help with leds

bobsmith

I am making a emergency light bar with leds. Is it possible to connect many led leads to one pin and how much power do I need?

Jon Williams

Probably not -- what you may want to do is some sort of buffer/driver like the ULN2003/ULN2803.· The attached schematic shows one line connection.· The 330 resistor is based on putting about 20 mA through the LEDs.· At 1.5 volts each, there is 6 volts across the resistor.· 6 / 0.020 = 300, 330 is the closest "standard" value; this results in 18 mA through the LEDs.· The resistor should be 1/4 watt (0.018 x 6 = 0.108 x 2 = 0.216, hence 1/4 w).

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Jon Williams
Applications Engineer, Parallax
Dallas, TX· USA

Post Edited (Jon Williams) : 1/3/2005 5:37:01 AM GMT

Martin Hebel

Using a common transistor (Radio Shack should have) may save you some cost and space also.



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Martin Hebel

Electronic Systems Technologies
Southern Illinois University Carbondale
Personal Links - ·Lot of BASIC Stamp info
and
SelmaWare Solutions
StampPlot Pro Version 3 Release 4
Graphical Data Acquisition for your Micro and Imagination!
Now allows additional controls to be added, or developed.

Jon Williams

My assumption was that he would animate several sets of LEDs to get attention, hence my choice of a multi-stage driver. I showed just one stage for simplicity.

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Jon Williams
Applications Engineer, Parallax
Dallas, TX· USA

chemania

Thanks for this post....after reading the nuts and volts article on the uln2803 and spending a couple hours trying to figure out why I could not get an led to flash your image said it all.· The article had pin 10 listed as a + supply.· When trying to run it this way (as a current source I suppose) it just would not work.·· Using it according to your diagram (current sink??) it works perfectly.· I guess I should have searched the forum earlier.· Lesson learned.·Now I have to rewire my 8-bit relay board to use the chip properly!

Heating up the iron,

Chemania