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.
(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.