A little while ago I acquired a Walkera Devo 7e from a friend, for use with my assorted R/C aircraft. This is a pretty dandy little radio in its own right, which speaks the Walkera Devo radio protocol. By doing nothing but reflashing it with Deviation firmware, you can add support for the Spektrum DSM2 and DSMX protocols. But if you're willing to pick up a soldering iron, you can accomplish a whole lot more for not too much money.
When I built my R/C models previously, I wanted a TX for each and I wanted them to be cheap, so I went with the Hobby King 6 channel radios. These are just some of FlySky's cheapest radios re-marked, but they are six channel radios with two analog pots and two two-position switches on top, they're only twenty-five dollars, and you can buy additional receivers for them for just ten dollars. But the range is not exactly fantastic, and six channels can be a bit limiting. A bigger flaw with these radios is that you need the wonky specific USB to serial cable to program them. The transmitter also only supports the one basic FlySky protocol, and the receiver lacks a PPM output (which used to be included!)
By contrast, the Devo 7e not only has a screen on which you can edit mixing settings, but it also has a mini-USB jack and will appear as a USB storage device. But it's the Deviation software which really unlocks the full potential of the hardware, and makes it the device to have. If you started out with the HK-T6 equipment, never fear; you don't even have to replace your RX later unless you want more channels or additional features, like telemetry. The Devo 7e with the 3 in 1 radio module installed (or at least an A7105 RF module) binds easily with the HK-T6A v2 RX using the FlySky protocol.
Deviation is replacement software for the Walkera DEVO series, including the 6/6S/7E/8/8S/10/12/12S/F7/F12E. Installation is trivial, and performed by flashing with a Java-based tool. Deviation is chock-full of features you wish every TX had. You can edit model configurations with a text editor, as well as the hardware configuration. It's the latter part that's interesting right now. The software knows how to utilize hardware not included in the original design, and by editing the configuration file (hardware.ini) you can enable it for use in your configurations. I have done three of the four mods I have planned, specifically the diode short mod, the 2x3 switch mod, and the 3 in 1 radio mod.
Besides the excellent protocol support, Deviation's primary benefit is its advanced mixers. They are somewhat harder to comprehend than the stock Devo mixers, but they are immensely powerful and can do basically whatever you might imagine you want to do with a mixer. The attached example MODEL.INI files give some reasonable basic examples of how you might use a Devo7e with the switch mod.
2x3 switch mod
By far the simplest mod is the 2x3 switch mod which involves sourcing appropriate switches and putting them into the case. This requires removing stickers on the case front or drilling through them, the latter of which is easier and cleaner-looking... but which is not what was done on this TX. Finding appropriate switches is easy, because they are basically all of a sort and readily available as spare parts. If you want to spend extra you can get flat switches, but I don't find that to be necessary for the long ones. I found mine on eBay in an auction labeled "FrSky Taranis X9D Replacement 3-Position Long Switch". The benefit of buying switches sold for RC use is that they tend to come with a nice retaining nut that matches the other nuts on your gear, as these did.
Two diodes are also attached to each of the switches,2 and then some wires are run to the front of the main circuit board. I was able to run these around the perimeter of the case, which keeps them completely out of the way. Then one line is twiddled in the hardware.ini file, and the TX is restarted. It's trickier making intelligent use of the switches; for example, one common purpose is to mix a two-position switch with a three-position switch to emulate a six-position switch. This involves a "complex" mixer with four "pages" in Deviation. Example model files are attached below.
The change to the hardware.ini file is fairly obvious:
As mentioned, two example model config files are provided. The first example is a simple eight-channel setup with the two three-position switches assigned to channels 7 and 8. The second is a bit more fiddly; it uses a four-way "complex" mixer to emulate a six-position switch. This lets you select six different flight modes from your model without any confusion about which mode it is in due to trying to find the right rotation on a knob. (Adding knobs to the Devo7e is something of a PITA anyway.)3 The left three-position switch is passed through as channel 5, and the left two-position switch is set up as a rate limit for the elevator and aileron. I didn't rate limit the rudder, as my focus is on multicopters.
Diode short mod
The diode short mod is probably the trickiest; you have to remove the heavily-soldered RF shield from the stock RF module, then short out an extremely tiny diode (even for surface-mount) which is in between other, taller components. The shield is then reattached. This gives access to approximately 150mW of transmit power. Between the high-power diode mod and the 3 in 1 radio mod, you get access to practically every 2.4 GHz R/C protocol — certainly all of the popular ones. A small ini file tweak is made when the mod is complete, to reflect the changes in transmit power.
has_pa-cyrf6936 = 1
I have a fairly skinny soldering iron, but even so it was too large to actually solder a bridge across the pads. I removed the diode in the hope that I would be able to do so, but then I couldn't actually do it. Be warned! I then had to bridge the pads, and I used conductive paint applied with the end of a hypodermic needle in which it was shipped to me. I could have done this without actually removing the diode! On the other hand, while this is the trickiest mod, it is also the cheapest if you are handy with a soldering iron and have a nice pointy one already. Even if you aren't and/or don't, you can get some conductive paint for around five dollars. That won't buy you much, but you'll use even less. Surely you can come up with some use for the rest.
3 in 1 radio module
The 3 in 1 radio mod is a small board that has been made specifically for devo7e and devo10 radios which comes with a little 2.4 GHz antenna and the leads needed to attach it to the transmitter. On the devo7e it is necessary to solder directly to a pin on the MCU; I used some knockoff Kapton tape ("KOPTAN") to shield the adjacent wires and then soldered a bit of 27ga magnet wire to the appropriate pin and secured it with hot glue, then soldered my flexible lead to that. I bought the board on eBay for about thirty dollars.4 Some hardware.ini changes are required to let the radio know how to make use of them.
enable-a7105 = A13
has_pa-a7105 = 1
enable-cc2500 = A14
has_pa-cc2500 = 1
enable-nrf24l01 = A15
has_pa-nrf24l01 = 1
4 in 1 radio module?
An alternative to performing the diode mod when installing a module into your TX is to install the 4 in 1 module instead. In the Devo 10, it plugs into the TX in place of the original radio module, and then you only need to solder three wires (the CS pins for the other three radio modules) into the unit. In the Devo 7e, you have to first desolder the original radio module, then solder all of the same connections as in the 3 in 1 mod; plus one more wire going to the CS pin which activated the stock CYRF6936 module. For the Devo 10, this is pretty much a no-brainer; you should definitely install the 4 in 1 module. It's ten dollars more expensive, so if you have to buy conductive paint to do the diode mod, you might as well budget another five bucks and get the 4 in 1 module. It also eliminates the factory antenna entirely, as all four radios on the module share the same antenna jack.
TX power regulator replacement
The fourth mod (which I have planned but not yet executed) is the replacement of the transmit power regulator. This is generally inadequate to the task of running at the full 150mW. It can simply be removed and bypassed. The AMS1117 3.3 is not suitable because the maximum input voltage is too low, and I have a wider input buck converter on the way. Another power mod option is to switch to 2x18650 cells with USB charging; the cells will fit with only minor modification to the battery storage area. It's not safe to charge LiPos in the unit, and there's no provision for a balance charging connector anyway.
If you feel like you should have external antenna jacks, it's easy enough to get a 15cm RP-SMA solder pigtail (for the onboard CYRF6936 module) and/or a 10cm RP-SMA to IPEX pigtail from eBay, as well as some RP-SMA antennas. You can also often get appropriate RP-SMA antennas from 802.11b/g wireless access points, which use 2.4 GHz frequencies. This will also let you install a substantially larger antenna in the field if needed, or a directional antenna for long-range flights. Another popular modification is the adding of a power LED. You can take off switched power from a two-pin solder pad very near the switch and power regulators, but it's probably wiser to draw from the main 3.3 volt regulator. (Don't take power from the RF module, that regulator is taxed as it is!)
For some years, the go-to transmitter for those looking to save a buck was the Turnigy 9X, a competent 8 channel PPM or 9 channel PCM radio which has a socketed RF module on the back meaning it's simple to make it support different radio protocols by swapping it out, plus a screen on the front for programming. The Devo7e is around $20 more expensive, and it doesn't speak any more protocols out of the box, but a simple reflash gets you DSM2/DSMX protocols with telemetry — for free! But for around $120 all in, you get every popular protocol and all the switches you need. It's also a slightly higher-quality radio in general, and it's smaller and weighs less. There's no reason to throw rocks at your 9X, but the Devo7e is a better buy today.
- 1. I went with 5.0.0-1d8c208 from 2016-10-15, which is currently the most recent. This build seems to be working fine. On the other hand, if you want the Walkera WK2x01 protocols, you need a version of Deviation 4.1 or lower. Not sure what that's about.
- 2. I used 1N4001 diodes for the switch mod.
- 3. The easiest approach is to add an attiny or similar which outputs PPM to the PPM input, but if you do that you can no longer connect a PPM input such as head tracking.
- 4. The item was called "CC2500 NRF24L01 A7105 3 IN 1 Wireless Module for DEVO 7 10 Transmitter Multi RF" and I bought it from eBay user millionshare who got it to me within a couple of weeks.
|Simple 8-channel example with 2x3 switches on channels 7 and 8||739 bytes|
|More complicated 6-channel example with 6-pos and rate limit||1.25 KB|