help! Led Matrix Row Transistor

[cuber18]

ok, thank you for the clear explanation sir.. what if i only use 1 npn transistor?

 

[DineshSL]

Hi cuber,

You use 74ls138 to drive transistors. Outputs of 74ls138 is active Low; however you need to on/off cathode line of LEDs. This is not possible without 2 transistors NPN & PNP. Otherwise you should change 74ls138 to use only 1 npn.

Thank you,

 

[cuber18]

if i use 2 transistors what would be its schematic from 74ls138 to cathode row of led matrix?

 

[alexan_e]

ok, thank you for the clear explanation sir.. what if i only use 1 npn transistor?

I have explained that in post #8 https://www.edaboard.com/threads/202816/#post854881

if i use 2 transistors what would be its schematic from 74ls138 to cathode row of led matrix?

I already gave you a schematic in post #10 https://www.edaboard.com/threads/202816/#post854895

Alex

 

[DineshSL]

Hi alex,

Cuber has a little problem here;
He use 74ls138 to drive the base of transistor. It's a 3-line-binary to 8-output decoder/demultiplexer, it outputs Low when an output is selected. (check data sheet for further details).

Hence to light up particular set of LEDs 74ls138 outputs Low. So using 1 npn to switch cathode of that LEDs is not easy. ( 74ls138 can't sink much current to connect LEDs-cathode directly). That is why 2 transistors are essential for his application.

Your schematic on post_10 is applicable.
Thanks

 

[cuber18]

i have 14 cathode rows.. so it means i will use 2 NPN each rows? am i right? having 28 NPN all in all? and what resistors will i use here? thanks for all of your help..

 

[alexan_e]

Yes you need 2 transistors for each row but i don't know anything about your circuit,
how do you drive the columns?
Is your led matrix like this

How many rows and columns do you have?
What is the specified current for each led?
What is your power supply?
Do you use multiple displays?
Do you use any multiplexing or you drive them all in the same time?

Alex

 

[cuber18]

yes sir, thats my led matrix.. i have 14 rows and 40 columns.. 10mA or 4mA will do.. i have DC supply here having 12v and 1.2A... yes i do use multiplexing.. i have 74HC595 in my columns as drivers.. i dont know about multiple displays.. but my matrix is scrolling...

 

[alexan_e]

do you have 40 columns in each row that should be able to light in the same time,
if so you need to be able to provide a max of 10mA *40 =400ma in each row.
Is this correct?

Do you light only 1 row at any given moment (multiplexing)?

Alex

 

[cuber18]

yes sir... its correct.. so what can you suggest sir ? no i dont light1 row sir because i am scrolling lettters here..

 

[alexan_e]

I'm trying to understand the max current that you will need,
if you light more than i row at a time then your total current may be more than the specification of your psu.
You have 560 leds, if you light more than 120 leds simultaneously (10ma each) then you power supply will not be able to provide the current.
How does your multiplexing work, i would expect it to light row1..2..3......14..1..2.... at a fast rate so that the eye can't see that switching.

Alex

 

[cuber18]

ahhmm.. yes my supply is not enough.. so i canvass another supply for my led matrix.. 9v with 3A.. is it fine.. but 7805 can only accommodate 1A in its output.. so i have to bypass it in order for me to have a 2A output..

 

[alexan_e]

This is why multiplexing is used, to lower the current consumption.
Do you drive the leds with a microcontroller, if so you can do multiplexing.

Alex

 

[cuber18]

yes im driving it using pic16f877a.. so any suggestions sir? i worry so much on what drivers will i use in my rows and columns..

 

[alexan_e]

The driver will be the same as i posed before but I'm trying to tell you that you may have problem with your power supply.
You said earlier that you are using multiplex, but you didn't explain to me what kind of multiplex you are doing,
are you using multiplexing or not?
in rows or on columns?
if you use it in columns then each transistor will only need to give a max of 10ma beaus only one column will be on.

If not do you know what it is and how to use it?

Alex

 

[cuber18]

im using row scanning sir.. and im using 74hc595 in my columns which a shift register.. i dont have transistors in my columns sir..

---------- Post added at 17:01 ---------- Previous post was at 16:58 ----------

https://www.edaboard.com/threads/199151/

in this thread i have my attached simulation in isis..

 

[alexan_e]

I tried finalTest3.DSN but almost all the leds are on and i don't see any characters,
Which file should i use?
if you have made changes upload the fixed project

Alex

 

[cuber18]

the finaltest only sir.. not the finatest3..

 

[cuber18]

here is it sir..

 

[alexan_e]

there isn't a 74ls138 in your schematic, will you use it in place of the 74HC154?

Alex