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Common Anode display problem

 
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Nginuity
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Joined: 13 Nov 2006
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Location: Dallas, Texas

PostPosted: Thu Mar 15, 2007 10:47 am    Post subject: Common Anode display problem Reply with quote

Hey everyone.

I am feeding a common anode seven segment LED display with a 74LS164 driver, which has the main function of sinking current from the cathodes of each of the pins on the 7 segment LED display.

Since they are common anode, I am running into an interesting problem in regard to current limiting. I can't use a fixed value resistor, because the LED will be real bright when just one segment is lit, and really dim when all of them are lit. Does anyone have a decent idea I can use to limit current to the display, but not in a constant state? I toyed around with using some sort of transistor current supply configuration, but I can't seem to nail it down how I want it.

Ideas?
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Alan
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Joined: 29 Jan 2006
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Location: Winnipeg, MB

PostPosted: Thu Mar 15, 2007 12:38 pm    Post subject: Re: Common Anode display problem Reply with quote

Nginuity wrote:
Hey everyone.

I am feeding a common anode seven segment LED display with a 74LS164 driver, which has the main function of sinking current from the cathodes of each of the pins on the 7 segment LED display.

Since they are common anode, I am running into an interesting problem in regard to current limiting. I can't use a fixed value resistor, because the LED will be real bright when just one segment is lit, and really dim when all of them are lit. Does anyone have a decent idea I can use to limit current to the display, but not in a constant state? I toyed around with using some sort of transistor current supply configuration, but I can't seem to nail it down how I want it.

Ideas?


Most people just use 7 resistors.
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Nginuity
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Joined: 13 Nov 2006
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Location: Dallas, Texas

PostPosted: Thu Mar 15, 2007 2:05 pm    Post subject: Re: Common Anode display problem Reply with quote

Alan wrote:
Nginuity wrote:
Hey everyone.

I am feeding a common anode seven segment LED display with a 74LS164 driver, which has the main function of sinking current from the cathodes of each of the pins on the 7 segment LED display.

Since they are common anode, I am running into an interesting problem in regard to current limiting. I can't use a fixed value resistor, because the LED will be real bright when just one segment is lit, and really dim when all of them are lit. Does anyone have a decent idea I can use to limit current to the display, but not in a constant state? I toyed around with using some sort of transistor current supply configuration, but I can't seem to nail it down how I want it.

Ideas?


Most people just use 7 resistors.


Even on a common anode? I've seen that done, in fact, it is the way to do it on common cathode where you have 7 separate anodes...but the common anode only has one lead. I only ask because I've always used CC instead of CA displays.
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Alan
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PostPosted: Thu Mar 15, 2007 5:17 pm    Post subject: Reply with quote

Hey Nginuity,

I would wire it up in this fashion:
http://chaokhun.kmitl.ac.th/~kswichit/2051/7-seg/7-seg.htm
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Nginuity
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Joined: 13 Nov 2006
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Location: Dallas, Texas

PostPosted: Thu Mar 15, 2007 5:34 pm    Post subject: Reply with quote

Thanks a bunch! I don't know if the transistor is really limiting any current, since it looks like it is just a power switch of sorts from the controller, but I am definitely going to breadboard it to see what I can come up with tonight!
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Alan
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PostPosted: Thu Mar 15, 2007 6:45 pm    Post subject: Reply with quote

That's right, if you are only controlling one of them just tie the common anode to hard power. The transistors are used to strobe multiple units with the same data lines. For example you could turn on transistor 1 and display the value 6 on the data lines, pause some short time then turn on only the second transistor and change the data lines to another number that will then be displayed on the second display. You just repeat this very fast and you only need n+7 data lines where n is the number of 7 segment displays to drive.
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Nginuity
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Location: Dallas, Texas

PostPosted: Thu Mar 15, 2007 6:55 pm    Post subject: Reply with quote

I never quite thought of doing it that way. It is DEFINITELY pretty slick.

The model that I am using for the driver is a 74LS162N, which has 2 serial inputs, not sure what I am going to use them both for yet...but, it may be just as easy to do the same with a dual display. I;'ve got 25 of those drivers though, so things should be kind of neat to play with.
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Alan
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PostPosted: Thu Mar 15, 2007 7:10 pm    Post subject: Reply with quote

Nginuity wrote:
I never quite thought of doing it that way. It is DEFINITELY pretty slick.

The model that I am using for the driver is a 74LS162N, which has 2 serial inputs, not sure what I am going to use them both for yet...but, it may be just as easy to do the same with a dual display. I;'ve got 25 of those drivers though, so things should be kind of neat to play with.


Sounds like fun, let us know what you end up making. Smile
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