Proportional pan Tx pot

edited November 2012 in Control Systems
Can anyone tell me what value pot I should use to replace the one on the end of the transmitter pan lever?
I want to make up a proportional dial to operate a geared unaltered pan servo, similar to Cris's SLR rig.


  • The one in my transmitter is a 5k pot. But its worth checking the one in your Tx with a meter, just to be sure.

  • Andrew - I just went looking through my old notebooks to see if I kept a record of the potentiometer used. I didn't. I recall taking the transmitter apart and checking components during disassembly with a multimeter, writing the relevant values on the Bristol board covering my workbench. But like Tom, I recall the transmitter using a 5K pot for each channel. Depending on the transmitter there might be trimming resistors fore and aft of the control potentiometer.
  • Many thanks. I did measure mine and was getting 1 to 2k but your confirmation is reassuring.
  • edited November 2012
    Here's what I wrote on pots and trimmers in '99 it discusses why to use lower range pots with trimmers instead of the standard 5K. Also describes setting up for limited shutter servo movement to avoid straining servo and camera button..

    "Some study of how potentiometers and trimmers work by using a resistance meter will help here. Some articles in Aerial Eye have described variations on this method of control. It is an excercise in logic for some long winter's nights with a soldering iron. For the pan and tilt, I found that 2K pots with trmmers set at approx 1.2 K on each side give the optimum control, though a lot of fiddling with the trimmers when the rig is set up will help give the perfect settings. By using a narrow range (2K) pot with trimmers allows the working range of the servo on the rig to be covered by the full angular deflection of the pot. Most servos will immediately give an increased angle stop- to- stop as well through this method. The shutter servo is controlled by a switch rather than pots, so I wired the switch to add in the correct resistance value via a trimmer to move the servo just enough from the resting position (another trimmer) to just press the shutter button. I found that I used four trimmers for this circuit, but I could not get the delicate control, resting and triggered positions with less. that I am sure is down to my lack of understanding of all but the simplest electronics. My friend Jan insists that what is needed is a capacitor to "smooth" the action when the switch kicks in the extra resistance. Luckily for me, my arrangement works as smooth as silk as I set everything up with the trimmers to only create just enough servo movement to trigger the shutter and no more."

    The article can be found on way back machine :
  • 5k worked perfectly. It was a very easy mod with the old lever pot unit just unplugging from the board. I reused one of the plugs for the 5k pot and plugged it back in.

    I also shortened the tilt lever, stiffened the action and limited the range. Very happy with this rig now.

    KAP radio mod
  • Yup. 5k pot will do it - discussed here. You can also buy ready-made gizmos from Brooxes and KAPshop.
  • Simon, thanks for posting that excerpt. That makes for good reading.

    The only radios I've done this mod on were programmable, so getting the exact range I needed was a matter of software rather than hardware. But I started doing KAP with a venerable 72MHz Futaba I since gave to a friend and fellow KAPer. I haven't touched base with him on KAP for some time. Next time I talk to him I'll ask if he'd like a modification on his transmitter like Andrew's. Your 2k+2x1.2k trimmers sounds like a really nice solution.

  • Here's a demo of the rig.

  • Funky. Now you need to add a built-screen for the video downlink!
  • If this done with an unaltered servo what's the maximum rotation? It would sure be nice to have something close to 360 degrees.
  • edited July 2013
    Rick, depending on the tx you may get up to 180 degrees I guess... For this purpose I use a 'one turn sail winch servo' (unmodified). This will get you a bit more than one turn.

    Here's an example:
    You can find cheap ones on ebay or hobbyking
  • It looks like in Andrew's video he's getting > 270 or so...
  • Yes it's a standard unaltered servo for pan and I get a bit less than 360º rotation with 5:1 gearing (I think. Will measure it more closely after work tonight)
  • Sorry for my confusing comment, I didn't look carefully at Andrew's rig. What the sail servo does is without any external gears.
  • It's just dawned on me...I'm running Brooks DELUXE REDUCTION GEAR PACKAGE so I'm at a huge disadvantage at 1:4 reduction compared to Andrew's 5:1 multiplier. Guess I need to rethink this all.
  • Take a look at the gearing offered on Servo City. You may be able to keep the Brooxes hardware, but swap out just the two gears. Servo City offers a lot of their gear and belt hardware with Futaba or Hitec splines, so they should just drop into place if you can find a pair that has about the right spacing and ratio.

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