PIC 16F84A 10x7 Display Project LED Falling Brightness.

I built a 10x7 led display project using PIC 16f84a.It is the extended version or the 5x7 display.

My 10x7 display has 10 columns & 7 rows.Circuit is running well.I added some runing messeeges & some designs in it.

My problem is

* when column scaning the brightness of the leds are good.
* when the row scaning the brightness of the leds going low.
* when the letters displaying continuously the brightness falls.

The PIC output pins can draw only 25mA current. Its not quit enough to drive more leds...

Anybody knows how to prevent this??? I hope to extend this circuit to 20-25 columns to display a whole word...

I have attached the connection of LED's

Add PNP drivers feeding the rows (and invert the logic in the program).

RE: PIC 16F84A 10x7 Display Project LED Falling Brightness

Nigel Goodwin said:

Add PNP drivers feeding the rows (and invert the logic in the program).

Click to expand...

I Didn't Add PNP Drivers First Bcuz I need to change (invert) my whole program.
My Program is too big about 14 pages so many patterns......So I added NPN Transisters to each ROWS.(feeding 7 NPN Transisters).The circuit works pretty well..But...
In my next LED display having 30 columns.So one Transistor is not enough for each ROW.So I'm Planing to add NPN transistors to each 5 LED's in a ROW.Signal is giving to the base from the PIC through 100 Ohm Resistor.

ThankX

If your still working on it, try this:

In place of your row resistors, insert the MIC2981/2982 chip from Micrel. It's an LED driver and should source 500ma to each row, more than enough for you right now.

Datasheet: http://www.datasheetcatalog.org/datasheets/37/186976_DS.pdf

Good Luck!

Hi I m new in this world...so i want to know regarding PIC 16F84A.
If I use a NPN transistor in each row and each row contain more then 100 leds in parallel ,so what should be the input voltage and how much ampere of current should we provide? if possible provide the circuit diagram also... plz help

Thanks in advance...