Author Topic: 74HC595 weirdness  (Read 709 times)

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Offline konzillTopic starter

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74HC595 weirdness
« on: October 13, 2019, 11:27:33 pm »
Edit: looks like I have a couple of bad chips, either broken or just dodgy. I replaced them with another one bought at a different store on a different day and the circuit works right.


Hi. I've been learning how to wire up this shift register. Specifically I'm using the Texas Instruments SN74HC595.


But I've had some weird behavior. The circuit had no output until I added pull up resistors to +5v on the output pins of the chip. As soon as I added these my LED array, also wired to the output, started producing the expected output.  The thing is that none of the tutorials for how to wire up this circuit seem to need to do this.  I've now built the circuit from scratch using new components twice and they both behave this way. 


the circuit on each output pin looks like this:

Code: [Select]
chip output------- LED ---- R1 --- Gnd
                |
                R2 --- +5v
                   
EDIT: I've attached a photo of my actual breadboard. Seeing as its been asked I have OE low and MR high in this setup.

The Arduino code is set up to count from  0 to 255.  And with the R2 resistors in place I'm seeing the green LED's count up exactly as I would expect. 
« Last Edit: October 14, 2019, 09:22:36 am by konzill »
 

Offline amspire

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Re: 74HC595 weirdness
« Reply #1 on: October 13, 2019, 11:42:51 pm »
The IC should be able to put a few mA through a led without a pull-up. How much voltage is across the LED when it is on? If you have a LED that needs a particularly high voltage or more then 1 LED in series, that could be the problem.

Do you have the OE pin connected to 0V?

Richard
 

Online wraper

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Re: 74HC595 weirdness
« Reply #2 on: October 13, 2019, 11:49:22 pm »
SN74HC595 has 3-State Output. What you say means that IC output is in high-Z state, not logic high.
 

Offline konzillTopic starter

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Re: 74HC595 weirdness
« Reply #3 on: October 14, 2019, 12:45:43 am »
So does that mean things are working correctly? Or that I need to change one of the other inputs for things to be working correctly? Do I need to change my output enable? The pin Is currently pulled low.
 

Offline thinkfat

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Re: 74HC595 weirdness
« Reply #4 on: October 14, 2019, 04:31:02 am »
OE low is fine, but remember to also pull MR high.
Everybody likes gadgets. Until they try to make them.
 


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