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Pic and dual brightness 7 segment display

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superflux

New Member
Can someone tell me if there is any potential issues with my layout?

I have a superbright 7 segment LED display. I want to do a dual brightness via transistors and want to make sure that I don't blow out the transistors some how or ruin the PIC. I figure this is the least burdensome on the PIC itself. The PNP transistors are 2N3906s. I didn't note it on the schematic.

The 7 segment is a common anode, so I send a 0 to the respective PORTB port to light it up on that segment. The 7 segment LED can take up to 160ma.

Thanks in advance!
 

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be80be

Well-Known Member
I'm reducing the current flow to the 7 segment display for a dual brightness situation.

Not going to get much light with a 470 and the 220 your limiting it to
7.2 Millamps and the 3 k will lower to 1.5 millamps
I would leave the 3 K off and test with different values for the 470 resistor
I didn't see Bills post but till mine but he has the right idea
 
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superflux

New Member
I modified it and put a 200 ohm resistor directly off of PORTA,6 and between PORTA, 7, I put a 4k resistor to the Base of the transistor and removed the 3k resistor. I simply switch between the two and have the perfect bright and dim situation.

Works like a charm without a big software change. Its not that I have anything against PWM, I just don't want to go through the hassle for 2 stage brightness.

Thanks guys!
 

spindrah

New Member
you will not have the same brightness with a 1 as you do with an 8, the current that flows through the trany resistor will be devided between each segment.

and don't forget the LED voltage drop, you will never even get to 7ma
 

superflux

New Member
I get 84ma with 6 of them lit. I have a 39 ohm resistor on each segment. It is working very well so far. I'm getting the results I'm looking for anyway.
 

spindrah

New Member
I get 84ma with 6 of them lit. I have a 39 ohm resistor on each segment. It is working very well so far. I'm getting the results I'm looking for anyway.


lol, no you don't

most of that is the pics current or something else, unless you ALSO changed the 470ohm resistor you could not possibly have more than about 6mA even with the 200ohm resistors changed to 39ohm
 

superflux

New Member
I think you missed some of the changes I mentioned in an earlier post. Here is the current schematic with those changes:

**broken link removed**
 

be80be

Well-Known Member
Your 16f88 is a short timer when that 2n3906 is on your sinking 128 millamps
 

superflux

New Member
Your 16f88 is a short timer when that 2n3906 is on your sinking 128 millamps

Error on my schematic which should be corrected. I do believe when PORTA,6 is set to Input it will only sink ~25ma.

Instead of using that 200 ohm resistor, can I simply add another 2n3906 to PORTA,6? That should elminate any issue with too much current going to or from the PIC.

I sincerely appreciate the help!

**broken link removed**
 
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spindrah

New Member
what is the voltage drop of the LED? unless it is like 4v you are going over the MAX current and will end up popping that chip.
 

superflux

New Member
The 7 segment that I'm using is actually 6.5-7v at 160ma. The 160ma is with all segments lit. It can go brighter than what I'm giving it.
 
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