of which 14 can be used as PWM outputs) and 16 analog inputs.

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