Fly Sky i6 as a flight simulator TX

These days I cannot fly physical RC, so I thought about flying with a simulator. There are very nice simulator software out there. One of the best is Picasim. It has great flight dynamics and graphics. More importantly for me, it can easily run under Linux using wine (give it a try!). Next is the transmitter box. While it is possible to buy a transmitter box for flight sims, they are generally not good quality or offer much configuration.

Looking around the web, I found Fly Sky i6, an entry level transmitter which offers a great number of options. Better still, it is available from China without a receiver or any other accessories, for about $35 including shipment. This is about the same price as "dumb simulator transmitters". The i6 is very interestig because it has a great followers group who are modifying its firmware to make it better. I would have hesitated to use it for a real flying RC model, but when I opened it up for the first time, I was surprised with the good quality. It can be trusted upon to fly simple real RC models.

Back to the topic. One of the most interesting features of the i6 is its "trainer port". It is documented well. The trainer port is a "female mini DIN 4" shape, A.K.A. "S video", and has 5 connections available: GND, PPM OUT, PPM IN, RX, TX. A matching mini DIN 4 male connector looks like this FYI:

Mini DIN 4 male connector (A.K.A. S video)

If you build (see below) or buy a PPM to USB joystick converter module (for around $11), you can immediately use the transmitter straight away. However, there are two "problems" with that! First of all, where is the fun of building if you are simply buying stuff and plugging them together? Secondly, of you buy a sim module, then you will need to run the transmitter on batteries which you must remember to buy replacements or to charge. The purpose of this post is to describe the modifications to the i6 electronics so that you can power it directly from your USB port.

Two modifications are required to the i6:

1. Modify the trainer port pins so that one of them becomes a power input pin,
2. Reduce the power consumption so that it will run from the 100mA available from the USB port.

Modification of the tranier port functions

You need to eliminate one of the functions on the trainer port so that you can make room for the power input pin. I selected to eliminate the "PPM IN" pin (of course another can be selected). In essence, all that you need to do is cut the wire to the PPM IN pin from the connector and solder it to the power switch and you would be done for this step.

However, there is a smarter way! The trainer port is connected to the rear case with a cable and a nice connector. It is possible to make a few changes to the components so that the cable is intact. The changes are shown below:

Changes to the PPM IN connection to make it a power supply pin.

I labeled the changes in the photo to match the item numbers below.

1.  The PPM IN signal goes to connector J8, second pin from top. (The top pin is GND by the way, and the third from top is PPM OUT pin.)
2. There is a reverse biased protection diode D1 that shorts the PPM IN to 3V3 supply on the board if it exceeds the allowable voltage. Remove that diode. Connect a wire to its left pad (next to the buzzer). This is a bit fiddly because of lack of room and you can see some burn marks on the buzzer. Well, it still "buzzes"...
3. Connect the other end of the wire to the battery (+) pin. There is actually a more convenient pad for that labeled "BAT+" just to the left of the battery connector.
4. If you find it difficult to solder to an SMD diode pad, there is also another easy location for the PPM IN pin here.
5. Finally, you will need to remove the in-line protection resistor R13 that used to connect PPM IN to the processor. I generally lift up one pin and still leave the resistor connected, so that I can take back the changes in the future. In case you need, this is a 1k Ohm resistor.

That is all. Now the "PPM IN" pin on the trainer socket is connected to "battery in" circuit. CAUTION! This has two implications: The USB 5V is now connected directly to the battery box output. If you have batteries in the case, their voltage will be applied to the USB socket of your computer. Always remove batteries from the transmitter before using it. Second; because the USB supply voltage is only 5V, there is no room to use an in-line voltage protection. The i6 starts low voltage alarm when the supply voltage falls to around 4.5V. If you apply a high voltage to this pin (12V or similar) you will burn out the electronics. However, a simple reverse protection is installed, in the form of reverse biased diode D2 across the power supply. I find it good to have batteries installed for a balanced feel. So you may want to install only 3 of the 4 cells, or keep the power connector of the battery box unplugged, or cover one of the cells with tape etc. to keep them all installed. I selected the last option.

Reduction of power consumption

The second item is to reduce the power consumption. The transmitter draws about 100mA with the backlight on. The USB joystick module also requires about 20mA. The total of 120mA is above USB power supply specification and may cause a reset of the USB port on some PCs. The simplest way to reduce power is to remove power to the transmitter module of the i6. However, the microprocessor must communicate with it, and probably will not run without it. If there were a firmware upgrade that disabled the transmitter module, it would have been the best, but I did not come across one. The next biggest load of the i6 is the backlight. It consumes about 20mA and it is simple to disconnect. So that is what I did.

 

All you need to do is cut one trace so that the backlight is disabled. You can also remove R13 but i my case, it was firmly glued down. Anyway, if you want to re-enable the backlight, simply bridge the cut. BTW, the backlight is supplied directly from battery voltage.
 

Cut this trace to disable the backlight.

So, that is all. Now the i6 can be powered directly from USB, and the PPM IN pin has become a power input pin. If needed, it is still useable as a regular transmitter.

FWIW, my Fly Sky i6 PCB is labeled i6-M VER 2.4  20190623

The interface

Finally, the simulator interface (A.K.A the cable) itself. This converts the PPM (Pulse Position Modulation) signal which is standard across most RC transmitters, through the USB interface, into a joystick emulator for the PC. If you look around, there are many candidate projects. If you have an Arduino Leonardo or similar laying around, you can search for such projects. Since I am currently doing development with STM32 series microcontrollers, I had a blue pill board (STM32F103 breakout) laying around and chose to use that. Again, there are many good projects for it especially in Github. I found this one: "STM32-RC-USB-Adapter" by "voroshkov" and did not even bother re-compiling it; used the binary straight from the repo. It worked like a charm. It only supports 4 analog channels. I may in the future upgrade it to 6 all channels if I have time. I have modified the firmware to a contemporary version of MX Cube and added the 5th channel. I will put it up to GitHub later and put a link here. However, there is really no use for an extra analog channel in the simulator. 

The connections to the Blue Pill Board -> i6 are simple. Just buy a mini DIN 4 pin male plug and use that:

1. GND -> GND
2. 5V     -> The newly created Power input pin
3. A8     -> PPM OUT

The photo is below:

The completed PPM to USB joystick adapter

Finally, you need an adjustment. Gamer joysticks have poor gimbals with a lot of electrical noise, so that most OS'es set the default dead zone at the center to a large value and you will not have precise flight control. In your OS remember to calibrate the joystick for good performance. Typically the OS will require that you move the sticks to extremes and then center them while it tries to detect the amount of noise. There is an easier way to calibrate an i6: Manually set each joystick minimum to '0', maximum to '1000', dead zone min and max both to '500' for the best performance.

The setup works well with my 2016 vintage Linux laptop running Ubuntu 16.04 as well as a contemporary Windows 10 machine. There is really no difference to the USB joystick definition across OS'es.

Have fun!