Needed: 9v, potentiometer driven, LED circuit

Discussion in 'The Projects Forum' started by nctech, May 18, 2009.

  1. nctech

    Thread Starter Member

    May 18, 2009
    15
    0
    greetings,

    I hope you all can help. I stumbled across your forum while searching for the solution to a project that I've been put in charge of. It involves finding a way to control a multi light LED position indicator. (red/yellow/green) The best way to detect our device's position is with a pull string potentiometer.
    (see http://www.celesco.com/_datasheets/sp1.pdf ) I need a circuit that will light up more lights as the position of the potentiometer changes. If that's not tricky enough, I would need to be able to adjust where in the stroke the high and low lights come on. It would only be activated by the push of a button (that part of the circuitry I can handle) and will need to run on a 9v battery. I've searched your projects and experiments but haven't found quite what I need. Any advise, schematics, links and suggestions would be greatfully considered.

    Thank you in advance for your time and consideration.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    Take a look at SgtWookie's simple PWM circuit:
    http://forum.allaboutcircuits.com/showthread.php?t=9016

    In place of the load you can place the led with the current limiting resistor.
    In place of theresistor R1 you can place the sensor potmeter (if the resistance is correct).

    Greetings,
    Bertus
     
  3. nctech

    Thread Starter Member

    May 18, 2009
    15
    0
    Thanks Bertus for the link but I'm not seeing how I would adjust my upper and lower limits for the LED's. That doesn't mean it's not there, it just means I need to be told & shown.
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    This sounds like an application for an ADC.
     
  5. nctech

    Thread Starter Member

    May 18, 2009
    15
    0
    Thanks CD but I was hopeing it could be done without the use of too many acronyms. I'm assuming that's an analog to digital converter. The thing is I don't need that kind of precision. I need to know it is in one position or the other (device is completly engaged or completely disengaged) or somewhere in between. Thus the need to tune in the "in" position and the "out" position. And I was hoping to do it with a line of LED's. Kind of like an audio level indicator. Pull the string make more lights light up. I thought it could be a lot easier (and cheaper). But keep in mind this is my first excursion into the world of circuitry. That's why I'm here. Most everyone here is better informed about these things than I am. Again, Thanks for your help all.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
  7. nctech

    Thread Starter Member

    May 18, 2009
    15
    0
    The 0-5v bar graph meter looks like exactly what I need. Thanks Bertus. My question now becomes can I tune in the first and last lights with this. Would I use a rheostat in series with my string pot? or would I use it instead of the 1.21k r1? Is that how do you use that REF. ADJ? You see, I know just enough to be dangerous. I can see this should work. Just not sure exactly how.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You would supply one "end" of the pot with 5v, and connect the other end of the pot to ground. This would give the wiper of the pot a range of 0v to 5v. You'd just connect the wiper to the input of the LM3914.
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    As Sgt. Wookie (best avatar ever) stated you need 0 to 5V for the Sig input to the ADC.
    Here are two methods of supplying pin 5 (Sig) of the LM3914 with 0 to 5 volts from a 9V battery. I think the resistive divider will suffice for you though. P1 is your pull string pot.
     
  10. nctech

    Thread Starter Member

    May 18, 2009
    15
    0
    Tanks Sarge,

    Now I'm getting anxious. Gonna order some parts today to see if I can get this puppy to work. Does anyone know of a supplier that might carry the LED display in stock? I was hoping I wouldn't have to make my own LED board. But then I'm hoping I don't have to do much at all. Don't suppose this is a kit somewhere? :)

    But seriously. Thanks all for your help. I'm a lot closer to a finished product than I was yesterday at this time. Please keep the suggestions coming. If I get this to work ya'll be the second people to know. Thanks again.
     
    Last edited: May 19, 2009
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    LM3914 has a built-in programmable reference voltage which is much like an LM317.
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Oops, I didn't catch that.... Duh!!
    OK, so all he needs to do is change the values of the (Vref divider) R1 & R2 as per the given formula. Then he can connect his 10K string pot across the 9V battery and connect the wiper to pin 5.

    Thanks for correcting me Ron.
     
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    The battery voltage will change over its lifetime, causing the trip points to change. I personally think it might be easier to use 3 sections of a quad comparator, and connect the string pot and the reference divider (with pots for the end point adjustments) across the battery. Then the whole thing would be ratiometric, independent of battery voltage. The down sides are
    (1) you probably need transistors to drive the LEDs, and
    (2) LED brightness would drop as the battery voltage drops, unless you add more complicated circuitry.
    The LM3914 reference could be applied across the string pot, with rheostats on the ends of the internal divider string to adjust the end points. This will drift somewhat with temperature, and I don't think the resistance tempco of the internal divider string is specified. With added complexity and cost, a dual op amp could buffer the end point adjustments.
    I doubt that he is going to be happy with the relative short life of a 9V alkaline battery. Lithium might be satisfactory.
    I would work on designs if I had time, but I am committed to cleaning my hot tub today.:(
     
  14. nctech

    Thread Starter Member

    May 18, 2009
    15
    0
    Wow Ron thanks for that post. I was surprised that I understood a small portion of it. I really like that plan because of the need to adjust top and bottom positions. I don't care if the LEDs dim with a weak battery but I can't have the end points drifting. I don't think temperature will be a factor. I'm not worried about battery life, this is a push the button to read then done type of thing. Never on for more than 3 or 4 seconds two or three times a year, maybe. I hope you can get your hot tub cleaned today. And I'm really hopeing after that you could get me a schematic of the design. I'm a patient man and can wait on your schedule. This is a little over my head and you covered some things I didn't think about. But I think I can build it with a working schematic. I appreciate your time, consideration and understanding of what I'm trying to do. Thank you all for your time to answer my posts.
     
  15. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    As a percentage of the string pot's range, how much adjustment do you need for the end point pots? For example, you might need to cover 0-10% from each end, or 10%-20%, etc. I need to know before I can design something.
    Also, do you have some LEDs selected, so you can tell me how much current each one needs in order to provide adequate brightness?
    What is the end-to-end resistance of your string pot, or is that something I can choose?
    Do you not need a pot for the midrange indicator?
    I'm sure I'm not alone in saying we would love to know what your application is, if you can share that.:)
     
  16. nctech

    Thread Starter Member

    May 18, 2009
    15
    0
    probably about 0-60 or 70% from .25"-3" of the strings 4.75" travel will be used. I haven't calculated the exact amount of travel I'll need yet and I can get that tomorrow. Gotta run now. I'll get you more later. Thanks again guys.
     
  17. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    OK, if you can't tell me percentages, then tell me in inches where you want the end points of your adjustment pots. For example, you might want the lower pot to cover 0.25"-1", and the upper pot to cover 2.25"-3". You tell me the ranges.
     
  18. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Here is one way to do it, with resistor values to be filled in when you answer the questions in my previous post.
     
  19. nctech

    Thread Starter Member

    May 18, 2009
    15
    0
    Thanks Ron for your ideas so far. Sorry I had to leave so abruptly yesterday. Here is a detail of what I'm trying to do. I'm trying to represent the position of the lever in the drawing. It is for a large amprage switch (1200-2000 amp). The lever accuratly depicts the position of the switch. (fully open, fully closed or somewhere in between) The object is to push a button and read an LED bar graph of the switches position. There is little room for any other type of measuring device thus the pull string mounted high. As you can see the travel of the pull string is around 1/2" from fully retracted to around 4 11/32 of the 4 3/4 fully extended device. So to answer your previous question, I'd like one to adjust the .25 - 1" range and the other to cover the 4.25 - 4.75 range.

    I've been shopping for bar graphs. Can't seem to find what I want and am afraid I my have to make my own. I want a 7 or 10 light red on the bottom and green on top with yellow in between. I also want it fairly large and easily visible. Say 4" long. I thought the arrays that are available are a little small. I'm open to suggestions.

    Again Ron thanks for your input and time. It is appreciated. If any other information is needed please ask.

    p.s. Did you get your hot tub to sparkle?
     
    Last edited: May 20, 2009
  20. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    OK, forget my last post and schematic. As you can see, I somehow got the impression you wanted 3 LEDs.
     
Loading...