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OT: I give up, anyone want to give me 'voltage dividers for dummies'?

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  • OT: I give up, anyone want to give me 'voltage dividers for dummies'?

    I'm setting up a RC servo on a hardwired 'off the shelf' driver. It's circuit is likely something like this:
    Click image for larger version

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    So the position of the servo is controlled by the 10k pot voltage divider. What I need to do is mess around with that analog signal. Lets say that 270 degrees rotation on that pot drives the servo 90 degrees. I want to set up trimpots to be able to do something like drive the servo 60 degrees with 30 degrees of pot rotation, and be able to set those endpoints. All this is easy to do using software on a modern aircraft RC transmitter, but that's not the project.

    I've been messing around with circuits on this simulator site http://lushprojects.com/circuitjs/circuitjs.html, but am not getting what I need. Here's what I have, but it ain't right. The only thing working is the position trimmer. I have no idea how to expand the movement range from the primary rather than compress it other than start with a higher resistance pot, and how to make the end trims affect only one end of the range.

    Click image for larger version

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    It's just been too long since I had to learn this stuff in HS! I know this is child's play for some of you guys, so any help would be appreciated. Thanks.


    Location: Jersey City NJ USA

  • #2
    Did you program the Pic?
    There is quite a few Pic circuits out there for a RC Servo.
    Also I believe Picmicro have app notes on it.
    Easier to configure it in the Pic. !
    The range may be configured in

    Comment


    • #3
      Originally posted by MaxHeadRoom View Post
      Did you program the Pic?
      There is quite a few Pic circuits out there for a RC Servo.
      Also I believe Picmicro have app notes on it.
      Easier to configure it in the Pic. !
      No, I'm just buying a cheap off the shelf controller with a pot. I'm sure if I wanted to get into it, arduino etc would be great, but I'm trying to keep this foolproof and simple to just tweak the trimpots. Perhaps I should have used a 555 based circuit for the example to keep the idea simple.
      Location: Jersey City NJ USA

      Comment


      • #4
        Yes, I meant to suggest if not pic familiar or have the original program, a 555 based circuit may be better, also configurable.

        Comment


        • #5
          You will get better results if you use lower value trimpots for end adjustment - perhaps 1k. And you might want to add a resistor to the arm of the position trimpot. I assume that is just a fine trim for the position set by the primary control pot. You could add a quad op-amp like an LM324 so as to provide a high impedance on the trimpots and a low impedance output voltage that won't be much affected by the loads.
          http://pauleschoen.com/pix/PM08_P76_P54.png
          Paul , P S Technology, Inc. and MrTibbs
          USA Maryland 21030

          Comment


          • #6
            It will take me a little while to do the circuit but there are some details that I will need. WHICH end of the range do you want to keep? The low or ground end? Or the high or V+ end? And can I assume that mechanical adjustment will be used to "zero" that fixed end of the range?

            And how accurate must that 60 degree range be? And what is Vcc and is it a regulated Voltage?

            I can give you some initial thoughts now. The value of the 10K pot is a very common one that is used when the variable Voltage will go to an IC. Most linear/analog inputs on ICs are in the area of 1 MOhm or more. The 10K value is 1% or less of that value so there will be little "loading" effect on the Voltages generated by the pot. There is a lot of room for slop in this, but it is a good idea to keep the overall impedance of the pot circuit, as seen from the IC's input, somewhat in that range. Generally, lower values will be OK, but will draw more current from the V+ supply. Higher values will start to show loading effects due to the input impedance of the IC so they should be avoided - within reason anyway.

            The general idea of they type of circuit you will need is:

            1. You presently have a ratio of 270° of pot rotation for 90° of servo rotation. That is a ratio of 3/1. More important is the 90° of servo rotation per Vcc. Since I do not know what Vcc is, I can't go further there. But it is probably 5V or perhaps 12V. You want 60° of rotation and it would seem that you need about 2/3 of Vcc as your adjustment pot's range.

            2. Inexpensive pots (under $50) will not have a precise angular range and often their resistance curve goes flat at one or both ends for several degrees of rotation. Also they can often never reach zero ohms at the two ends. This is due to the way that inexpensive pots are made.

            3. Since you only need to adjust one end of the servo's movement range, you only need an adjustment on one end of the main pot. This adjustment will be in the form of a series resistance. If a trimmer pot is used, it will be wired as a simple variable resistor (center wiper is connected to one end).

            4. The main pot will need to drop 2/3s of Vcc and the trimmer pot will drop 1/3rd of Vcc. It is probably that easy. So the value of that trimmer pot AFTER ADJUSTMENT will be about 1/2 of the value of the main pot.

            5. Working from the 10K recommended value of the main pot in your circuit, the desired value of the new, main pot would be 2/3s of that 10K or 6.67K. You will search far and wide and never find that value. Generally speaking the available values of pots will jump from 4.7K (or 5K) to the 10K value so in the real world you are stuck with one of those two values. Lets say you keep the 10K because you already have that value.

            6. Then the trim pot, AFTER ADJUSTMENT will need to be about 5K. Now, you must decide how much adjustment range you will need and how accurately you will need to make that adjustment. You do not want just a 5K pot as it would be at or very near one end of it's range. One option would be to use another 10K pot which would have that 5K value around the center of it's range. But this would give the coarsest adjustment. Another option would be to use a 5K pot and an additional 2.4K fixed resistor in series with it. The added 2.4K of resistance would place the desired adjustment point at approximately the center of the 5K pot. (2.4K is a standard resistor value while the calculated 2.5K value is not and would be more expensive for no reason.)

            Many other values for a fixed, series resistor can be used as long as they are less than that 1/3rd value of the main pot (10K). For example a 4K fixed resistor (3.9K for a standard value) would work in conjunction with a 2K pot to put the 5K total at the center of the 2K's range. As you approach that 1/3rd value for the fixed resistor and simultaneously decrease the value of the trim pot, the fineness of the adjustment will increase while the range of that adjustment will decrease. The general idea here is to keep the 1/2 of the trimmer pot's value plus the additional, fixed resistor's value close to that 1/3 value (5K). Any such combination should work.

            I will draw the circuit and post it as soon as I can. That will probably be a few hours as I have a prior commitment.
            Paul A.
            SE Texas

            And if you look REAL close at an analog signal,
            You will find that it has discrete steps.

            Comment


            • #7
              Thanks Paul Alciatore I'm going to have to read that a few times more to understand. I ordered a variety box of pots to play around with. As for your range of motion question, the answer is I won't know till the rig is built. This is for a puppet mouth, so I want to be able to tune it shut with one end trim pot and set the max open with the other one. I'll need to rig the main control with a spring return to all the way to one end of it's performance range, but I expect that range to be only 30 deg or so, which is why I need the range trimmer. This is all stupidly simple with an RC aircraft controller! If I had time to reinvent the wheel and program an arduino or such it would be awesome, but I don't. When it was RC I told them I needed a week and that's what they expect now that its hardwired.
              Location: Jersey City NJ USA

              Comment


              • #8
                I'm sure you've thought of this but any chance you can adjust the range mechanically?

                The servo arms have multiple holes in them to allow various degrees of throw. You can also mount the servo arm "off center" to give more or less throw in either direction.

                Comment


                • #9
                  Originally posted by C_M_H View Post
                  I'm sure you've thought of this but any chance you can adjust the range mechanically?

                  The servo arms have multiple holes in them to allow various degrees of throw. You can also mount the servo arm "off center" to give more or less throw in either direction.
                  In this business its not accurate enough. Same reason hobby vehicle controllers have trims. Problem is the simple driver boards are made for testing not operating.
                  Location: Jersey City NJ USA

                  Comment


                  • #10
                    Expanding the movement range will require an amplifier of some sort-

                    either electronic or mechanical.

                    Can you modify the linkage to make 30' of movement give you 60' of pot rotation?

                    Or:

                    use a linear pot instead of a rotational pot?

                    Last ditch- a pot that goes from 0k to 10k in less than 270 degrees.

                    A long time ago, electromechanical things like this were common, but not so much any more.

                    Oh, wait-

                    there IS one way to 'amplify' the range, and I've done this: instead of +6v for the voltage divider reference,
                    (270' for the pot and 90' the servo, let's say)
                    use a higher voltage to get the center point of the range to where you want it.
                    Let's say you use 12v. Ground is ground for both. Now, at 135', your voltage divider
                    will be putting out 6v, and the servo will be at 90'.

                    Since the pot will never travel above, say, 120 degrees, you'll never over- voltage
                    the input, BUT you've just reduced your pot rotation relative to the voltage swing.
                    If you then use the above circuit, you can trim and position from there.

                    I've used zener diodes as 'overvoltage' protection on inputs like this in case something
                    goes wrong in the wiring, but I've had a lot of success doing it to turn a 70 degree
                    throttle position sensor into a 0-5v signal for logging, using 12v on the 'high' leg of a pot.

                    hth

                    t

                    rusting in Seattle

                    Comment


                    • #11
                      Are synchros available in electronic form or are the thing of the past. Seems a micro-miniatur synch would or could be useful. Just asking. I dont know JR
                      My old yahoo group. Bridgeport Mill Group

                      https://groups.yahoo.com/neo/groups/...port_mill/info

                      Comment


                      • #12
                        All the range stuff can be done in software. Pick up on of the cheap arduino nano clones are there are a bunch of rc servo libraries. All most servos look at is a pwm signal and it’s duty cycle. 50% puts it in the middle. 20% puts it most t of the way in one direction, 80% puts it the same amount in the other direction.

                        yeah. You can do this with a 555 or a PWM ic but it’s way more configurable with a microcontroller. You just read in analog voltages on the pins and then you can set the scaling in software. Add a couple more pots and you can set endpoint and even add another to set the scaling.

                        badically you do an analogRead of all the pots and save those as variables. The do a little math to them and saved as another variable. Then that variable is used to set thr pwm duty cycle which depending on the uC is just an integer. Say 0-1023. Setting it to 512 would be about 50%, 768 would be 75%, and so on.

                        Comment


                        • #13
                          Originally posted by macona View Post
                          All the range stuff can be done in software. Pick up on of the cheap arduino nano clones are there are a bunch of rc servo libraries. All most servos look at is a pwm signal and it’s duty cycle. 50% puts it in the middle. 20% puts it most t of the way in one direction, 80% puts it the same amount in the other direction.

                          yeah. You can do this with a 555 or a PWM ic but it’s way more configurable with a microcontroller. You just read in analog voltages on the pins and then you can set the scaling in software. Add a couple more pots and you can set endpoint and even add another to set the scaling.

                          badically you do an analogRead of all the pots and save those as variables. The do a little math to them and saved as another variable. Then that variable is used to set thr pwm duty cycle which depending on the uC is just an integer. Say 0-1023. Setting it to 512 would be about 50%, 768 would be 75%, and so on.
                          That sounds totally cool, and I'd like to learn it, but I just don't have the time for the learning curve. I can barely make out the Chinglish manual to program the gun style RC car controller for the other part of the project.

                          I found this: http://www.precisionsales.com/joysti...rocker-pot.htm

                          Click image for larger version

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                          It's described as a 30' single axis joystick of sorts. It comes sprung both centered and to an end. and It's a little big for the palm of the hand as this project requires, but might work.
                          Location: Jersey City NJ USA

                          Comment


                          • #14
                            Originally posted by macona View Post
                            All the range stuff can be done in software. Pick up on of the cheap arduino nano clones are there are a bunch of rc servo libraries. All most servos look at is a pwm signal and it’s duty cycle. 50% puts it in the middle. 20% puts it most t of the way in one direction, 80% puts it the same amount in the other direction.

                            yeah. You can do this with a 555 or a PWM ic but it’s way more configurable with a microcontroller. You just read in analog voltages on the pins and then you can set the scaling in software. Add a couple more pots and you can set endpoint and even add another to set the scaling.

                            badically you do an analogRead of all the pots and save those as variables. The do a little math to them and saved as another variable. Then that variable is used to set thr pwm duty cycle which depending on the uC is just an integer. Say 0-1023. Setting it to 512 would be about 50%, 768 would be 75%, and so on.

                            "Then that variable is used to set thr pwm duty cycle which depending on the uC is just an integer. Say 0-1023. Setting it to 512 would be about 50%, 768 would be 75%, and so on"

                            To me that is as good as it get so to say... JR

                            My old yahoo group. Bridgeport Mill Group

                            https://groups.yahoo.com/neo/groups/...port_mill/info

                            Comment


                            • #15
                              I am actively working on an off the grid type project.
                              My old yahoo group. Bridgeport Mill Group

                              https://groups.yahoo.com/neo/groups/...port_mill/info

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