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Author Topic: Multiplexing leds, faint leds  (Read 3331 times)
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I had similar issues when I made a 4051 multiplexer setup. What I found was the chip control lines were floating when in the low state. I put a high value resistor (~47k) on each of the control lines connected to ground. This solved the issues. You might want to try it on your A, B, C, and D control lines. Putting resistors on the chip I/O lines might introduce errors if analog signals were being multiplexed.
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What I found was the chip control lines were floating when in the low state.
What do you want to say???
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What do you want to say???

What was said:
1) Unpredictable outputs may occurr with a 4051 type chip (this is the chip I have experimented with) when the input control lines (A, B, C, and D in the post) are either not driven at ~5v input or not connected to ground.
2) If the arduino digital output pins do not sink to ground when in the "low" state, then the control lines to the multiplex chip my "float" and produce undesired results.
3) The solution provided might just be a workaround for the real problem.

Easy enough to to test. In the origional wiring configuration, jumper the low control pins to ground and see if the dim leds go dark.
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If the arduino digital output pins do not sink to ground when in the "low" state,

Ah but they do.
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Ah but they do.

So, what is causing the problem other than your usual "..and this is not the way to use an analogue multiplexer."? I've never tried driving transistors with the 4051. The fact that the transistor is connected to another 5v source could be introducing issues by having some backfeed to the 4067 output pins. One could check this by adding a diode between the chip output pin and the current limiting resistor, and removing the grounding resistor. Also, pin 15 (E bar) on the 4067 chip probably needs to be connected to ground if not already done so. I'm interested in finding a cause and not just a fix.
    
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1) Unpredictable outputs may occurr with a 4051 type chip (this is the chip I have experimented with) when the input control lines (A, B, C, and D in the post) are either not driven at ~5v input or not connected to ground.
Well, of course - this is somewhat typical for all CMOS chips....
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2) If the arduino digital output pins do not sink to ground when in the "low" state, then the control lines to the multiplex chip my "float" and produce undesired results.
See Mike's answer.
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3) The solution provided might just be a workaround for the real problem
???

WRT to your last posting.  The "not enable" pin must of course be connected to ground - also this is somewhat typical for all enable or not-enable pins...
« Last Edit: September 26, 2010, 03:48:55 pm by mpeuser » Logged

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So, what is causing the problem
zoomkat you are obvious so clever why can you not work it out? You are clever enough to use a chip in a way it was never intended to be used so why are you not clever enought to work out why it will not work?
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zoomkat you are obvious so clever why can you not work it out? You are clever enough to use a chip in a way it was never intended to be used so why are you not clever enought to work out why it will not work?

At least I'm researching the issue instead of just being a peanut gallery heckler. I've looked at the TI data sheet for the 4067 chip and can't find any thing that would substantiate your claims that this is an inappropriate use of the 4067 chip as long as the operating limits are not being violated.

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???

You believe the chip is operating correctly.

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See Mike's answer.

I'm new to this chip, but I think I saw something to to the effect that the I/O pins can be set high, low, and high impedance. Perhaps the undisclosed code being used is invoking the third option.
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Oh come off it zoomcat. Your "peanut gallery heckler"s have probably forgotten more about that chip than you know.

I agree that: 1) I don't know much about "that chip" (which ever one you are refering to), and 2) no comment on your comment as to what may be going on in the peanut gallery.

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So you think "your usual "..and this is not the way to use an analogue multiplexer." is some sort of conspiracy by the knowledgeable and experienced to withhold arcane knowledge from newbies?  Electronics is not magic. That chip didn't just appear last week. People have been using it for decades, and it seems doubtful that someone who is "new to this chip" is going to find some mysterious new feature or function.

So what is the point of this ramble? Electronics generally "works the way it is wired". A general statement has been made about this being an inappropriate use of the 4067 chip. I'm sure the "knowledgeable and experienced" will step up to the plate and provide the technical basis for their claim. Perhaps they can explain why there appears to be sufficient current flow thru the transistor base to allow current flow thru the transistor and partly illuminate the LEDs. I agree that "magic" is probably the answer.  



 
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The presenting symptoms of dysfunction aren't enough proof for you?  The argument is pointless.

(I'm not going to waste my time proving that the sun appears to rise in the east, either.)

Then you will find the below "proof" spot on!  smiley-wink

http://tinyurl.com/2cbdzw9
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The 4051, 74hc4051, 4067 e.t.c. are very fine chips that work exactly as to be expected. Note that there can be minor differences between the original 40xx chips and their modern 74HC versions.

The main difference to relays is, that they have a quite high closed impedance value (around 100 ohms @ 5V); the datasheets recommend to not have more than 0.8 volts drop at that switch, which means 8mA @ 5V. (It is considerably more (25mA) with higher voltages, the 40xx types can be used upto 16 volts generally..)

I can imagine that "crosstalk" will occur when leading 20 or 40 mA through them @ 5volts.

Otherwise the open pins have a resistance of >10Gohms, which can lead to funny effects with following FET transitors or Arduino inputs, if you forget about that.

Hans mentioned another thread, where he most likely messed it by untidy multiplexing (and also drawing too much current)  smiley-wink

zoomkat's insertion is quite valid of course, that the selection pins have to be set to defined values.
« Last Edit: September 27, 2010, 04:15:59 am by mpeuser » Logged

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Damn, alot of replies... Like i said it works fine now with a analog multiplexer. And ofcourse im setting all the selectlines. (even when using only 3 bits).
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