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Thread: Phidget LCD 8/8/8 Shuts Off Because of Potentiometer?

  1. Phidget LCD 8/8/8 Shuts Off Because of Potentiometer?

    Hey, so, some of you may have seen my thread about turning analog inputs into multi-digital inputs with the use of resistors connected in parallel with switches. I've finally started to put the circuit together, and so now I'm trying to find the right resistor values, and I had something happen to me that was kind of disturbing and I was hoping I could get some feedback and some help.

    In the 8/8/8 manual, the following diagram is provided of a simple analog sensor you could build to use an analog input:


    Well, it just so happens that I had a 1K pot lying around, so I hooked up the circuit exactly as shown here (at least, I think I did -- the center connection is the common one, right?) and plugged in the 8/8/8.

    For a while, it seemed to work okay. The value started around 972, and as I slowly and gently turned the pot, the number slowly went down. Then, understandably, the decrease accelerated, until it hit 800 exactly. Then it didn't go any lower, and a split second later, the Phidget turned off completely... as in I got the "BING BONG" sound from my PC that told me the USB device was no longer connected, and the interface went completely blank...

    ... then I turned the pot back in the opposite direction where I had been a second before, and the Phidget turned back on like nothing had happened. Same behavior, performance, readings... and then, once more, I turned the pot "below 800"... and the Phidget shut off /disconnected again. I turned the pot back up, made sure the Phidget was still working, unplugged it and thanked my lucky stars I hadn't burned the thing out. I would have inevitably blamed myself, assuming I had done something wrong, but the circuit is too simple to mess up... and like I said, until I went below that 800 threshhold, it was absolutely working as advertised.

    So, question -- what might I be doing wrong? Is the pot likely too high an amperage rating? Should I, despite it being an official Trossen manual diagram, use a much higher (like 1M) resistor rated pot? Should I add an additional current-limiting resistor right before ground?

    Thanks for your help, everyone. As you can imagine, discovering that a suggested circuit right in the device's manual seems to be capable of straining the limits of the device (and perhaps even breaking them) is a little disconcerting...

  2. Re: Phidget LCD 8/8/8 Shuts Off Because of Potentiometer?

    Man, this is confusing. I've tried putting current-limiting resistors right before both the ANALOG INPUT and GROUND connections twice (with two different resistor ratings), and both times, the result was that the analog input interface showed a static value that didn't change when I turned the pot! What's the deal?

    ADDENDUM: *And* I just tried the circuit again, with just a current-limiting resistor before GROUND, so now it doesn't shut off any more, but I'm also not getting any ... okay, hold on... I'm going to try a lower-resistance resistor right before GROUND...
    Last edited by Clavis; 09-06-2009 at 09:55 AM.

  3. Talking Re: Phidget LCD 8/8/8 Shuts Off Because of Potentiometer?

    Okay, never mind, I've got it figured out... I've got it down to a pre-GROUND resistor of 220 ohms and the analog sensor input is now reading (when I turn the pot) between around 987 and 995... so everything is working okay, I just need to "tune in" the right resistor values... sorry to clog up the message board... Happy Labor Day!

  4. Re: Phidget LCD 8/8/8 Shuts Off Because of Potentiometer?

    Hey, guess what? Turns out, even if you take care to put a current-limiting resistor into your circuit, if you don't have a spare 3-hole analog sensor plug lying around (and I don't), and you try jerry-rigging connections into the 8/8/8 Analog Input so as to try out your new circuit, that you can accidentally (I assume this is what happened) short between +5V and GROUND inside the jack and blow out the 8/8/8 completely! Now it doesn't show up in the Ph USB menu at all! ZAP!!!

    I was going to rant and rave about sending a kit without the necessary connectors to connect into the thing (all the other connections have little screw terminals), but I assume it's a standard jack and I just shouldn't have risked it... but I worked really hard to protect the contacts from one another, and I wanted to test the damn thing to make sure it worked. Well, I tested it, and it worked, and I tested it some more, and now it doesn't work at all.

    Oh, well... add another $99 to my investment on this project. [Excrement]


    P.S. I don't care to what degree this was my fault right now. I really don't. [I'm off to work on the depth and breadth of my vocabulary now.]
    Last edited by Adrenalynn; 09-06-2009 at 06:35 PM. Reason: You might notice the lack of f-bombing around these parts. I understand you're upset about what you've done, but... -A]

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    Re: Phidget LCD 8/8/8 Shuts Off Because of Potentiometer?

    Don't toss the 8/8/8! It may be repairable. I suspect what happened was that you connected the pot according to the diagram above and blew out the regulator, which can happen if you have the pot adjusted too low. Get a voltmeter and make sure that the reg is putting out the right voltage.
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    Re: Phidget LCD 8/8/8 Shuts Off Because of Potentiometer?

    Just curious. Do you need your project to read more than 1 key at a time?
    Also, does anyone happen to know what the resolution is on the analog input for the 8/8/8 input device (ie 8 or 16 bit read)?
    Sidster . . . . . :^)

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    Re: Phidget LCD 8/8/8 Shuts Off Because of Potentiometer?

    ten bits, I do believe.
    I Void Warranties´┐Ż

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    Re: Phidget LCD 8/8/8 Shuts Off Because of Potentiometer?

    ten bits, I do believe.
    Thanks Adrenalynn.
    You can use this circuit to allow for 8 inputs to be read independently by using this D/A converter.

    Attachment 1471

    Essentially it will be like taking an 8 bit word and converting it to an analog value, then the CPU will convert it back to an 8 bit value. The 8 bit value will depend on which switches were pressed and will work even when more than 1 switch is pressed at a time.

    For 32 inputs, you will need to use this on 4 analog inputs (4 x 8 = 32). Although, I still think digital inputs (as opposed to analog) are a better solution, since you don't have to go through all this.

    By using only 2 digital outputs and 8 inputs, you can accomplish the same thing much easier.
    Sidster . . . . . :^)

  9. Re: Phidget LCD 8/8/8 Shuts Off Because of Potentiometer?

    Sid, your help and suggestions are very much appreciated.

    Now, if only I could have any confidence in the 8/8/8 at this point... how can I be sure that the resistor values I come up with aren't going to be too low and result in another blown-out 8/8/8 unit? Or too high so that every button press results in an identical "999" reading? I haven't the slightest idea how to work out the calculations, and while normally, I would experiment with different values until I zoned in on the correct ones, that apparently is how you destroy the 8/8/8, so that strategy is out. I guess I have to use my psychic powers to magically know in advance what resistors to use in order that the Analog Inputs aren't a complete waste of money.

    (I swear, I know I'm being electronically naive, but when you build a unit where using the Digital In consists of connecting + directly to -, it shouldn't be a 100% guaranteed way to break the unit to directly connect the + and - to each other in the Analog In, either...)

    I'm sure there are formulae for calculating such things -- lnxfergy offered "V = 5V*(R0/(Rx+R0))" in another thread for calculating the "voltage at the junction" of my earlier 5-switch circuit -- but I'm really not in the mood to try to decipher such heiroglyphics at this point. I'm fine with using a purely Digital system. Screw the Analog Inputs. They're worthless.

    So, Sid, how much for you to design a foolproof 38-button circuit? And can you guarantee it will work with the Digital Ins and Outs of the Phidget LCD 8/8/8? Or will it result in another obliterated unit?


    Quote Originally Posted by Sid723 View Post
    Thanks Adrenalynn.
    You can use this circuit to allow for 8 inputs to be read independently by using this D/A converter.

    Attachment 1471

    Essentially it will be like taking an 8 bit word and converting it to an analog value, then the CPU will convert it back to an 8 bit value. The 8 bit value will depend on which switches were pressed and will work even when more than 1 switch is pressed at a time.

    For 32 inputs, you will need to use this on 4 analog inputs (4 x 8 = 32). Although, I still think digital inputs (as opposed to analog) are a better solution, since you don't have to go through all this.

    By using only 2 digital outputs and 8 inputs, you can accomplish the same thing much easier.

  10. Re: Phidget LCD 8/8/8 Shuts Off Because of Potentiometer?

    Is that the thing that looks like a tiny black rectangle? 'Cause there's a lot of those...

    Quote Originally Posted by jes1510 View Post
    Don't toss the 8/8/8! It may be repairable. I suspect what happened was that you connected the pot according to the diagram above and blew out the regulator, which can happen if you have the pot adjusted too low. Get a voltmeter and make sure that the reg is putting out the right voltage.

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