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 ( 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 (

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 ( 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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