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1  Using Arduino / LEDs and Multiplexing / Re: LEDs without the use of current limiting resistors on: January 10, 2013, 09:30:36 am
So, is this a valid way to drive a display of 4 digits, 7 segments common anode?

a 74HC595 conected to the A...G with resistors on every segment to limit the current and a direct pinout from arduino to every digit 1...4?

or should I put a transistor on every digit? I think it is an obvious question but I'm not sure if the output current of  every pin on arduino is limited to 40 mA or it is just a limit that it shouldn't reach though security.

PS: In the case of using transistors to drive the digits, should I use resistors too at the gates?

Thanks for all the answers.
2  Using Arduino / Microcontrollers / Re: Attiny84 wiring, using arduino Duemilanove as ISP on: January 08, 2013, 06:36:55 pm
I'm asking this because I've seen different schematics for duemilanove and uno.
3  Using Arduino / Microcontrollers / Attiny84 wiring, using arduino Duemilanove as ISP on: January 08, 2013, 06:13:00 pm
Is it true that the 10 uF capacitor isn't needed to program an attiny84 from an arduino duemilanove?
4  Using Arduino / LEDs and Multiplexing / LEDs without the use of current limiting resistors on: January 07, 2013, 08:41:04 am
Hi, this is my first post and I'm sad to introduce myself with a question. But I'd really appreciate an answer.

Well, I'm in a project and I was searching some info on the internet when I found this code: http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/LED/_7Seg_Example.pde

The thing is that I would love to know wich rules/relations/equations does this guy use to get the current average just with the amount of time that the led is on and off, in this part of the code:

Code:
//Display brightness
//Each digit is on for a certain amount of microseconds
//Then it is off until we have reached a total of 20ms for the function call
//Let's assume each digit is on for 1000us
//If each digit is on for 1ms, there are 4 digits, so the display is off for 16ms.
//That's a ratio of 1ms to 16ms or 6.25% on time (PWM).
//Let's define a variable called brightness that varies from:
//5000 blindingly bright (15.7mA current draw per digit)
//2000 shockingly bright (11.4mA current draw per digit)
//1000 pretty bright (5.9mA)
//500 normal (3mA)
//200 dim but readable (1.4mA)
//50 dim but readable (0.56mA)
//5 dim but readable (0.31mA)
//1 dim but readable in dark (0.28mA)

I wonder that this may be related to joule/second = w, but I'm not sure.

Thanks in advance.
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