• APRS with just an Android phone and an HT that supports VOX

    Objective:
    Small and easy solution for bi-directional (TX and RX) portable APRS via RF (no Internet based APRS-IS) with as few devices as possible and a rich user interface capable of showing stations on a map.

    Solution Narrative:
    APRSDroid is an Android APRS application capable of encoding and decoding AFSK for portable APRS operations. While it supports APRS-IS (Internet based APRS), I really wanted a solution that would allow for bidirectional APRS communications in the field without being dependent on an Internet connection. I also wanted a rich user interface with offline maps and the ability to EASILY send and receive messages over APRS. The solution is really quite simple. APRSDroid can be configured to communicate via the Internet (APRS-IS), Bluetooth (requires an external TNC with bluetooth to serial connectivity), or audio input/output. In the spirit of KISS, I really didn't want to mess with any intermediate devices to facilitate keying the transmitter, so a VOX capable HT is used to key the transmitter when audio is detected. This keeps the solution very clean and simple. Two independently powered devices connected with a single passive cable.

    Required Equipment:
    • Android Device with headset jack and APRSDroid
    • HT with Speaker/MIC input and VOX
    • Custom audio cable


    My setup:
    • T-Mobile G2 Android Phone (already owned) with APRSDroid
    • Baofeng UV-3R from Amazon for $53 (small, cheap, and supports VOX)
    • Hacked up 4 conductor 3.5mm audio cable (already owned) soldered to the earpiece that was included with the UV-3R


    Details:
    The hardest part was really creating the cable. I looked up the pinout of a standard smartphone's headset jack and dissembled the earphone / mic / ptt headset that came with the UV-3R. Then I simply connected the speaker out of the UV-3R to the MIC in of my phone and the MIC in of the UV-3R to the left channel speaker out of my android phone. I then configured the "connection protocol" in APRSDroid for "AFSK via Speaker/MIC" and pressed "Start Tracking", tuned the UV-3R to 144.390 Mhz Simplex, connected the audio cable, and set the UV-3R VOX level to "7". Voila! I instantly started seeing APRS message pop up on my Android phone and beaconed out my SSID and location everytime I hit the "single shot" button (as I wasn't moving and have SmartBeaconing enabled).

    Considerations:
    • I tested everything BEFORE soldering and heat shrinking anything.
    • Any notification sounds on your Android device will cause the UV-3R to key, so set the phone to silent mode before connecting
    • There seems to be a 3 second TX hang time after my beacon is sent, probably related to the VOX setup of the UV-3R. I haven't figured out anyway to solve this.
    • Sending headphone level audio signals into a microphone input probably isn't ideal, but hey, it works!
    • I used the UV-3R specifically because it is FCC Type accepted and has a standard SMA connector with single 3.5mm 4-pin Speaker/Mic connector. Other similar radios don't meet all of these criteria.


    References:



    This article was originally published in forum thread: APRS with just an Android phone and an HT that supports VOX started by KC7IPY View original post