Add Dual Rates to a Transmitter's Channel

Being able to control the maximum travel or endpoint of a channel is commonly called dual rates. On an airplane, it can be used to limit the travel of a control surface to make the aircraft more controllable for landing and take off. Then they can be expanded for aerobatics. On a surface vehicle, it might be used to limit the steering travel so that it doesn't make sharp turns and spin out. I originally found this guide in relation to a sail winch, but I can only guess at the utility in that application since I don't operate RC sail craft.

My personal usage of it is as a throttle endpoint adjustment. Many radios come with steering dual rates, but most of those don't also allow for throttle dual rates. My main reason for controlling throttle endpoints is to allow my young son to drive, he simply doesn't understand variable throttle and holds the trigger at wide open throttle all the time. It also makes casual driving nicer and of course is useful when letting someone else drive for the first time. I started out modifying two of the cheap transmitters that come with the 1/16 Traxxas E-Revo and Summit, a TQ. I then did it to the TQi that came with my full size Summit and most recently (for this article), a pair of Spekrum DX4S radios. These photos show the finished product, as you can see you'll end up with the knob of a potentiometer sticking out the side which you can dress up however you like.

The original idea: Transmitter Modification to add end adjustment for sail servo and a post in this forum thread titled Help!! technical Help Needed - Rate Switches

Step 1: Required Items

• Adjustment - 10k or 5k ohm linear potentiometer.
• Trim - 2.5k, 1k or 500 ohm linear potentiometer.

I've used three different types of 10k ohm potentiometers successfully for adjustment. On the throttle channel, 10k works very well and I'm able to turn it down low enough that my cars barely move. For steering, I'd use 5k since going straight isn't very useful. I've also only used single turn potentiometers, but a multi turn might be interesting. Most recently, 3310C-101-103L-ND because it has a small shaft and I hoped to put a knob on it. Previous to that, 987-1279-ND because it has a nice big shaft that just acts as the knob. I also used 987-1382-ND (which has detents) with the idea of having the main shaft act as throttle dual rates and the secondary shaft act as steering dual rates. But steering dual rates weren't important enough to bother with, so I never used the secondary shaft. Of these three, I prefer the first for the small size and the second for looking good and being easy to use. I wouldn't bother with the third. Any linear 10k or 5k ohm potentiometer will work, feel free to browse Digi-Key or your favorite electronics supplier and find the best one for your application. Go to town with Digi-Key's Rotary Potentiometers - Linear section, but be sure to filter the list down to only the values you want... and even then, the list is overwhelming.

For the trim potentiometer, I've tried 10k and 5k ohm so far and they're too much and it ends up being extremely sensitive. The problem with both 10k and 5k is that the range used is probably 1/10th of the total range so adjustment is very finicky. But I don't have anymore transmitters to modify, so I'm not sure what would be perfect. If I were you, I'd take a risk and order a 1k and 500 ohm potentiometer instead. And of course, the original article even suggests a 1k ohm trim potentiometer. Here I've also only used single turn devices, but a multi turn would probably be perfect for this application due to the higher precision. Most recently, I got 3386F-502TLF-ND because it has a nice, easy to turn knob. But this is a 5k trim pot, I only recommend the style. I would probably get 3386P-102TLF-ND, a 1k or 3386F-1-501TLF-ND, a 500 ohm potentiometer. At Digi-Key look in their Trimmer Potentiometers section and make judicious use of their filters to only show what you need.

You'll also need wire, shrink wrap, a soldering iron, screw drivers, scissors, and who knows what other tools.

Step 2: Disassemble Your Transmitter

In my opinion, the hardest part is disassembling your current transmitter. In the case of the DX4S, there are a number of screws hidden under stick on decals. In the case of the TQ and TQi, it was simpler since none of the screws were hidden.

Step 3: Find the Potentiometer for Your Channel

On this DX4S, I had to remove the trigger mechanism to see it's potentiometer and the connections. Then, because the wires weren't obviously marked, I had to figure them out. The wiper is easy, it is in the middle. To figure out which edge is positive and which is negative, you'll need a multimeter. In my case, positive is the green wire on the right, negative is orange on the left and yellow is signal. In the TQ and TQi, the wires were red and black, making identification trivial.

Step 4: Crude Schematic and Written Description of Connections

The above image from wikiHow's article on wiring a potentiometer should help identify which leg is 1, 2 and 3.

The diagram doesn't point out the existing connection on the main pot because it doesn't matter, you're not going to change this. The trim pot will simply piggyback its positive and negative connection.

Potentiometers:

• Main Pot - This is the potentiometer connected to the trigger or stick, the one already in your transmitter
• Adjustment Pot - This is the potentiometer that you'll use to adjust the dual rates.
• Trim Pot - This potentiometer will center everything so that the midpoint doesn't move when the adjustment potentiometer is changed.

Connections, as text:

• Adjustment Pot Pin 1 <--> Main Pot Pin 2
• Adjustment Pot Pin 2 <--> PCB signal input (former location of Main Pot Pin 2)
• Adjustment Pot Pin 3 <--> Trim Pot Pin 2
• Trim Pot Pin 1 <--> PCB negative (piggyback with Main Pot)
• Trim Pot Pin 3 <--> PCB positive (piggyback with Main Pot)

For me, looking at the diagram wasn't very helpful. Even reading all the text at once didn't help. They key for me was to look at just one thing and make that connection. Then move on to the next. By the end, everything is connected and you're set. For me, a combination of the pictures and text is what helped the most. For each transmitter modified after the first, I'd just open one up and copy the wiring. It was easier for me than trying to decipher the original description I found of this on the internet.

Step 5: Prepare the Adjustment and Trim Potentiometers

I start with the adjustment potentiometer and I'm using servo wire, so I put black on 1 and red on 3. I use yellow for signal on the wiper, 2. I've got heat shrink tubing on the legs, just in case. The trim potentiometer gets black on 1 and red on 3, this one will eventually be hooked up to positive and negative power. Finally, red from the adjustment potentiometer's leg 3 goes to the trim potentiometer's wiper, leg 2.

Step 6: Wire the Adjustment and Trim Potentiometer Into the Transmitter

This step finally involves modifying the electronics of your transmitter. You'll disconnect the wiper / signal wire from the existing potentiometer and connect it to leg 1 (black) of the adjustment potentiometer. You could wire it directly, but I've found the existing wire just isn't long enough to work with.

The wiper (leg 2) from the adjustment potentiometer replaces the main wiper connection on the circuit board.

And finally, from the trim pot, pin 1 (negative) connects to ground and pin 3 (positive) connects to positive on the PCB where the main pot is connected. It is a piggy back on the existing connection rather than a replacement. That might be a little hard to tell from the pictures.

Edit: In one of the images above, the blue Trim Pot is mislabeled Adjustment Pot.

Step 7: Trimming Dual Rates With the Trim Pot

I think the second hardest part is getting it trimmed. If you have a computer radio, you can put it into monitor mode and watch the channel involved. If you don't, you'll need to hook up a servo to the channel in question to watch for movement. I actually did all my non-computer radios with throttle hooked to the ESC and the car on the ground. Obviously, this can go terribly wrong... but if you're lazy like me, it is easier than pulling out a receiver and a servo.

To start with, put the trim pot close to the center of its travel range. Now, twist the adjustment pot to either one of its extremes. Move the main pot between extremes, in my pistol radio example this is the trigger. If the servo or monitor doesn't move much, you're at the minimum dual rate setting. If it moves all the way, you're at the extreme. Keep that in mind.

The goal for the trim pot is to have the center point not move while you're turning the adjustment pot. You'll need to move the adjustment pot back and forth while you fine tune the trim pot. Eventually, you'll get to a point where the servo doesn't move or the monitor barely changes. Once you're there, you're done with the trim pot.

Step 8: Drill a Hole and Reassemble

The hard part is done. Now just pick a spot and drill a hole that is the right diameter for your adjustment potentiometer, stick it through and tighten down the nut that holds it in place. I'd suggest mounting it on the half with all the electronics so that you can take it apart without a bunch of wires keeping the halves together.

Reassembly will of course be the reverse of disassembly. Pay careful attention to wires inside and make sure they don't get crushed or cut by any of the pieces that come together. You should tuck the trim pot inside somewhere it won't get bumped by moving parts and ruin the adjustment.

I like to give everything a little test during parts of reassembly, to make sure it is coming together correctly. That way, if something went wrong you'll find it before you get everything back together.