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Topic: 3-8 line decoders (Read 1 time) previous topic - next topic

treebykooba

Hello,

I want to try out the arduino ShiftOut tutorial (I can't post links yet) because I want to use around 50 or so RGB LEDs on one duemilanove (I know it's a lot...is it possible?). Could anyone point me to any info explaining what the numbers in these chips mean? In the tutorial it says to use a 74HC595 3-8 line decoder. Sparkfun sells a 74HC238 and I just ordered a sample from fairchild of MM74HCT138N. But they are all 3-8 line decoders, all appear to be CMOS and they all have that 74HC in them. What is the difference? Would I be able to run that tutorial with the chips I ordered?

Thanks so much for any help!

Grumpy_Mike

#1
Feb 09, 2010, 10:55 pm Last Edit: Feb 09, 2010, 10:56 pm by Grumpy_Mike Reason: 1
A 74HC595 is not a 3-8 line decoder, it is a shift register.

Quote
point me to any info explaining what the numbers in these chips mean?


http://en.wikipedia.org/wiki/List_of_7400_series_integrated_circuits

Quote
Would I be able to run that tutorial with the chips I ordered?


Sorry no.

MikMo

The 74HC595 is a 8 bit serial in parallel out shitregister. It will give you extra digital outs. These are good for controlling LED's if you only need to turn the LEDs on / off.

If you need to control the intensity of each of the colors in the LED's (to get mixed colors) this is not a good way to do it. It is better to use an IC that gives you PWM control. It is possible to implement PWM control in software using shiftregisters but it is not easy and you will loose some of the LED's brigthness.

This is an example of a specialized PWM IC that can drive 8 RGB LED's

http://macetech.com/store/index.php?main_page=product_info&cPath=6&products_id=17&zenid=d3b4d5fd62588a8d25a8a67b374968cc

But you will need quite a few before you can control 50 :-)

treebykooba

Hello,

Wow thanks for the quick replies! So I'm curious...what are those chips I ordered (the MM74HCT138N)? Is there anything I could do with them? (it's okay they were just free samples) On the wikipedia entry for 138 it says "3 to 8-line Decoder/Demultiplexer"  (no idea what the MM means also) and the 595 is "Serial-in Shift register with Output Latches." I don't actually know what any of that means...the 238 on sparkfun says it's "1-of-8 Decoder/Demultiplexer, Active High Outputs." Could I run that ShiftOut (http://www.arduino.cc/en/Tutorial/ShiftOut) tutorial with the chip from Sparkfun?

I'm not looking to do any PWM quite yet...just want some DIP packaged chips I can slap on a breadboards...

Thanks so much again for any help...

Grumpy_Mike

#4
Feb 09, 2010, 11:43 pm Last Edit: Feb 09, 2010, 11:44 pm by Grumpy_Mike Reason: 1
Quote
Could I run that ShiftOut (http://www.arduino.cc/en/Tutorial/ShiftOut) tutorial with the chip from Sparkfun?


No, a shift register is a set of boxes connected together, you put something (a zero or one) in the first box and hit the clock input and all the boxes shift up by one place.
Read this:-
http://en.wikipedia.org/wiki/Shift_register

A 3 line to 8 decoder has 8 outputs and 3 inputs. Depending on what binary number you put on the input that number output goes low. So if you had all the inputs low then output number 0 will go low with all the other outputs high. If all inputs are high then output number 7 would go low with all the other outputs high. That is for a 138 for the 238 the active output goes high will all the other outputs low.
It's like this:-
http://en.wikipedia.org/wiki/Decoder

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