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Topic: Problem with simple transistor circuit (Read 3 times) previous topic - next topic

wossname

Please see the attached diagram, which shows the circuit twice but the second time a switch has been toggled.

This is a much simplified version of part of my LED multiplexing circuit.  In the real world circuit, I cannot seem to have just one LED (on it's own) lit up, I get a bunch of adjacent ones all glowing together.  Some bright, some dim.  I checked all my connections which seem to look sensible so I have discounted short curcuits in the system.

I went back to my circuit simulator to try to reproduce this problem and it does seem that certain high/low combinations of the top 3 switches can cause current to pass through to the LED.

I've clearly got a flaw in my design here, but I'm not sure about how to go about fixing it.  Am I overloading the transistor somehow?

The real world circuit uses 5 arduino pins to source and sink 5v.  To be totally accurate... the real world circuit has 192 such LEDs, driven by 3 separate multiplexing curcuits.
This system needs 15 IO pins from the arduino, all pins are set to outputs at the start of the program and I want to use various combinations (5 nCr 15) of these pins to select an LED to light up.

My multiplexing boards use common bases and common collectors in order to address the particular LED I want.  It seems that current is leaking through by finding phantom circuits that I never expected could exist.




Is this making any sense?  I'm not sure I've explained properly, but anyway please let me know your opinions, I'd appreciate a bit of a helping hand on this :)



Adam.

RuggedCircuits

You have current flowing from base-to-emitter in the second transistor from the top. Remember that the base-to-emitter junction of an NPN transistor is essentially a diode. Try redrawing the circuit with the top transistor removed (since it's off) and the second transistor from the top replaced with a diode (from base-to-emitter). That will show you why current is flowing through the LED.

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The Aussie Shield: breakout all 28 pins to quick-connect terminals

wossname

So if the collector is tied to GND, then the base voltage always flows out of the emitter?

Now I'm confused about how multiplexing is supposed to work.  I was under the impression that you could use common bases horizontally and common collectors vertically on a grid of transistors and thus use an x,y coordinate to cause the emitter on the relevant transistor to supply the collector current to one of many LEDs.

Do you think there is any way I can get around this without tearing up and starting again?


RuggedCircuits

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So if the collector is tied to GND, then the base voltage always flows out of the emitter?


Errr...not sure how to answer that. Voltage doesn't flow, current flows. Current will flow from base to emitter if the base voltage is 0.7V (or so) higher than the emitter voltage.

Quote
Now I'm confused about how multiplexing is supposed to work.  I was under the impression that you could use common bases horizontally and common collectors vertically on a grid of transistors and thus use an x,y coordinate to cause the emitter on the relevant transistor to supply the collector current to one of many LEDs.


I doubt that's possible. Do you have a link to where you saw this idea proposed?

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Do you think there is any way I can get around this without tearing up and starting again?


Personally, no...but I'd like to be proven wrong!

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The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected

wossname

I saw this concept explained in a video...
http://www.youtube.com/user/kdarrah1234#p/search/8/-1UPPHjR0vk

This guy seemed (to me, at the time) to know what he's talking about but he never actually demonstrates this physically in action, but he does use multiplexing in some of his other videos.  The principle seemed plausible to my novice ears.  I even had the same components that he described, 1k resistors and 2n3904 transistors.

How about if I use much higher ohm value resistors on the transistor bases, to ensure the base voltage never goes reaches 0.7v?  Would that still allow the transistor to act as a switch?

I feel so stupid  :smiley-red:

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