Low Cost String-pot Displacement Transducer


Introduction: Low Cost String-pot Displacement Transducer

Prototyping on a student budget is always challenging, especially when it comes to prototyping pneumatic robotics and wanting to implement linear motion feedback. To overcome the high costs of industrial linear displacement transducers I decided to build them myself and use them in a working principle prototype. The linear displacement transducers as shown in this instructable are low cost, flexible and versatile however they do not function according any standards.

Step 1: Materials

1) A potentiometer of your choosing

2) A badge reel of your choosing

3) A “servo mounting thingy”

4) A piece of aluminum square tubing of your choosing

5) A small screw

Additional stuff: Glue, Drill, Drilling machine, metal saw, (the nut comes with the potentiometer)

Step 2: Construction

1) Drill the appropriate holes in the aluminum tubing.

2) Take apart the badge reel and keep the reel and the spring.

3) Glue the “servo mounting thingy” to the reel.

4) Place the reel in the aluminum tubing and attach the small screw to the spring.

5) Put the potentiometer to the hole in the aluminum tubing and screw on the nut loosely.

6) Press the head of the potentiometer in the “servo mounting thingy” (it should make a tight fitting such that the potentiometer turns when you pull the string on the reel)

Lastly) Fasten the nut on the potentiometer and voila: A low cost string-pot displacement transducer.

Step 3: Voila

I used the above mentioned string-pot displacement transducer as position feedback in a servo pneumatic robotics working principle prototype and they did the job perfectly. In my next instructable I will show you how to build your own low cost servo valves (up to 8 bar) for servo pneumatic prototyping purposes.



    • Creative Misuse Contest

      Creative Misuse Contest
    • Clocks Contest

      Clocks Contest
    • Water Contest

      Water Contest

    14 Discussions

    is that a servo horn?

    if it's then where does that fit in.

    I'm interesting in making a steering angle sensor for my 1992 Porsche 968 track car, and this might be a good approach, in that I could attach it to the shaft on the steering rack that's attached to the tie rod. Just one question - could you please describe the "servo mount thingy" you used in a little more detail? Thanks.

    I need something like this to be able to measure the velocity of the wire. 2 types of questions:

    1. Where did you get that badge reel? It looks sturdy How long is it? I can't find any, the only thing I can think to do is to get a retractable dog leash and take it apart. I did it once but it broke :(

    2. Would a rotary encoder work as well?

    6 replies

    you can buy a badge reel at most office supply (web)stores. Depending on your application a retractable dog leash might be perfect. In my case a dog leash would have been too big.
    If you only need velocity, an encoder is perfect. If you never worked with encoders, this will require some time getting used to. There should be able to find some madlab or Arduino tutorials for this.

    I have this on hand

    However its a cheap one I intended to use just to practice and generate some code to read the signals until I buy a more robust rotary encoder

    This is the one I probably will buy, could you give me a suggestion for this one, or similar?


    The link to the dog leash is missing. The iiMash encoder has
    a rod that could support axial forces and possibly shear forces, I have doubts
    about the cheaper one. Since the forces applied by the reel are quite
    significant I definitely recommend the iiMash.

    Firstly, drill a hole in the housing of the dog leash to connect
    the housing of the encoder. In case of the iiMash encoder, four holes (one for
    the rod and three for the connecting screws). Next, it might be possible to
    drill a press fit in the reel to connect the rod to the reel. In any other case
    you need to search for an appropriate coupling.

    I hope this helps

    I am comfortable reading electrical output from the rotary encoder, I'm just at a loss at how I would attach the rotary encoder to the dog leash so that it spins as it retracts..

    Well, that really depends on the encoder you use. Probably you
    need an axle/rod to connect the encoder to the reel of the dog leash. If you
    send me a link of the encoder, I can think of specific suggestion to solve the

    Have a look here at a picture of the dog leash I have

    Good question! The inside end of the continues force spring was bent around the little screw to form a cylinder. Next, remove the screw and position the spring inside the reel. Align this sub-assembly with the holes in the aluminum tubing and insert the small screw such that it fits within the cylinder you made earlier.

    I hope this helps. In any case i will update this instructable to include this part.

    Kind regards

    Thanks, I think I get it but if you have pictures that would help a ton. I'm also attempting to locate a potentiometer with a shaft that would interface with a servo wheel. It appears from the pictures you have a Vishay potentiometer. I found this Vishay data sheet and this Vishay part. (Note that the Newark site calls out that the picture is NOT of the actual part. The potentiometer in that pic has a smooth shaft.) I'm just trying to source parts that will work together. Any references you can provide will be much appreciated.

    And now after re-reading I see that your Vishay potentiometer has the smooth shaft unlike the spline shaft that I assumed it was (like this one). It would be awesome to have a CAD drawing made for a wheel specific to a spline shaft potentiometer. But I digress. Thanks for this instructable.

    I'm just now getting into pneumatics and your Instructable will definitely help me a bunch! Looking forward to your next instructable!