Sunday, July 8, 2012

FPV Made Easy!

DIY: The RCModelReviews FPV Backpack


By now most readers will have seen the RCModelReviews FPV backpack in action and many of you have expressed an interest in building one for yourselves.

Well here is where that information will be published.

What does it do?

The FPV backpack is a simple device that contains a 200mW 5.8GHz video transmitter, a 5V UBEC, a voltage regulators, a voltage dropper/filter for popular 9V-12V cameras, a low-cost board-camera and an optional microphone.
In its cheapest form, this will probably cost you little more than $50 to build but if you want best results, I highly recommend investing an a decent board-camera -- which will likely bring the price closer to $80 or $90 - but the results will be stunning!
By building all this functionality into a single unit that can be attached to virtually any model by a strip of velcro, it becomes cost-effective to fly a whole bunch of different RC planes (or boats or cars) using FPV.
The system is designed to work from a 3S lipo, usually the same lipo that powers your ESC and motor but in the case of a larger model that might have an IC engine or a 4-6S lipo, a small separate 3S pack can be used.

What you'll need

The heart of the system is the tiny 200mW 5.8GHz video sender modules which can be purchased online from a number of sources. The price of these modules varies from US$15 to US$20, depending on your source.
It is this module that converts the video and sound signals into a radio-frequency transmission that is received on the ground and used to send the video to your FPV glasses, visor or screen.
Although these modules claim to be 200mW, I'm not so sure they're actually that powerful but right now I don't have any equipment to confirm that suspicion (test gear for working with 5.8GHz signals is very expensive). Never the less, these modules work very well for the kind of short/medium-range FPV for which this system is designed.
These modules require 3.3V of power -- any more and the magic smoke will come out -- any less and the range will be considerably reduced. For that reason, the backpack has a 3.3V regulator on its circuit-board that ensures the modules are properly fed and watered (so to speak).
While this regulator could have been designed to accept the full 11.1V of a 3S lipo, that would have meant making it bigger, heavier and wasting more of your battery's power as heat. For that reason, a cheap 3-5A SBEC is used to drop the full battery voltage down to 5V before it goes into the 3V regulator on the backpack. A simple solution to an otherwise complex problem.
The circuit board also contains a second regulator, this one delivers a safe and constant 9V for the camera. By using this regulator, the problem of interference from the ESC is eliminated, where the backpack is powered by the main flight battery. It also means that the camera tends to run a little cooler -- which is a good thing.
Of course no FPV system is complete without an antenna and the RCModelReviews FPV backpack uses the popular clover-leaf, circularly polarized antennas developed by a gentleman calling himself IBCrazy. These are super-cheap to make and work much better than the traditional black plastic-covered dipoles that ship with most commercial FPV systems.
And, of course, you'll need a camera. See the RCModelReviews YouTube channel for the reviews of several cameras and decide for yourself whether you'll be able to get away with a cheap "under $20" camera, or whether it's worth spending an extra $30 for the "top of the line" high-resolution CCD camera with "wide dynamic range".
Finally, there will be a few cables and connectors to make up -- but these are pretty straight forward.

Why the delay?

Right now I'm finalizing a couple of small but important details in respect to the backpack design (testing, testing, testing) to ensure that it's as close to a 100% reliable design as is possible to produce.
To this end, I have removed one of the regulators and replaced it with a simpler and more reliable method of reducing the voltage fed to the camera.
Under extensive testing at elevated operating temperatures I found that there original 3-terminal regulator was getting too warm -- so I've ditched it in favour of a charge-pump setup using a diode and capacitor (don't worry -- unless you have a background in electronics your eyes should have glazed over just then).

So, bookmark this page now and come back in a day or so when the final design, bill of materials and other details will be posted.
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