Hitec Lynx 3D Aux Channel Modification

I purchased my Hitec Lynx 3D radio from EBay just before BotBash 2002, since I needed a second radio to run Tangled Marionette, my returning 12lb robot, since Centrifugal Enforcer had laid claim to my ground-modified Futaba 6XA transmitter. I really liked the pistol grip interface, and even my wife (who was slated as backup driver in case Tangled Marionette and Centrifugal Enforcer had to fight in the same match) found the controls intuitive.

Well, the driving controls, that is. Fortunately Tangled Marionette was a Rammer, which meant I didn't need to use the 3rd channel to control a weapon. As you probably know, control of the 3rd channel is via a set of Up/Down buttons on the Transmitter face, far enough away that your "Steering Hand" can't reach them while holding the wheel. I've seen footage from Battlebots with one person driving and a second team member standing beside him working the Up/Down controls to fire a weapon. UGH!!! No way.

After attempting to drive my squirrely 2-wheel flipper-bot Scarab at Steel Conflict in February 2003, I came to the conclusion that the lack of Exponential settings on my Futaba 6XA were becoming a problem. On Tangled Marionette Mk1 I'd gotten by using the Expo2 setting on my Vantec ESC, but now I was using Hobby ESCs, Victors, etc. The aftermarket mixers don't offer a configurable exponential curve, so the only way to get expos is from the transmitter... I really like the exponential capabilities of my Hitec Lynx 3D, but that 3rd channel is just so hard to operate while I'm driving the bot. Well, I decided I wasn't going to lose any more matches due to a radio setup I wasn't comfortable with, so I found a simple modification that allows me to use the grip mounted DualRate Up/Down buttons to control Channel 3!

DISCLAIMER: I take ABSOLUTELY NO RESPONSIBILITY for anything you do to YOUR radio. I'm sure the FCC has some sort of rule against this sort of thing, even though this doesn't touch any of the actual radio components. The things you need to cut and solder to are fairly big and easy to get to (by PCB standards), but if you're clumsy with a soldering iron, or are worried about damaging your radio, find someone who isn't!

Still interested? You'll need the following:


The face of the Lynx3D. I've circled the offending Channel 3 Up/Down Buttons. I added a switch on the side of the housing to the right of the steering wheel. The arrow points to where it is on mine.
Start by removing the 5 phillips head scews on the back. The lower two screws are the ones you would remove to convert to left/right hand use. You might want to remove the frequency module and antenna too, although I didn't remove mine. In blue I've circled the backside of the Dual Rate rocker switch that will also function as Channal 3 Up/Down. The switch operates on both sides of the grip, for left/right handed use.
After pulling off the back panel you will see a row of plugs, some from the grip and some from inside the head unit. CAREFULLY document the order the plugs are inserted by color, and then unplug the row that connects to the grip. I've already removed mine here in the picture. Unscrew the connector PCB via the two screws circled in blue. The PCB mounts on a set of posts, pull it off of these, then remove the expansion ring via the four screws circled in green.
Then remove the head unit panel by removing the seven screws from the back, AND ONE SCREW FROM THE FRONT (to the right of the steering wheel). You don't need to disconnect the connectors from the small PCB as I did here, I was just trying to trace all of the signals. Once you've got the case out, be careful not to let the main PCB fall out, and DO NOT touch any of the chips or the connections for the LCD screen.
This is looking from the back with the unit "upside down". (If the antenna were still connected it would be pointed down.) There's a space to the right of the steering wheel pot and spring mechanism that you can fit a small switch. Using a small drill bit and a file I made a rectangular hole for the body of my switch and just friction-fit it in. You could add a dab of glue if you're paranoid. I soldered on the 3 leads from some extra servo cable I had lying around. I waited until after I'd inserted the switch to solder the leads on, this was probably a mistake. Solder this end first and then feed the cable through the hole.
I used Radio Shack part number 275-409, but any mico-size SPDT switch that will fit will work fine. This switch has a "Center Off", but it was all my local Radio Shack had in stock. An "On/On" switch would be better. I couldn't screw through one of the mounting ears even if I wanted to since the screw post for fastening the case together is right behind one of them, but its a tight friction fit.
OK, the meat of the project: It turns out that the Grip Mounted Dual Rate Up/Down switches already share two wires with the Aux +/- buttons that control channel 3. How lucky! Each up/down switch is just two momentary switches with one in common. The other two poles are connected to a "high" and "low" voltage respectively. The microprocessor samples the "common" pin to determine if either of the switches is closed. (When one is closed, the common pin is shorted to ground, when the other is closed it shorts to V+.) So, all that's required to get the Dual/Rate switches to function the same as the Aux +/- buttons is re-routing the common wire from the actual switch pair to the trace on the PCB that lead to the processor. You will probably still need access to the Dual Rate controls to setup your radio, but personally I don't mess with them in the middle of a fight, so I used a switch to set whether the center pin on the switch is routed to the "Aux +/-" common pin or to the "D/R +/-" pin it was originally wired to. This way, you can still use the D/R switch to setup your values, and then as the fight starts change the switch and use those buttons to actuate a weapon.
In blue I've circled the pins for the 3 wire connector that goes to the D/R switch in the grip. On my radio these are all green wires to the "patch" pcb, and then to the "Red/White/Yellow" set into the grip. The center pin is "common" and is shorted to either of the outside pins when you move the D/R rocker switch. In green I've circled the "common" of the Aux +/- buttons. You can actually see the trace that connects the 4 poles that are ganged together (the PCB uses DPST switches, so there are four pins that are the "common" for the aux switch circuit"). Solder the wire leading from one side of the SPDT switch to any of these four pins. (Black wire in my case)
Then you need to cut the center green wire after it comes off of the connector to the main PCB (leave yourself enough slack to splice into both cut ends). The end that is still connected to the main PCB goes to one side of the SPDT switch. This is the white wire connected to the green one (left arrow in the blurry picture). Lastly, connect the "common" or center pin of the SPDT switch to the common wire of the grip D/R switch (the other side of the green wire you just cut). This is the red wire at the right arrow in the blurry picture. Be sure to insulate each splice with a short length of heat-shrink tubing.
The wiring job, with the colors I used, probably easier than the blurry pictures. I didn't draw it in this picture, because I didn't want to confuse people, but the trick that makes this so easy is that the two outside green wires (left untouched) are already connected (one each) to the other pairs of switch pins for the Aux +/- switches. The microprocessor samples the "common" pin to determine if either of the switches is closed. With this mod we're simply redirecting the Dual Rate switch common to either the original microprocessor input (Dual Rate) or to the Aux +/- input.
That's pretty much it! You can temporarily hook everything back up to test without having to screw the case back together, if you want to. When reassembing, take care not to pinch any of the wires connecting the various PCBs. You may also want to label the function of the switch (or at least which position does what), since you never know when someone else will pick up or inspect your radio.

Copyright © 2003 Puppetmaster Robotics. All Rights Reserved. Original Images and Text are property of Puppetmaster Robotics.
Please contact webmaster@puppetmaster-robotics.com to request permission before reusing.