# 7 segment Led display

Discussion in 'Homework Help' started by RdAdr, May 19, 2013.

May 19, 2013
214
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So after reading on the internet i figure it out what A,B,C,D,E,F,G,DP does.Also the role of the resistances in series with the 7 segment-diodes(to minimize the current so that the diodes dont burn out).I know that combinations of COM1,COM2,COM3,COM4 give different numbers.
What i don't understand is the role of the transistors and the resistances.Is that a BCD to 7 segment decoder?I guess not because the decoder converts COM1,COM2,COM3,COM4 to 7 segments.

PS:those are pnp transistors.The quality of image is not very good.

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2. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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Its a common Anode led display, they feed the segments (a to g, dp) in the module and illuminate each led, using current limiting resistors.

display

The transistors switch on each Anode separately to enable multiplexing.

Last edited: May 19, 2013

May 19, 2013
214
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So wait...i have there 4 digits?Because i have 4 transistors.
I thought that i only have 1 digit with 7 segments.

4. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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yes four separate displays that have their cathodes tied together, and each has a transistor to drive the Anode.

5. ### MrChips Moderator

Oct 2, 2009
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5,359
You would think that by just looking at the display you can tell how many digits you have.

May 19, 2013
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So inside that LED1 there are 4 digits that have their cathodes tied together and each has a transistor to drive the anode.If com1 is 0(transistor 1 is blocked) then 1st digit doesnt show anything,no matter what A,B to G are(0 or 1).
And each digit has 7 segments.So there are 4*7=28 segments.So there are 28 diodes.

But i have only one set of A,B to G.Aren't these 4 digits all show the same thing together?

http://www.thelearningpit.com/lp/doc/7seg/7seg.html
2nd truth table
I thought that SW3,2,1,0 are like my Com4,3,2,1.And different combinations of these give the desired numeral(0 to 9) to appear on the 1 digit.How come there are 4 digits in there?

And further down the page for more than 1 digit they show that by multiplexing u can realize that.But there u have more than 1 set of a,b to g.I only have 1 set.

Last edited: May 19, 2013
7. ### MrChips Moderator

Oct 2, 2009
17,362
5,359
I think you need to start from square one. Give us the model number and link to the display you have.

May 19, 2013
214
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The professor said that the circuit will be designed as an adapter for the development board A1847-V0.And my job is figure out how that scheme works and what does it do.He said also that this is a 4 digit LED display.

The model number is SSAC V0.

The display i have uploaded here(images.jpeg).I dont know if this is the display u're reffering at.(this is from the internet...not from the professor like the other two)

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