Most radio-control transmitters are set up for the big-volume market, r/c aircraft. They have one or two 2-axis joysticks, and maybe some additional knobs or switches for extra functions, like flap trim and landing gear.
But what if you need something different? Like twin parallel-stick control for independent control of port & starboard boat motors? One way is to find an old Ace Nautical Commander, or a newer Robbe F14 Navy, an expandable radio that has an available twin stick unit:
But you'll lay out some big money for this unit.
Another way? Hack your own radio!
Here's an example of one I did. This one hacks the steering instead of the motor
controls, but it shows what's possible. I wanted steering more like a ship's
wheel, with several turns of a free-spinning wheel needed to move the rudder
from side to side. A bit of a challenge to use, not unlike a real ship's wheel.
I also wanted a center indicator, which I built into a sector and sight
glass.
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The mechanism is made up from scrounged gears and bits of hardware. A simple rotary pot is hiding beneath the big gear. The slot and pin limit the travel of the pot. An old single axis stick is used for speed, and a Radio Shack case houses it all. The guts are from an inexpensive Futaba 2 ch car radio. |
WARNING: This is not for the faint-of-heart, or the electrically-inexperienced. It's also against FCC rules to modify the internals of an r/c transmitter. But I've chosen to interpret the FCC's clauses regarding "remote operation" of the transmitter section to allow the joysticks and case to be modified, as long as the transmitting section isn't messed with. This is the de facto situation with radios like Robbe's, and aftermarket channel expanders like MCD's.
OK, first let's be clear- we're only talking about modifying the joystick
mechanicals, and potentially the radio's case.
Joysticks are nothing more than one or two potentiometers mounted in some
sort of frame with a lever control. With a 2-axis joystick, left-right movement
"turns the knob" one one pot, and up-down motion works the second pot.
In doing so, a variable voltage is presented to the radio's inner workings,
which then decide what kind of signal to send out on the antenna.
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A simple single-axis stick from an old radio. The pot has three wires. The lever-and-spring arrangement provides centering. |
What's a pot? Just a variable resistor, where we take an adjustable voltage off one tap. All pots are set up with three leads... one at either end of a resistor, and the third to an adjustable tap that slides along the resistor. A fixed voltage is applied across the resistor, usually 5 volts in logic devices. Since the voltage is being dropped along the length of the resistor, you can put a probe at different places and measure different voltages... close to 5v near the 5v end, closer to 0v near the opposite end. The adjustable tap does just this, allowing different voltages to appear on the center lead.
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It's important to note that the radio may be designed to use only a portion of the pot's travel... say between 1.5 and 3.5v. If higher or lower voltages are sent to the radio, odd behavior can be expected, like changes on another channel. So it's a good idea to check the min & max voltages that can appear on the center tap. Take a voltmeter, attach the ground (black) lead to battery negative, and attach the red lead to the pot's center connection. Move the stick through it's full motion, including trim adjustment, and note the voltage range. You'll want to reproduce this range with your hack. I've been told that Futaba Skysports, which are available on ground frequencies, use more of the pot's travel than most radios.
What to use for pots and levers? You can make up your own hardware, like my steering gear above... but it's probably easiest to use salvaged joysticks from old radios. These will have spring centering and/or detents, without which it can be hard to control things. But if you want you can start with a plain pot:
The 1-inch variety from places like Radio Shack works fine; make sure you get
a LINEAR wind, with 5k-ohm value, NOT a pot with a nonlinear/logarithmic wind.
You'll connect the two outer leads from the original pot to the outer leads on
the new pot, and center to center. If your radio has servo reversing, no problem
which outer lead goes on which new tap; otherwise, you may need to swap the
outers to get the desired direction of operation.
The Radio Shack pots come with long 1/4" shafts, which can be cut off as
needed. You'll need to come up with your own lever hardware; or panel-mount the
pot, and use a knob.
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You can also get
"slide" pots, which may suit your needs.
Check sources like Jameco and Digikey for much greater selection than found at the rat-shack. |
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You could also use a toggle switch
to merely flick between two resistance values, say for quick control of a
servo that is one position or another. The resistor values shown here
MIGHT work for you.
The 8 and 16 channel "expanders" from MCD are variations on this theme, with multiple switches and a computer on the receiver end to interpret the signals, and to operate multiple on/off devices. |
So that's the basics, the hardware is left to you. What about that twin-stick control for two boat motors? Try finding two single axis sticks from some toy or other old radio, and set them side by side. You might even try making new levers that have bends, bringing the two closer to each other.
There is one other way for controlling two motors- try a special mixer from someone like ActionKit. He has one that takes the input from 2 channels (throttle & steering), and depending on the combined position, will speed up or slow down the ESC's for either outboard motor, even to the point of reversing one when you steer at zero throttle (e.g., turn-in-place).