Low-Power Remote Control/Echolink Node

At certain times of the year I migrate a bit North of Southampton to an area where the the hams on VHF are grumbly and seem to be few and far between. Add to this that the local repeater only uses tone burst and that the QTH is right next to some noisy power-lines, which makes getting further afield difficult and HF deafening, and my time playing radio takes a nose dive. My solution to increase my air time was to take advantage of one of the perks of having a full license - remote control using, any method of communication, to control a radio in the Southampton QTH.

 

At the start of this project I had a spare HP T5720 "thin client" lying about. These are low-power (~1-2A at 12V), small computers which have a 1GHz AMD CPU, 512MB RAM, 512MB/1GB of IDE-based Flash storage and run XP Embedded and can be picked up for very little money. The flash can be replaced with a 2.5" hard drive with a bit of hacking and full XP can then be installed. The system has serial and parallel ports, 10/100 LAN, audio in and out, 6 USB ports, VGA, PS2 connections and a PCI slot. The first step was to decide which rig to use. My first thoughts were to use a Tait T500 which had been thrown my way by G0WFQ however this turned out to be a red herring as the audio in and out of it was atrocious so I settled for my Baofeng UV-3R. For those of you not familiar with this amazing little radio, it is a 2W 2m/70cm handheld which you can pick up for £30 or less on a well known auction site and this project prompted me to buy a second one. They are so low power that it is possible to power one solely from a USB port and there are cables available to do just that. The headset jack is a 4-pole 3.5mm affair with fairly standard pin out (of tip to ring) Speaker, Mic, PTT and Ground.

 

With the rig chosen I needed some method to control it. Ham Radio Deluxe offers remote control functionality and has been used by other people however you need to have a supported radio/interface which is obviously a no go for this. Another option would be to use Skype and then VOX on the handheld but, while this would work and has again been used by other Hams, my concern would be accidentally triggering the transmit with other sounds from the computer. In the end I went for Echolink which allows for generic PTT control over a serial port using either RTS or DTR and, while not intended for private links, it can be locked down to restrict access to certain named users. Echolink also has the benefit of being less complex on a software front than a HRD setup as no VPN is required.

 

Finally, an interface board of some description was required to control the PTT and to get audio in and out of the radio. Full opto-isolation of the PTT was not required as the radio was being power FROM the PC however widely available Eagle LT700 (1.2K to 2x 3.2R) audio transformers were still used to cut out any hum. RTS was used to control the PTT with a BC-547 diode.

Circuit schematic for Echolink interface

This solution allows me to use the GB3SH repeater from practically anywhere - from my PC at the alternative QTH to a 3G connection on my iPhone/iPad - with minimal cost and work.

The down side to this solution is that there is no way to control the frequency so you are stuck to a single channel or repeater.  I have also found that it can be challenging to get into a flowing  conversation without tweaking Echolink's VOX receive settings to reduce the cut out time.

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