KAP-header14

My 2.4 GHz transmitter goes to the chop shop

Cris

About a year ago I acquired a new radio for controlling aerial camera cradles that I lift using kites. For the previous ten years I used an Airtronics Infinity 600, a programmable transmitter that dated from my RC sailplane days. While it was well time to update this 1991 vintage radio, my principal motivation for change was concern about using the 72 MHz frequency assigned to RC airplanes. Recently radio control systems have appeared using the general use 2.4 GHz band, a frequency that is available for general use worldwide.

My new radio is the Spektrum DX6 from Horizon Hobbies, a product that is special for two reasons. First, it is the first commercially available system that operates in the unlicensed 2.4 GHz frequency band. Gone are concerns that I might interfere with RC aircraft and foreign travel for KAP is no longer a bother (at least in terms of radio frequencies) Second, the radio employs a digital spread spectrum technique to transmit the control signaling. In theory, and happily in practice, this technique isolates the Spektrum system from a 2.4 GHz environment that must be chaotically awash with signals from WiFi, Bluetooth, baby monitors, and remote-controlled tie racks. What could be better? The radio is programmable for 10 cradles and the system costs less than $200 new with receiver and four tiny servos But wait you say, surely such a bleeding edge technology will be idiosyncratic and trouble prone, good for early adoption junkies but not for me. After a year of use I can report the Spektrum has performed superbly as a KAP radio, nary a glitch to report.

You can find a detailed and interesting description of the Spektrum DX6 at the Spektrum DX6 Resource Center, a hobbyist’s site.

Transmitter package modification

The stock Spektrum has the dual thumb stick geometry typical of RC aircraft radios.

Over the last couple of weeks I have worked on repackaging my Horizon Spektrum DX6 transmitter to allow operation with a single hand (left) while leaving the other available for handling the kite. I designed and made an umbilical hand controller for my old Airtronics radio, completing the project just before my shift to 2.4 GHz made it obsolete. This time around I decided against the umbilical approach and opted to reduce the size of the transmitter package itself while revising the control potentiometers and switches. The modifications have not altered the FCC certified 2.4 Ghz transmitting component.

Thinking the project might be on the bench for a while my first step was to buy a second Spektrum transmitter. A quick trip to EBay yielded a new TX with charger for less than $100. I then programmed the shiny new transmitter to be my working radio and took the old one to the workbench.

Stock Spektrum DX2 transmitter

The stock Spektrum DX6 transmitter. I have used a standard aircraft radio for virtually all of my aerial photographs. The two-hand geometry that suits RC aircraft flying so well is sub-optimal for KAP since we generally have a kite to fuss with.

Taking the back off of the DX6 revealed three moderately complex circuit boards supported by a variety of small printed circuits for switches. Two of the circuits appeared to handle control inputs and user interface while the third is the 2.4 GHz transmitter. It took about an hour to disassemble the radio. Afterwards I carefully taped the electrical components to my workbench (to avoid shorts) and booted the transmitter. It still worked.

Circuit boards removed

Circuit boards removed.

At one point I was going to build a new housing from scratch. While measuring the heights of the standoffs in the transmitter’s empty plastic case it occurred to me that it was silly to recreate this geometry anew. So I reassembled the transmitter’s plastic case and (gulp) headed over to the band saw. The idea was to trim the case down to a volume that would be compact enough to hold in one hand while still accommodating the circuit boards and new interface switches.

Transmitter case surgery complete, I turned my attention to building new surfaces for the sides and top of the transmitter. These were assembled from small blocks of wood and model aircraft plywood that were glued together and shaped with a sander. As the enclosure took shape I kept looking at my hand and thinking about how it might fit the transmitter and where controls might be placed. This helped me plan enough internal space for control placement.

Spektrum enclosure parts

The wood portion of the enclosure begins to take shape.

It was also clear that there would be no place for the stock 600 mAh NiCad TX battery. After measuring the space available I browsed onlybatterypacks.com for a pack that would fit and a 1500 mAh 2/3 A NiMH option fit the bill at $40 (ouch). This was my only project purchase beyond the transmitter itself.

With the case modifications complete I painted the plastic portions of the project with a nice red spray paint found during the end-of-spring school studio cleanup. I stained the wood portion red followed by a couple of coats of water-based varnish. After everything dried I gingerly (re)installed the circuit boards in the plastic portion of the case Once again the radio booted properly when powered up (sigh of relief).

Starting to sort out new control inputs

Wires a’plenty as I cut off the old potentiometers and switches.

I mounted my potentiometers and switches to their assigned locations on the wooden portion of the transmitter case, soldered pigtail leads as required, and tidied the lead routing with small wire ties and a dab or two of hot glue. Turning to the transmitter circuit boards I cut the wires to the transmitter’s original control inputs while methodically tagging each with a bit of tape and keeping notes. Controls leads that would not be used (e.g., rudder and aileron channels, buddy cord connector) were clipped and terminated with electrical tape.

Chopped Spektrum with circuit boards installed

This portion of the project was entertaining in puzzle-like ways. The digital voltmeter was my friend.

There followed a satisfying period of joining wires (solder splice then heat shrink tubing) and tidying up here and there. Before too long it was time to fit the pieces together for good and fit they did. There is not much wasted space in the final results and it fits nicely in my hand, particularly after I added a velco-fastened hand strap to the back.

Modified Spektrum w/ hand for scale

The finished transmitter in hand. My fingers can reach behind the unit to turn the large pan wheel or slide the tilt potentiometer. Either of these actions can also be accomplished with a single finger from the kite-flying hand – no opposed grip necessary.

I have now used the repackaged transmitter during several long KAP sessions and it “works the treat”. I can liken the experience to switching from a mouse to a trackball. The functions are basically the same but the interface is quite different. I felt clumsy and inaccurate for the first session or two but that is all smoothed out now. It is VERY nice to have a singled hand control device.

Transmitter in use

The new transmitter setup in use — Pacific Beach, June 2007

2 Responses to “My 2.4 GHz transmitter goes to the chop shop”

  1. Antti - RadioControlledAeroplanesBlog.com Says:

    I’m surprised there’s no comments on this post as that’s one cool build. I’m a RC plane flyer myself, so no experience on kites, but that new modded transmitter looks so sweet I started thinking about modding a transmitter myself 🙂 not for one handed use thou, but make it look nice and just right for my hands.

  2. GS Says:

    I love to see well done stuff, and this is just another one. Great work

    I’m also thinking though, you could’ve bought a lanyard, hung the transmitter around the neck and operated it with one hand!. Your final picture on the beach leads me to think that 😉

    Good work, but!