Fatshark Goggles 5.8GHz Modification

The Fatshark RCV922 Goggles are designed for use in FPV (First Person View) RC piloting. They contain two small VGA LCD screens giving a 42 degree field of view.
They also have 2.4 GHz A/V receiver built in, so you can receive the A/V stream from your plane/helicopter/ufo directly to the goggles.
Unfortunately, using a 2.4Ghz A/V link is a really bad idea if you’re using a 2.4 GHz radio control system like the Spektrum or JR.
2.4GHz systems have become really popular lately, because of the lack of frequency clashes on the field, smaller antenna e.t.c.
So, what to do?
Rebuild the Fatshark, of course.

Receiver Module

The Fatshark were using an Airwave AWM634 Receiver module. The size is 41x31x5 mm. I found a 5.8Ghz module, also from Airwave, the AWM682. It is slightly bigger, 51x31x5mm, but the shark is indeed fat, so there was some spare room.
The module to the right, is my reserve receiver, it has a 1/4 wave wire antenna. The module I built into the Fatshark, use the “real” Fatshark antenna, which you’ll see later.
You can find more information about the Airwave modules here at the Active Robotics website.

Some reverse engineering required

The new 5.8GHz module was a completely different pinout than the original module, so I needed to reverse engineer the board layout back to a schematic so I could see what I was doing. But first, the old RX module had to be removed.

This is definitely not for the faint hearted. The RX module has 20 pins, plus four large corner pins. Desoldering this from the main A/V board was a bitch and then some. I tried to get the module off without damaging the PCB pads, so I could remount it, if the need should arise. I used a combination of regular soldering tool, hot air tool, desoldering pump and soder wick, but eventually, I destroyed 3 or 4 of the holes slightly, as I realized it would be almost impossible to get the module off otherwise. As the new module is completely different, and need to be connected with wires, this doesn’t really matter for getting the new module to work, but, as I said before, you will not easily be able to restore the Fatshark to 2.4HGz operation.

A Schematic Emerges

I then traced all tracks and draw up the schematic for the A/V board.
Here’s a quick overview on the main componenents and how it works:

  • LM4880 – This is a stereo headphone amplifier. It get it’s input data either from the received audio stream or from the A/V input plug. The audio is amplified and output to the headphone plug.
  • MAX8211 – This is a voltage monitor. When the voltage on the THRES pin goes below 1.15V, the LED with light up, indicating a low battery condition. The voltage divider on the input translate the threshold by 5.7 to (5.7*1.15) so tha actual battery volatage being monitored, is 6.5V.
  • L4941 – This is a 5V, 1A voltage regulator, supplying a stable 5V to the A/V board, the receiver and the external display board.
  • Switches – One switch select which channel the receiver should be tuned to. The other is a power switch and also select various A/V modes. There is also a mini joy-stick for controlling brightness and contrast.
  • Other – An NPN transistor gates the video out from the receiver module to the display board and the A/V plug. The Video switch control whether it is open or closed. The N-channel FET is used as a switch to turn the power on or off for the display board that drive the LCD displays. Here is a peculiar design – there is a 200 Ohm pull down resistor on the gate to the FET. In combination with the zero Ohm resistor from the 5V supply, this means that a current of 25mA is continously “wasted” in this resistor. It’s “wasted” because it’s totally unnecessary, this resistor could be much bigger. I’ve contacted Fatshark about this, but they do not reply. I haven’t had time to experiment with a more suitable value, but will update when I get to that.

The birth of a new shark

Now, this is what it’s all about. The first picture here show the new receiver mounted on the back of the Fatshark A/V board. You can see the grounding wires and the piece of coax I used to connect the module to the Fatshark antenna connector.

Next is a picture of the other side of the A/V board, you can see a few yellow wires coming from the back. You can also see the solderpads from the old module. The pads that were below the antenna connector were really difficult to handle.

Here’s an unfortunately pretty bad picture looking in between the boards. The coax goes from the antenna pads on the RX module to the antenna connector that is mounted on the A/V board.

Here the board are installed in the goggles. Here you also get a good view of the display board. It’s important the modification doesn’t affect the movement of the displays.

And here’s a view from the other side. You can see that I added some double sided tape between the boards. Not so much for keping them in place, but more for isolation.

I have only tested the googles with the 5.8GHz indoor, as there’s sh*tloads of snow outside. Performance was not that great, but that was to be expexted when the signal had to go through walls e.t.c.
As soon as I’ve tested them outside and got some range data, I’ll add it here.