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 Posted: Jan 05, 2007 - 12:10 PM |
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Joined: Jan 02, 2007
Posts: 2
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Hi,
im a complete beginner with avrs, but i would like to build my own led-matrix, because i think a matrix is really interesting and its not so difficult to start with the software because i could only use one row first and then more, then try pwm and so on.
Programming isnt a real problem, but i dont have much experience with avrs and electronics in general. A bit from school, but nothing more.
I want to build a 8x8 led matrix, with a atmega8. After reading many threads, webpages etc. i think i know a bit, but i have some questions left. The 8x8 matrix should be almost the same like the circuit here: http://home.wanadoo.nl/electro1/avr/dotmatrix.htm (if its 5x7 or 8x8 should really matter?)
I dont want to use an external chip to control the leds, but use the atmega directly by multiplexing the 8x8 leds, the atmega8 has enough output pins and its fast enough.
But there are a few things i havent fully understood by now:
Do i need resistors for the leds? Is it a problem to have up to 8 leds on one pin?
And what type of leds exactly could i use? What else do i need for the circuit, what power supply etc?
I read much about programming an atmel and so on, but these things are probably too simple so they arent mentioned anywhere, but i dont know this.
And, isnt it a problem that the leds are only on for max. 1/8 of the time when i multiplex them? Arent they too dark?
I know how to program with bitwise operations and so on, but im missing experience with electronics in general. But im very interested in doing this and i have time, so why not 
Greets,
sid |
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Posted: Jan 05, 2007 - 12:37 PM |
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Joined: Oct 30, 2001
Posts: 1563
Location: Manchester England
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You do need a current limiting resistor for the leds.
8 leds is a bit much unless low current devices. It is normal to switch the leds using a transistor.
You need to multiplex the leds at at least 60Hz to avoid flicker.
Led choice at this point; just get a hand full of red ones. There are hundreds of types out there just find the 'cheapest' from your supplier. Do not freak out and get high brightness devices they cost a lot more.
Look up the spec for the devices you choose to find the max current, and limit to just below that. |
_________________
Keep it simple it will not bite as hard
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Posted: Jan 05, 2007 - 01:16 PM |
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Joined: Jan 02, 2007
Posts: 2
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Ok, then i think i need 8 transistors and 8 resistors. When i only use one row at once, every row needs a transistor, because then max. 8 leds per row -> 8 leds on one pin. I dont need a transistor for each column because theres only one led at once, which isnt a problem, but every column needs one resistor.
The leds i found need 2,1V and 20mA. I think i have 5V (the atmega needs 5V?) so i need 3V at the resistor, r=u/i -> r=3V/0.02A = 150 Ohms
Is this right? |
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Posted: Jan 05, 2007 - 01:21 PM |
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Joined: Oct 30, 2001
Posts: 1563
Location: Manchester England
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As close as it needs to be.
Don,t for get the base resistor fto the transistor. |
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Keep it simple it will not bite as hard
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