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View Full Version : Phidget LCD 8/8/8 Shuts Off Because of Potentiometer?



Clavis
09-06-2009, 08:35 AM
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:

http://i28.tinypic.com/10p1t8z.jpg
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... :confused:

Clavis
09-06-2009, 08:48 AM
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...

Clavis
09-06-2009, 09:05 AM
:rolleyes: 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!

Clavis
09-06-2009, 04:21 PM
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.]

jes1510
09-06-2009, 10:04 PM
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.

Sid723
09-07-2009, 01:22 AM
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)?

Adrenalynn
09-07-2009, 01:50 AM
ten bits, I do believe.

Sid723
09-07-2009, 02:41 AM
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.

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.

Clavis
09-07-2009, 06:55 AM
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. :genmad:

(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?



Thanks Adrenalynn.
You can use this circuit to allow for 8 inputs to be read independently by using this D/A converter.

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.

Clavis
09-07-2009, 06:57 AM
Is that the thing that looks like a tiny black rectangle? 'Cause there's a lot of those... :eek:


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.

Clavis
09-07-2009, 07:10 AM
Say, would this --

http://www.trossenrobotics.com/store/p/5891-84-Channel-USB-Servo-Controller.aspx?a=blog

-- be an easier way for me to have ~40 digital inputs? Just buy a board with a bunch of digital inputs and leave it at that?









The SD84 is a USB interfaced controller for up to 84 servos, which maintains a 20ms refresh rate irregardless of number of servos or servo positions. Each of the 84 channels can be set to Digital Input, Digital Output or Servo Output. Additionally 36 of the channels can act as 10-bit Analogue inputs, making this a great I/O controller as well.

Sid723
09-07-2009, 11:26 AM
All depends on your application. Do you really need 84 digital inputs? I thought you only needed 32?

Clavis
09-08-2009, 06:22 AM
Good question. Well, I gave 32 as the # because it was the minimum I needed for my project *if* I used all the remaining inputs as button inputs to make up the difference... my full plans include a 35-button keypad, 3 large separate pushbuttons and an unspecified number of internal rocker switches, contact switches, interlocks, rear panel reset buttons, etc. So theoretically, there could be 40-50 switches needed... PLUS, I assume the Digital Out on the SD84 works as well as the Digital Out on the 8/8/8 does at illuminating LEDs, which means that I can use any of the leftover connections as LED outputs... and given that I've already used up all 64 of the LED outputs of the LED-64 I bought, I could use them!


All depends on your application. Do you really need 84 digital inputs? I thought you only needed 32?

Clavis
09-08-2009, 06:49 AM
P.S. Sorry for my earlier potty-mouth. I was too upset to do research into the standards and practices of the board at the time. Thank you for not dumping the entire message...

Sid723
09-08-2009, 10:19 PM
The SD84 is a USB interfaced controller for up to 84 servos, which maintains a 20ms refresh rate irregardless of number of servos or servo positions. Each of the 84 channels can be set to Digital Input, Digital Output or Servo Output. Additionally 36 of the channels can act as 10-bit Analogue inputs, making this a great I/O controller as well.If this works for you, then go for it. Less complication in the details always worked for me. But.... if you are up to the challenge. Then here is a circuit that will scan 64 keys and only use 3 output pins on one port and 8 input pins on another:

1490

What you will have to do is scan through the 8 rows by outputting a number from 0 to 7. Then read the 8 bit output port. The keys that are pressed will show a high (or 1). Good luck.

Clavis
09-09-2009, 11:09 PM
Dude, I appreciate the schematic and all, but I wasn't capable of avoiding blowing up my 8/8/8 even after all the warnings. Do you really thing I'm going to be able to manage to build what you're proposing? I'd be lucky to build a non-working version without stabbing myself in the eye with a soldering iron.

Seriously, I very much appreciate what you've presented, but I'm afraid I never managed to develop the enviable electronics knowledge necessary to build complex circuits. That's why the Phidgets are such inviting tools in the first place: VBA programming -- THAT, I can do!

Again, I admit I may be a jerk for saying this, but it just seems ridiculous to me that one could blow up a $99 device by connecting it to itself. It would seem to me, novice though I am, that built-in current-resistance would be near or at the top of my To-Do List for any of the Inputs, Digital, Analog or otherwise. :tongue: )

Adrenalynn
09-10-2009, 12:41 AM
Many "[obvious features]" seem a "ridiculous [oversight]" when my understanding of what I'm asking for is incomplete.

Right from the start it seems silly that I can blow up any of these microcontrollers simply in the process of picking them up off the desk on a pleasantly breezy Fall day... Or setting them down on said desk. Or exposing them to air at all.

Playing with electronics has inherent risk. Risk to self, risk to the components, risk to the bank account. Over the course of thirty plus years of playing with this stuff, I've decimated more components than I can count.

Point being that there are risks, and there's nothing any designer can do to negate 100% of those risks. If one wants to play with electronics design, one accepts the risks while doing what one can to minimize them. If you continue down this path with this hobby that won't be the last piece of electronics you fry, I'm afraid...

Go ahead and ask me how I earned the right to be called "Sparky" on a job site. ;)

As was pointed out - there's even a pretty good chance you didn't even smoke the whole board. Although I'd be surprised if you still had eight functional inputs...

It stinks. I feel for you. Very much. But at some point ya kinda gotta get a little zen about it and source more components or go back to just software engineering. Hardware engineering isn't for everyone, although I believe every software engineer should be forced to do a little hardware so they have more respect for what the hardware engineers have to go through.

Clavis
09-10-2009, 07:15 AM
Thanks, Adrenalynn. You're right, of course.

As for the board... HOLY COW! IT WORKS AGAIN! I just decided to try it one more time... and it worked! It showed up in the Ph menu, and I was able to bring up the two interfaces, put stuff on the LCD screen and test a *Digital* In, and it all worked! I don't know if there was an issue with my PC, or whether the thing needed to cool down a LOOOONG time, or what... but it works! I don't need to order a replacement! Woo hoo!

Thanks, everybody! Maybe it was your "good vibes" that fixed the thing! :veryhappy: Now I'm glad I didn't throw it against the wall!

Adrenalynn
09-10-2009, 07:22 AM
Congrats!

You probably "just" boiled the regulator and after it reached thermal equilibrium it decided it didn't hate you any more. :)

Connor
09-10-2009, 09:52 AM
FYI, I have a phidgets 8/8/8 (not the one with the LCD though) and I've hooked the onboard 5v straight into one of the analog pins.. They're 5v tolarant.. and the demo with the POT should have worked just fine. The ONLY thing I can think of is, some how you shorted the +5v to the ground some way.. that would have caused it to go shutdown and not come back up till you uplugged it from the USB port and plugged it back in.. (or maybe even having to reboot your computer).

Thanks, Connor

Robert
10-02-2009, 01:32 PM
The analog inputs have an input impedance of over 900k ohms. They'll accept being tied to 5v all day long. You won't need any sort of resistor on your analog input line.

Shorting the 5v to ground however can be a little more destructive.

As for hooking up potentiometers, I've had no problem with 1k pots on 888s before.