Almost every modern radio system offers the choice of servo reversing in every channel so that the servo controlled by the reversed channel reverses it's movement. But, there are some cases the above function doesn't work. Something like that typically happens when two or more servos are connected to the same receiver channel (using a "Y" cable). Then, we may need to reverse only one of the servos without affecting the other (so we cant reverse the whole channel from the TX). This usually happens in airplanes with dual servos when they are used to control the same control surface. If the reversing could be done mechanically (by changing the control arm's position) then the problem could be solved, but usually its not that easy.
Solutions
1) The first solution modellers "found" to reverse a servo was quite
complex:
The servo had to be disassembled and the wires on the control
potentiometer and motor needed to be reversed. This wasn't a very
easy job, and sometimes ended up with the servo "fried". So, this
method is no longer suggested as a solution (see the other solutions below)
2) A second (and of course better) solution was given by the servo manufacturers who build versions of their servos which are reversed from the factory. The main disadvantage is the fact that these servos may be hard to find and may also cost more.
3) Another solution is to use servos from different manufacturers. It is generally known that servos from different manufacturers work in a reverse way compared to each other, but still they are totally compatible to the same receiver input. (for example, Hitec and Futaba servos) This fact also eliminates the 2nd solution's "hard to find" problem.
The above solutions (2 and 3) have one big disadvantage: They need another servo to be bought. If there's already a servo available and to eliminate the extra cost of buying another one, we can use a servo reverser. Its a small device (circuit) connected between the servo and the reveiver. These kind of devices can be found in hobby shops and dont cost too much too. Below there is info about the circuits involved in one of the many versions of servo reversing circuits.
Construction
The "theoritical" logic circuit is the one below:
Operation theory
(Under constuction)