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[SOLVED] seven segment driving

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zeroskj1988

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i want to drive 12 digit seven segment. i am using common anode (positive) seven segment. i connect segment to directly to pic16f877a's portb. since due to deficiency of pin. i use cd4514 demux to drive common anode of the seven segment. since output current of cd4514 is very much less. seven segment light dull. can anybody suggest me remedy for this.
 

alexan_e

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cd4514 can only provide a few ma (about 3mA) and you expect it to provide up to 80mA to the common display supply, it can't drive the display correctly

Alex
 

zeroskj1988

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thanks for reply.i spot the problem. i thinking of using uln2003 something like that in series with 4514 to get good brightness. is ot possible
 

alexan_e

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uln2003 has an open collector output, in can only sink current (provide gnd) so it can only be used with a common cathode display

Alex
 

alexan_e

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yes you can use ULN ...
No he can't, he is using common anode displays and he wants to use the uln2003 to control the common supply of common anode displays, the segment cathodes are connected to the PIC
i am using common anode (positive) seven segment...
i connect segment to directly to pic16f877a's portb...
Alex
 

zeroskj1988

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thanks for reply.can you suggest something to boost the brightness of seven segment?
 
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zeroskj1988

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multiplexing also important along with brightness of display. my dear friend
 

alexan_e

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use cd 4093
And what makes you think that it will be able to provide more current compared to cd4514?
I would suggest independent transistors for each display.

multiplexing also important along with brightness of display. my dear friend
Yes, if the multiplexing frequency is too fast you can also get a dimmed display even if the driver is able to provide the current.

Alex
 

zeroskj1988

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thank you alex. conversation with you give me an idea.so i am going to use UDN series current source drivers. so that you for your help
 
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zeroskj1988

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is it possible to drive the cathode of common anode seven segment through uln2003? thank you in advance
 

alexan_e

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is it possible to drive the cathode of common anode seven segment through uln2003? thank you in advance
Yes it is , ULN2003 can be connected in any load that has the other end connected to the positive supply, in your case the ULN2003 drives the display cathodes and the common anode is connected to the positive supply (directly or through a transistor switch to be able to do multiplexing)

Alex
 

zeroskj1988

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i have a problem with uln2003apg.i connected 5v to common. ground to GND pin. connected 0v to one of input supposing high at output,.but i don't get it.
whether uln work fine with 5v?
may i have to pull up input?
thanks in advance
 

alexan_e

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The internal stages of ULN2003 look like
ULN2003_stage.jpeg

Your connection is correct but the output will sink current when the input is high.
When the input is 0 the output will be off (floating), if there is any load working as a pullup then the output will be high.

Alex
 

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