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Author Topic: Super Bright Red Led - BC547B Transistor, but which Resistors?  (Read 2890 times)
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Brazil
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:-p

Too bad I don't have those. But I will see if the local shop carries it... (doubt it)
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You can use ULN2803 as buffer on 74HC595.
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Sounds interesting, but how can I do that? Do you mind teaching me, pretty please? ;-) ;-)
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Brazil
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Can't I just use 2 x ULN2803 chips and 8 pins on the Arduino? (I don't need many free pins, only 4 for a multiplexer, and 2 for UART)
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I thought you wanted 16 non-multiplexed LEDs?
Connect up the 74HC595 & ULN2803 like I show at the bottom of this schematic.


* 74HC595-8x8LED.jpg (95.22 KB, 960x720 - viewed 27 times.)
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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.
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It doesn't matter, I can do both, just wondering on what I could do with the stuff I already have that will get my ultra brightness.  smiley-mr-green
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Non-multiplexed for ultra brightness smiley-cool
I'll be in the next room smiley-wink
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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.
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Maybe I'm not explaining things right or I just got lost.  smiley-red smiley-eek-blue

Anyway, as I asked, can't I just use 2 x ULN2803 chips and 8 pins from the Arduino?
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Yes you can.
Post a schematic when you're ready.
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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.
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Ok, here's the final deal, a friend also helped me out with the math.  smiley-mr-green

I will drive 2 x 74HC595 in series, so I can get 16 outputs.
Them I will drive those 16 outputs directly to 2 x ULN2803A

On each output of the ULN2803A, I will put a 100 ohm resistor + UB LED. (2.3Vf 20ma)

In my tests it works perfectly. And if I want a second LED to make it more bright, I just put another 100 ohm resistor and add in parallel.

Now I need to get some White-Frost Plexy to test how this will look like.

Here's something some guys did that are similar to my idea. But I plan on using RED-Only LEDs (Ultra Bright) and a 4 x 4 pattern instead.

http://e2e.ti.com/group/microcontrollerprojects/m/msp430microcontrollerprojects/447779.aspx

The difference is that in my project, each square will be touch-sensitive. ;-)
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That'll work.
2 LED/resistors in parallel per outpin will work fine too.
(5V - 2.3V - 0.7V)/100 = 20mA (0.7V is Vce across the ULN2803, rises to 0.9V typical at 100mA)

How are you making them touch sensitive?
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I'm using the CapSense Library and a 16ch Analog Multiplexer. I had to mess with the code around a bit, in order to get only true positive touches.

http://arduino.cc/forum/index.php/topic,121508.0.html

As soon as I have the first prototype ready, I will post about it.  smiley-mr-green
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