One wire reads the keypad from the APRS radio mic

[Shane Burrell] decided to spend some time learning how the keypad on the his Kenwood TM-710A APRS radio mic works. It uses a different technique than you might think. Normally a grid of buttons is scanned as a matrix to detect keypresses, but this hardware actually counts pulses on a serial wire to take each reading.

The stock radio sends a steady digital pulse to the handset and with each pulse the mic pulls the line low. It then uses a 4017 decade counter to see what comes back. If the edge count matches it means nothing is pressed, but a change in the number of pulses returning to the base unit can be used to extrapolate which button has been pressed.

[Shane] went on to implement this control technique using an AVR chip in place of the  radio base unit. He used the data gained from measuring the pulse behavior using an oscilloscope to write the firmware for the project. He filmed a bit of a demo after the break which shows his findings.

We’re not quite sure how this would translate into your own home-brew projects, but the thought of scanning a keypad with two pins of a uC is quite desirable. Sure there is the 555-timer frequency technique, but we’re always down with new ideas.

Continue reading “One wire reads the keypad from the APRS radio mic”

Turning a router into an APRS gateway

[Chris Kantarjiev] is an amateur radio enthusiast (call sign K6DBG) and does a lot with the APRS. We think his build, turning a WRT54gl router into an APRS gateway will be very useful for the APRS tracker builds we’ve been covering.

Setting up an Internet Gateway, or igate, on APRS usually requires a ‘real’ computer. [Chris] didn’t like that idea, so he took aprs4r, igate software for embedded devices, and pruned it down to fit on the 4MB of flash and 16MB of RAM in the WRT.

The actual APRS hardware is connected though headers soldered onto the WRT54gl’s board which go to a small PIC-Based TNC. [Chris] argues that the APRS ‘backbone’ is great, but there aren’t enough nodes on the network for full coverage. We thing this would be a great way to put cheap hardware out in the wild to cover those gaps in the APRS network.

Check out the video for a rundown of the modded WRT54g after the break. If you’re interested amateur radio, Field Day is coming up in just 2 weeks. Find a local club and check out what’s possible with amateur radio.

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Radio packets…from spaaaace!


[Bill Meara] was watching the International Space Station and the Shuttle Discovery pass overhead a few weeks ago, which rekindled an interest he gave up long ago – sending and receiving radio packets from space.

Years ago, he used to send APRS packets into space with a small rig powered by a 286 computer and HandiTalkie. These packets would drift off into space most of the time, but occasionally they would bounce back to Earth whenever the space station or PC Sat would fly by. The packets were often captured by other ham operators across the globe, who happened to be tuned to 145.825 MHz.

His interest renewed, he dug out his old HandieTalkie and Kantronics Terminal Node, aiming them towards the sky via an antenna situated in his back yard. When he returned 10 hours later, he found that he had collected all sorts of “space packets” from across the globe.

While not exactly a hack, it is definitely a neat exercise in ham radio operation. We can imagine slinging data packets off the space station would be an exciting experience for any budding operator (and OMs as well!)

Trackuino – an Open Source Arduino APRS Tracker

trackuino board

Trackuino is a new open source (GPLv2 license) Arduino APRS tracker designed by [Javier Martin]. If you are unfamiliar: APRS (Automatic Packet Reporting System) is an amateur radio method used to relay small packets of position-tracking data to an online database for easy access and mapping. In this case, GPS telemetry data is used to track latitude, longitude, altitude, course, speed, and time measurements in near real-time via

Although this reminds us of the WhereAVR that we covered previously, the Trackuino includes an onboard radio so no external handheld unit is necessary. Since the Trackuino was designed primarily for high-altitude balloon tracking, a number of useful related features are also included: dual temperature sensors, support for a humidity sensor, and a remote “cut-down” trigger really make this a complete package.

Initially there was some concern that the 300mW radio used would not be powerful enough to reach the ground-based receivers from peak altitudes. This was clearly not an issue however, as the signal was heard from nearly 600Km away during the maiden voyage. If this still doesn’t sound like enough power, a 500mW radio is also supported.

Make sure to check out [Javier]’s blog for some amazing high-altitude photos and everything needed to get your own Trackuino up and running in no time!

Thanks [Brad]!

Terminal Node Controller in a router

[Andrew] used a DSL router to make his own Terminal Node Controller. This will become part of an APRS-IS network, an Internet-based network built by amateur radio operators. The router used here is a Dlink DSL-502T with an AVR based TNC module attached to the serial port header. The phone line connector and its accompanying hardware have been removed to make room for the TNC module, which is supplied with 12V via that red wire. When the router boots up it sends data to the serial port header so the firmware on the TNC needed some tweaking to accommodate this (yay for open source).

Want some more APRS goodness? Check out this AVR APRS tracker.

WhereAVR APRS tracker


We posted a story about someone doing some APRS tracking recently. This is old news to some, but new fresh stuff for others. If you want to build your own tracker, here’s a great writeup on one.  The WhereAVR is low cost, low power and has plenty of I/O.  With all of the schematics and PCB files available on his site, you should be able to get one working in quickly. He does need a little help building a nice simple configuration tool to work in windows, anyone want to volunteer?

APRS tracking


We really wish we had a little more information on the construction of this, but [Jeff] made this APRS tracker several years ago. APRS, or Automatic Packet Reporting System is a system where shortwave radios put out small packets of data that are uploaded to a database available via the web. This specific one is relaying GPS data so his family can see where they are located. With current phones, you might think this is antiquated, but he notes that he took this through New Mexico and was able to transmit his position even when there was zero cell phone coverage.