Analog RSSI for FPV conversion

[jolyboy]

Today I managed to get RSSI working for FPV using a 2.4 GHz FRSky Rx . I want to post info here, since it was extremely difficult to find info on it this project . This would also work on futaba receivers.

What is RSSI:
RSSI will give you an indication of how much range you have remaining before failsafe kicks in. The trouble is, getting this info displayed on the screen is difficult, because it is in a digital format. Most OSD's need an analog signal.

How it works:
Every time the rx receives a good packet of information from the transmitter, the green led flashes. If 100% of the packets are good, the green led flashes quickly and constantly. As packets get lost with an increasingly bad signal, the green led flashes less. If we were to use this signal on our OSD, the signal would be rapidly changing between 100% and 0% with nothing in between.
The conversion takes the green led signal, and smoothes it out so the quickly flashing led produces a higher voltage (good signal), and the slowly flashing led produces a low voltage (bad signal).

I used the following info:
http://translate.google.com.au/trans...ed=0CDMQ7gEwAg
http://www.fpvuk.org/forum/index.php?topic=1955.0

The difference with my one versus theirs is that everyone else seemed to go for an external module to convert the signal. I HATE adding extra modules on my heli, since it is decked out with too much stuff as it is. I managed to build it small enough to fit INSIDE the rx. I used one of the spare analog ports as the output. Have a look:











The result is really good. The max voltage for me is 1.64V, and minimum before failsafe is about 0.2V. I tested it by removing the antenna off my tx (to exaggerate the signal loss) and went downstairs. My building is double brick, so I only got down 1 level before failsafe kicked in. Its really good, because by watching the voltage you can see just when it will drop out of range. The dropping voltage is gradual, not like the "cliff" you get with typical digital signals.

The max voltage can be changed by changing the 68kOhm resistor with a different value.

[jolyboy]

Just to follow up on this, I did an on ground range check. At 1.5km range, RSSI voltage sits at around 1.2-1.3V. Any trees or obstacles between rx and TX causes voltage to drop considerably, and causes the TX to alarm out, as expected.

Line of sight flying at maximum visible distance gives RSSI voltage between 1.50 and 1.60V, depending on conditions (antenna orientation, weather etc). Based on this, the frsky TX has a theoretical range far far greater than what is legal in most countries, provided there is nothing to block the signal.

Sent from my GT-S5570 using Tapatalk 2

[mbelley]

Hi,

Can you give me resistors value. I tryed to figure out these value but i need more practice!

Thanks, sorry if i ask two times, I was not certain if first time worked

[jolyboy]

See the link in the first post for more info. Resistor values are 68ohm, 10kohm and 100kohm.

[mbelley]

Thanks,

I will try it soon

mbelley