Build your own diversity controller!
Ever since I discovered that most RF electronics have an RSSI (relative signal strength) pin on them I have been finding different ways to use it to my advantage. In this case I can use them to make a diversity system for a fraction of the cost of a conventional diversity controller.
Sure this requires a little more work than simply buying a diversity controller, but my controller uses less power and also allows me to power my entire base station from a single battery. Even my video glasses and recorder can be powered straight from this controller! I even put a voltage monitor on it. Now more worrying about charging multiple batteries and worrying about any one going dead.
So rather than spend the $180 or so to buy a video diversity controller, I decided to design my own that I could share with the rest of the FPV forum. This tutorial requires a little bit of electronics knowledge and some soldering skills, but the basics are all lined up so a novice shouldn’t have any trouble. This unit is very low power and actually fairly easy to build. I encourage even a novice to try this. The project should take about 3-4 hours or so.
While I do not make any warranties to this system, I will happily assist all who decide to build it. This system works very well for me and essentially doubles my flying area.
UPDATE!: Version #2 has been built and works very well. It powers EVERYTHING I need which includes both sets of video glasses, both RX units and even my recorder all from one battery! It even has a voltage monitor. I added photos of the new station.
The parts list
Parts list for the Version #2 diversity controller:
2 - LM358 OpAmps
1 - Perforated cicuit board
2 - 100K potentiometer
4 - 15K resistors
2 - 41K resistors (or 2 - 22K resistors if you are building a positive scaling model)
3 - 2K resistors
2 - 47 uF capacitors
1 - 500K resistor
1 - MAX 4547 video switch
2 - LEDs (your choice of color)
1 - RCA cable (phono jack type)
1 - phono jack (female)
For the V1 Diversity switch:
1 - LM393 voltage comparator (an LM339 works as well and can be found at Radio Shack)
1 – LM358 Opamp
1 - 12V DPDT relay (I’d get the PC board type, but the automotive type works in a pinch)
1 - RCA cable
1 - RCA female output jack
1 - UA 7805 voltage regulator
1 - pcb board (2”X4” is sufficient if you are decent at packing electronics)
3 - 10K resistors (1/4 Watt)
1 - 2K resistor (1/4 Watt)
1 - NPN transistor (2n2222A, BC547C, or similar)
2 - 47uF Capacitors
1 – 100K potentiometer
For the system:
2 - video receivers
2 - Video antennas (I used 2 patch antennas, but you can use Yagis, Patches, Omin directional dipoles, or any mix)
A 12V battery
Voltage monitor or volt meter
Red LED/green LEDs
2 – 100k multi turn potentiometers
Perhaps the best source for these parts is Jameo Electronics. They seem to be centered around the electronics hobbyist. Another good vendor I deal with is Allied Electronics. However, I find myself ordering from Ebay very often when I need just a few parts and need them cheap. Most of these items can even be bought at your local Radio Shack.
How it works
The diversity switch works by tapping the RSSI (relative signal strength indication) pin and comparing the voltages between the two RX units. It then selects the one with the highest signal strength and switches to that feed. It will remain with that feed until the signal strength drops below that of the other receiver, where the unit will switch back.
The unit can be broken down into 2 parts: The signal comparator, and the channel selector.
This unit does the actual selection of signals. Its inputs are the two RSSI voltages and it outputs a 5V or 0 V output. This tells the channel selector which unit to select. Once selected, the output is then buffered through an adjustable Schmitt trigger circuit (made by adding a single potentiometer to the LM393) to keep from switching unnecessarily fast between the two units which may cause disturbance.
EDIT: Version#2 - This version no longer needs a 5V regulated supply. It uses 12V.
This is nothing more than a relay controlled by the output of the voltage comparator. Nothing fancy here except that it is driven by a transistor so that a 12V relay can be used with the 5V output from the comparator.
EDIT: Version 2 - The relay is now an IC. This promotes faster switching.
Building the signal comparator
To make this system work you must have to very similar if not identical receiver modules. The modules can vary even within the manufacturer, so check you receivers RSSI pins to verify that they have the same voltage range. Locating the RSSI pins on your receiver modules is not difficult. If you donít know where they are, you can easily find them by checking your voltages on each pin and turn on and off your video TX and see which one changes. Itís usually pin #8 if you have a Comtech module, but Iíve also seen pin #6 on other models.
Now that you have located the RSSI pin, you must identify the scaling direction. If the RSSI volage INCREASES with signal strength you have POSITIVE SCALING. If RSSI voltage DECREASES with stronger signal, then you have INVERSE SCALING.
In the case that the RSSI pins do not have the same range, you must use a potentiometer to adjust the voltages so that they are identical (at least on low signal out). This is simple. All you need to do is connect the RSSI wire to one of the fixed terminals of the potentiometer and the other end of the potentiometer to ground. Connect a wire to the adjustable pin of the potentiometer. Now adjust the potentiometers so that the output voltage is the same when there is no signal (ie. TX not powered up). The high signal strength adjustment isnít critical since if you have a good signal, there is no need to switch.
Voltage regulator: (no longer used in V2)
The first thing you need is to obtain your 5V source. Solder in the uA7805 regulator. It will share a common ground with your 12V source. I had my 12V source also connected to the jacks for my 2 receivers and also had a voltage monitor I bought from DPCAV. This way I could monitor my entire system and power it from a single battery. I also connected a cable to the 5V source to power my video glasses as well. To make it, I simply cut and old cell phone charger end and used that.
You should need to have a buffer in the circuit to reduce the impedance of the RSSI and disconnect it from the circuit when it changes state. For this I used a LM358 OpAmp. It is a dual unit chip and can be found for dirt cheap on Ebay. This can be powered from the 12V source or the 5V source. It doesnít matter. I chose the 5V.
Connect your RSSI wire from your module to either (+) terminals of the LM358. Connect the (-) to the output terminal with a jumper. This is a simple opamp buffer.
The voltage comparator: (V2 uses the LM358 instead of the 393)
Despite having the same circuit description and denotation as a OpAmp, the LM393 is not really an OpAmp. It is more an impedance amplifier. Thus you need to provide power to itís output terminal through a resistor. The voltage comparator works by either making a conductive path for this resistance (effectively bringing the output to ground) or by giving high impedance (making the output the full 5V).
Connect the (V+) of the LM393 to your 5V source, and the ground should be connected to the (V-) or (GND) terminal of the LM393. Now connect a 2K resistor between the V+ terminal and the output of the LM393.
Connect the 10 K resistors to the (+) terminal of the LM393 and the other to the (-) terminal. Connect the other end to the output of each buffer. A capacitor should be placed from the (+) and (-) terminals of the LM393 to ground to allow some delay in channel selection. The smaller the capacitor, the shorter the delay. Instantaneous selection will likely cause frequent switching and therefore some delay is desired. Now install the potentiometer with the variable pin connected to the output of the LM393 and either of the other pins (but not both) to the (V+) terminal of the LM393.
Schematics are attached.
This is a bit more advanced, but offers more versatility.
The buffer now has a potentiometer before the RSSI's enter the LM358. This must be calibrated. The way to do this is to turn on both VRx units, but not your VTx. Adjust the potentiometers so that the RSSI readings are equal on the inputs of the LM358.
There is also a gain feature. This is the feedback resistor network (the 22 and 15K resistors). This gives better sensitivity at low signal strength. The 22K resistor can be replaced with a potentiometer for instances where you have two completely different modules in which the RSSI values scale differently. In the event that the RSSI values scale in different directions, you can reverse the input of either of the LM358 inputs and it will scale the opposite direction.
Version 2 no longer uses the LM393. It now uses 2 LM358 chips. This means it can now be run completely on a 12V source. No 5V regulator is needed.
Building the channel selector
The channel selector is simply a DPDT relay the is activated by the LM393 through a transistor connected to itís collector. Install your 12V relay. Connect one end of the coil to your 12V source. Connect the other end of the coil to the collector of the transistor.
Now cut your RCA plug in the middle and strip off about an inch of the covering. Connect one of the cables to the NO contacts of the relay and connect the other cable to the NC contacts making sure that your wires go to their respective terminals. Install your RCA jack and connect each pin to the (Common) pins of the relay via hook up wire.
Now connect a 10K resistor to the base of the transistor. Connect the emitter directly to ground. Run a jumper wire from the output of your LM393 of your voltage comparator to the 10K resistor.
And there you have it! All you need to do now is test it.
There have been a few improvements made here as well.
Video IC chip
The relay has been replaced with a Max4547 chip. this chip can be had as a free sample from maxim. It's part# for the PDIP version is MAX4547CPE. The MAX4547EPE also works. Here is the MAX4547 dtatsheet. This greatly increases switching speeds. All of the video negative terminals (the bare wire in the cable) can be soldered together. The positive wires (the insulated ones) are sent to the inputs.
Added in V2 is a manual override. This is a DPST switch with a center off position. I would velcro it to you RC transmitter for ease of use. There may be times when you want a manual switch of video feeds or when the controller switches and you didn't want it to switch, so you may now simply throw a switch and get the video feed you want manually. This in an optional part, but I highly recommend using it.
Also improved is the signal identifier cicuit. It is no longer used in conjuntion with a relay, but now may be driven directly by the channel selector. This is still an optional part and may be omitted.
Testing and caibrating
Power up your system with both RX antennas connected. Verify that you have a video feed. Now move the video transmitter away (20 feet should be sufficient) and change the channel on either of the RX antennas. If your video feed remains, then change the channel back and change the channel on the other RX. Again, you should have a stable video feed (you might get a short static). If not, you need to either reverser your RCA cables to your receivers or reverse your RSSI inputs to the diversity controller. It doesnít matter which.
Now for the selector sensitivity. This is set by turning the potentiometer. Move the video transmitter as far away as possible. Aim one of your directional antennas directly at the transmitter. Now slowly turn the directional antenna away from the transmitter. You should hear a click on the relay (if you added the LED indicator it should change). Now bring the first antenna back to the transmitter. Again you should hear a click of the relay and the indicator should change. If in this process the relay being to switch rapidly, turn the potentiometer to a higher resistance until it stops. If the relay doesnít change states then turn back the potentiometer.
You are now ready for flight. You can adjust the potentiometer to suit your flight needs. The final adjustments will be made after a flight or two. I have mine turned almost all of the way up to the maximum resistance.
Again, please forgive me for the poor drawings. I don't have any drawing software. All I have is MS paint
Let me know how this works out for all of you. If there are any questions, feel free to post here or send me a PM.
Keep your wings in the sky!
I won't be building these and selling them. I hate soldering up parts. If you can find the RSSI pin on your RX, you probably can build one of these. Just get a good soldering iron with a narrow tip and keep it hot. I use this adjustable heat soldering station, but a 30 watt iron works fine if you coat the parts in soldering flux.
The explanation is provided as additional information for educational purposes and trouble shooting. If you read and understand basic logic controllers, you can figure out how to modify this for even more Rx units to increase your system reliability even more.
I will take pictures of my circuit board when I get home (I'm out of town for the week).
+1 where to buy ...... Digi key?
This was build on a 2"X4" perf board from RadioShack.
All of the parts can be bought from Allied Electronics or Jameco Electronics. However I find Ebay very useful for quickly finding what I need any buying it for cheap. Digikey is also a good source. Heck, in a pinch even Radio Shack carries almost every part except for the LM358 and the voltage monitor. Jameco even has voltage monitor kits. They seem to be centered around the hobbyist electronics gurus. Probably the best source is Jameco.
There are many tutorials in my blog. Everything from antennas, to scratchbuild airplanes to motor construction is there and it's not clutterd with junk either. My purpose on here is to educate and be educated for the benefit of those who enjoy the RC hobby.
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