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Digital Pot setup

Discussion in 'General Electronics Discussion' started by JPU, May 19, 2012.

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  1. JPU

    JPU

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    May 19, 2012
    Please can someone help me with this design. I need to control a Brushless motor controller. The controller has the pins for a potentiometer to be connected to adjust the speed and it also has pins to accept a 0 - 5V input that will also adjust the speed.

    As the unit is to be sealed in a water proof case I will have to use the push buttons to allow the user to control the speed and therefore I will need to use the 0 -5V input pins.

    Can someone please help me with the digital potentiometer. Can I just buy this bit of circuitry so that all I have to do is add buttons and connect it to the brushless motor controller. Or is it simple enough to build. I have looked on the net and I cant seem to be able to buy one anywhere.

    Any help would be appreciated and I am very very new to electronics.

    The power source available is 12V or 24V. The control input is 0 - 5V

    Regards

    Justin
     
  2. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    This is going to be easier to do and provide more options if we can facilitate low side switching. Please post your schematic so we can see if low side switching is applicable.
     
  3. JPU

    JPU

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    May 19, 2012
    Thanks for your help,,cdrive..

    Im not sure if my schematic will make a difference. The motor will be controlled by this controller

    http://www.anaheimautomation.com/pr...-item.php?sID=278&serID=8&pt=i&tID=100&cID=23

    If you click on the link and then the specifications tab you can see a V control. This is where I intend to connect the digital Pot (if thats what I need?).

    I have a 12V lipo battery powering the configuration and Im using a 12v to 24v dc -dc module found at http://www.virtualvillage.co.uk/12v-to-24v-dc-dc-power-converter-module-003602-013.html

    What I want to be able to do is make use of the V control option so that a user can vary the speed of the motor using just two buttons to cycle up and down.

    Thanks again for your help with this.

    Justin
     
  4. BobK

    BobK

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    Jan 5, 2010
  5. JPU

    JPU

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    May 19, 2012
    Hi Bob

    Thanks for your links, I did come across those pages on the net when I was searching for help. However as I said I am very very new to electronics and was looking for help on how to wire in something like the AD5220. Also I notice that this chip has 128 positions and as I wish to control my motor through about, only 5 settings, is this the chip for me or is there something I`m not getting??

    If you are able to explain, in layman's terms, how to wire this up, I would be grateful for the help.

    Respectfully

    Justin
     
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Justin, before we veer off into a particular direction could you please describe where the Up/Down speed buttons are going to be? If they're inside the water tight case how do you access them? If they're not, how are you going to protect them from the water and how are you going to get the conductors from the switches to the interior of the case?

    BTW, at this point I'm not sure that the AD5220 is exactly what you want or not but most things are adaptable. It's a nice find by Bob and the 5 steps that you want can probably be realized, albeit with additional circuitry. First, let's determine if you need it. ;)

    Chris
     
    Last edited: May 20, 2012
  7. JPU

    JPU

    281
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    May 19, 2012
    Hi Chris

    I mentioned a water proof case as I thought this would simply convey the message of why I do not want to use a mechanical Pot (ie one with a shaft, sorry for the simplification).

    The unit I am trying to construct will be used in a wet environment, ie wet hands and fingers. I thought that in using a shafted pot there may be a risk of water getting into the pot.

    My solution was going to be to use two push button switches and I thought a switch like this would do the job http://www.maplin.co.uk/ip67-illuminated-push-switches-518455

    I did study the AD5220 Specification and found that the chips performance can be affected by quality of the buttons contact so I guess this is why you are asking this question??

    I have also ordered the following chip from rs-online..

    http://uk.rs-online.com/web/p/digit...2267573743D3730392D313535382677633D4E4F4E4526

    I`m not sure if I have wasted £5 buying 10 of these but I figure I need to try something.

    So to sum up, I would like the buttons to be placed on the outside of the case. The brushless controller would then be inside the case.

    Please take a look at this prototype I have been working on. You can see in thie video that I have a shafted Pot (is this called an analogue pot??) I would like to replace this pot with the buttons. (Just in case you are interested the tool is used to burr down horses teeth, in this video the motor used is not powerful enough and we have replaced it for a bigger motor, the controller is also different and I have already supplied you the link to the current controller).

    Thanks again for you help.

    Justin
     
  8. gorgon

    gorgon

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    Jun 6, 2011
    The components you have collected looks good. From what you have shown, the only item missing is a stable 5V supply for the pot and the 0-5V input. You need to take some care with decoupling this supply, to reduce the noise from the motor controller.

    To make the box waterproof should be no problem with the IP67 buttons you have bougth.

    TOK ;)
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Nice find. It has the de-bounce circuitry built in. Question though.. Unless I calculated this wrong, to sweep the pot from 0 to Max you will have to hold the Up button down for 8.6 seconds. Is this acceptable to you? If not I have some thoughts on a work around using a 556 to semi-automate control of the 5228.

    Personally, because you wanted 5 speed steps, I wouldn't have used the 5228 for your application. Instead, I would have used a Microcontroller (uC). Nothing complicated to learn. More on the order of a Picaxe, which I'm in love with. With a Picaxe it would be trivial to produce a voltage staircase 0-5V and just as easy to program it to produce 5 steps, where each button press produced 1 step.

    To answer your question: The answer is no. We just call them "Pots", which is short for Potentiometer. We don't preface them with "analog". Who knows? This could change though! ;) As a side note to further your electronics education, a Pot, where only one leg and the wiper are connected is termed a "Rheostat". Over the years though, I've noticed that most of my younger coworkers call this a "Variable Resistor". Evidently, Rheostat sounds like an archaic term to them. :D

    Chris
     
  10. JPU

    JPU

    281
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    May 19, 2012
    Hi Chris,,

    Wow, I think I will call them shafted pots and digital pots. I guess you know what I mean. LOL!

    No. 8.6 secs would be a ridiculous time to wait, and you are light years ahead of me. I would have to have built it to find that out!!!

    Please can you elaberate on your thoughts as I really want the ability for the user to "race" up the speed if they like, ie tap tap tap tap of the button takes you from 1st to last step..

    Regards

    Justin
     
    Last edited: May 20, 2012
  11. gorgon

    gorgon

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    Jun 6, 2011
    You don't need to wait for the autoscan to start, just push repeatedly on the button and it will count much faster. As far as I can see the debounce is 10ms. I can't see any reason to complicate the design with a microcotroller.

    If you need fewer steps, you could use a 4bit binary up down counter, a 3 to 8 decoder and some 4066 analogue switches and a resistor ladder. Or of course a microcontroller to controll the switches. Many ideas, but why not use the chips you have?

    TOK ;)
     
  12. JPU

    JPU

    281
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    May 19, 2012
    Gordon

    That sounds way out of my league. Do you think I could use the chips that I have and have only 5 step between 0V and 5V?? Please can you keep the language simple as I`m a newbee to electronics..

    Regards

    Justin
     
  13. gorgon

    gorgon

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    Jun 6, 2011
    The simplest way I could think of, would be to make a small circuit that puts out 6 pulses for each button press. You'll need 2 of them, one for each button, up and down.

    When you push the button, you'll advance the counter in the potmeter 6 counts instead of 1, and pressing 5 or 6 times max would span the 32 step range. This way you could use the chip you have.

    32 is the smallest number of steps I could find at AD at least.

    The counter circuit can be made from discrete logic or using a small microcontroller. You haven't said if you are proficient with microcontrollers, but if not, the logic approach would be easier.

    TOK ;)
     
  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Justin, have you ever done any programming an any of the 'BASIC' languages, like Quick Basic, Visual Basic, etc? Also, what is your time line for completion of this project? While Picaxe Basic is easy to learn, I don't think it's likely that you'll produce a working program without a couple of weeks playing with the code and the Editor/Compiler/Simulator. You have nothing to loose though because the software is download-able and free. It includes a very complete manual that includes sample circuits and the code associated with them.

    I'll post the links for you shortly.

    Chris
     
  15. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Picaxe has improved their chip line and added some new commands to their language. I have the last version, so I need to download this too! ;)

    http://www.picaxe.com/

    By the way,.. Their simulator is so good that you don't need a live chipset to write code and test it in the Picaxe Simulator. When I first started with Picaxe, I had about four vary basic programs written and simulated a week before I received the chip. In fact, I simulate all my programs long before I download them to the chip. And don't let anyone tell you that an Arduino or other uC family is as easy to learn and use as a Picaxe, cause it ain't so! But, of course, this is just my opinion ,,,,and anyone else that's used a Picaxe! :D:D
     
  16. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Unfortunately, I don't think that will work. The 5228 won't respond to rapid switching of its inputs. Anything under 600mS is considered one button press. If you hold it down longer than that it advances the pot only 4 resistance steps per second. Some of this, I'm think is de-bounce related. Did you read the data sheet?
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    At last, someone who agrees with me! :)
     
  18. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    We can form a coalition! :D
     
  19. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Sorry Justin, the answer is no. Why did you rush to buy ten of them? On the up side, they're not a large expenditure and I'm sure you'll have other projects that can use them.
     
  20. gorgon

    gorgon

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    Jun 6, 2011
    You need to take a closer look at the datasheet. The 600ms is the wait time to start the autoscan, not to debounce. Debounce time is listed as 10ms. So any push time more than the debounce time will advance the counter, no problem to advance 6 times for each push.

    Did a closer check on the numbers. The debounce time is 12ms(min) for a low(active press) and 1us(min) is needed between activations, so a clock of 50-60Hz for the pulsing would be more than slow enough. This would get you from zero to full speed in less than a second, depending on the debounce circuit you make for the logic.


    Justin, You CAN use the AD5228 chips you have.

    I'll be happy to design the logic for you, if you want. You'll have to build it yourself. Send me a PM if this is intersting.

    TOK ;)
     
    Last edited: May 21, 2012
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