Winter 2009 Issue

Up in the Air

Tracking Beacon

By Bill Brown,* WB8ELK

Photo A. Prototype board of the tracking beacon showing the USB converter, the Arduino Stamp, and the 434-MHz transmitter module.

No matter how many APRS trackers you may have on a balloon flight, there is always a chance that they all will fail. In that event, itís a good idea to have some backup. In those situations a low-cost, low-power transmitter can really save the day. This circuit also makes a great hidden-transmitter foxhunt device.
One of my favorite gadget supply houses on the internet is called SparkFun Electronics (www.sparkfun.com). Almost everything needed to build this tracking transmitter can be found on this website. The transmitter module we will be using is an 8-milliwatt transmitter module that operates on 434 MHz and currently costs just $3.95. There are only four leads on the module: Vcc, Ground, Data (the on/off control line), and the antenna output. Itís a natural for sending a Morse Code message since you can key it on and off with just one input line that can be controlled by a logic signal from a microcontroller. The module is listed under the ďWirelessĒ section and is called the RF Link Transmitter Ė 434 MHz, order number WRL-08946. One nice feature of using this module is that itís operating at a license-free power level and can be used as a radio-control (R/C) and model-rocket locator.

The Arduino Controller

The brain behind the tracking transmitter is based on the Atmel AVR Mega168 microcontroller and is called the Arduino Stamp (SparkFun #DEV-08164). The Arduino is similar in appearance and size to the classic Parallax Basic Stamp but is programmable in the C language. This is a very powerful system and a great way to learn the basics of C programming. The Arduino system puts all the low-level items that make C programming tedious into the background and provides you with loads of high-level routines for performing A-to-D conversions, timing delays, serial communications, and digital I/O with very simple and easy-to-use commands. The best part of the system is that the Arduino development software is totally free (www.arduino.cc).

In addition, no expensive external programmer is needed, since you can program the Arduino Stamp via a serial connection (via the internal bootloader program in the microcontroller). Keep in mind that the Arduino Stamp has logic level serial connections so you will have to use a serial converter IC such as the MAX232 to interface to your computer. However, SparkFun does carry a useful USB module that will connect to the Arduino Stamp with just two serial lines and a common ground. Itís called the Arduino Serial USB Board and is SparkFun item #DEV-08165. The Arduino website shows how to hook up the USB converter board to the Arduino Stamp on a prototype board (http://arduino.cc/en/Guide/ArduinoMini). Photo A shows my prototyping setup of the complete tracking transmitter, which includes the USB module to program the circuit. See figure 1 for the tracking beacon schematic.
 

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