In an attempt to get a cheap radio system that I could use with convoy vessels, I did a little hack and slash with a 4 channel DSM2 transmitter this week. The transmitter came with my ready to fly helicopter, but I now I fly the helicopter with my computer radio. The problem with the 4 channel transmitter is that it has no end point adjustment and no reversing. To add features to the transmitter a little de-soldering is required. Both joystick assemblies need to be removed by de-soldering the leads on the potentiometers (red ellipses) and where the assemblies are held down at the corners (blue circles). There is one nuance with taking the joysticks off. There is a grey plate attached to the joystick that extends under the switch right next to it. You can de-solder the switch and then solder it back on once the joystick is removed, or you can just cut the tab between the switch and the joystick like I did. To add the features, I had to add some additional circuitry and components. For the rudder and throttle channels, I added end point adjustment and a reversing switch. The reversing switch on the throttle probably isn't needed, but I put it there anyway. I eliminated the joystick assembly for the elevator/aileron channels and replaced it with switches. Since I wanted a self-centering throttle, I moved the joystick from the right side over to left side. I set up the aileron channel for a pump, using a toggle switch and making it adjustable so that I could set the pumping rate on an ESC. For the elevator channel, I put two pushbutton switches with endpoint adjustment for guns. Here's the circuits: I made two small daughter boards, one to put the endpoint and reversing adjustments for throttle, rudder and aileron, and one to hold the gun switches and adjustment pots. If anyone wants to try this modification and has experience with Eagle software, I can send you the circuit files. The circuits work well, allowing you to separately adjust each end point on all the channels. There is a small wrinkle in how the end points work: if you only adjust one side of the channel, the center point will move. As long as you adjust both endpoints on the channel the same amount (which should be normal practice) the center point stays in the middle. Here's the setup after everything is connected. It looks a little messy, but everything still fits inside the transmitter case. I had some trouble getting everything to work, having one open circuit problem and accidently flipping the transmitter from mode 2 to mode 1. Wires were de-soldered and soldered several times in the process of checking out the modification. But in the end everything worked. I cut some holes so you can reach the adjustment pots and reverse switches, and then put it all back together. Overall I'm very pleased with the project. For about $5 in components, I added all the features I needed to have a useful radio system. The transmitter will be great for convoy vessels and is even good for up to a two gun cruiser. I'll upload a picture of the completed radio when I get home today.
