Another 74HC595 Question
rr
Posts: 63
Hello
I am putting together a project that consists of using a BS2 OEM to control 24 leds. I want to daisy chain (3) 74HC595's together to control the leds and give me some left over I/O's to work with. My main question is can·I directly drive·one led from each output of the 74HC595 (20ma each with the possibility of powering all 24 at the same time). I saw the example from parallax (http://www.parallax.com/dl/docs/books/sw/exp/sw23b.pdf) but·I noticed it uses pretty large resistors before each led.·I looked at the data sheet for the 74HC595 (http://www.fairchildsemi.com/ds/MM/MM74HC595.pdf) and as far as·I could tell it has a max output of 35ma per pin, but does that also mean·I can power all the pins at the same time with 35ma max per pin? The reason·I am cautious is because·I know that the Basic Stamp can source 20ma per pin but not to all the pins at the same time.
Thanks for the Help...
rr
I am putting together a project that consists of using a BS2 OEM to control 24 leds. I want to daisy chain (3) 74HC595's together to control the leds and give me some left over I/O's to work with. My main question is can·I directly drive·one led from each output of the 74HC595 (20ma each with the possibility of powering all 24 at the same time). I saw the example from parallax (http://www.parallax.com/dl/docs/books/sw/exp/sw23b.pdf) but·I noticed it uses pretty large resistors before each led.·I looked at the data sheet for the 74HC595 (http://www.fairchildsemi.com/ds/MM/MM74HC595.pdf) and as far as·I could tell it has a max output of 35ma per pin, but does that also mean·I can power all the pins at the same time with 35ma max per pin? The reason·I am cautious is because·I know that the Basic Stamp can source 20ma per pin but not to all the pins at the same time.
Thanks for the Help...
rr
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If you need more current handling... it's not that much trouble to adapt the HC595 routine to use an Allegro A6821 Serial Latched Sink Driver (think: a serial shift register with power outputs that handle 500ma at EACH output!)
http://www.allegromicro.com/en/Products/Part_Numbers/6821/6821.pdf
It's a one chip solution that you can daisy-chain to get the number of outputs you need. DIGIKEY has these as part # 620-1095-ND
Post Edited (pwillard) : 6/25/2007 11:14:04 PM GMT