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Topic: Potential problems with large (50x30) led Matrix (Read 1 time) previous topic - next topic

CrossRoads

10 '595s is 80 bits, 80 transistors - just need more LEDs, and make one of the shift registers support 2 columns as well, just need to be a little clever coding it.
Will need 50 column current limit resistors, value determined by the forward voltage of your LEDs, and 80 base current limit resistors for the transistors so the HC595's don't burn out trying to drive them - the Base/Emitter voltage wants to go to 0.7V when driven, the resistor limits the current - pick a resistor that will limit current to the HC595's output level, which is 6-8mA if you want to ensure good voltage levels: (5V-.7V)/.008 = 560 ohm, 470 ohm at the lowest.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

aKiwi

Wow, i love this community.

You have surpassed my expectations 1000%.

Thanks heaps for the extensive reply, all the numbers are now starting to make sense (datasheets are more easier to read now)
I made a mistake in the subject, should have been 20 not 30.

I plan on making a proof on concept in the weekend, so for that ill attempt to wire up 8x8 leds and get the software started.


Here is alittle schematic i made up for the proof of concept.



With 8 leds on each row lighting up at one time, that works out to be a 82 ohm resistor on every led/ column.

If im reading your post correctly i need a 560ohm resistor between the base pins (of the row transistors) and their relative bitshifter pin?

--

Also if i do choose later to try to light all 50 leds) do you have a type of MOSFET in mind? My searching isn't proving so successful.. noone has thought to create a mosfet calculator ;)

If i use the parallel 2n3904 approach, how do you think i should power such an application? I intend on using a cell phone charger wall wart i have lying around for the test, but from it's readings i can (i assume its a max reading) draw up to 450mA from it.
So 450/20 = 22.5 -> so 20 leds. 

In which case if i need to light 50 at a time, then i need 10v for 40 (two strings paralleled, running 20 in series).. and thats where my knowledge on power supply breaks down..


Thanks so much for your time.


CrossRoads

560 between 595 & transistor base, yes.

Keep it at 5V, 10V just makes selecting parts harder.  You can get a 4A supply here for little money.
http://www.mpja.com/prodinfo.asp?number=18520+PS
I use this one for my projects.

Search at digikey.com
mosfet single n-channel
then start filtering one thing at a time:
- in stock
- logic level gate
- surface mount or thru hole

doing the above, with thru hole as the last, yields 12 pages.
View page 1, then sort on Rds.
Go thru the pages & find a price & Rds you can live with, such as
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=954-PSMN4R3-30PL127-CHP for 74 cents, Rds = 4.3mOhm.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=NTD4960N-35GOS-ND for 48 cents, Rds = 8mOhm

or, sort by price & scan down until you find a low Rds at a cost you're comfortable with.
I didn't look at surface mount; this should give you an idea of what to look for.



Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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