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Forum Index : Microcontroller and PC projects : optocoupler on uMite input

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viscomjim
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Joined: 08/01/2014
Location: United States
Posts: 925
Posted: 11:56am 23 Feb 2015
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Is it okay to hookup an optocoupler to uMite and use ONLY the internal pull up resistor as shown here???
(resistor shown would be the internal pull up resistor)




P.S. switching would be low speed stuff, less than 2Hz.
Edited by viscomjim 2015-02-24
 
redrok

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Joined: 15/09/2014
Location: United States
Posts: 209
Posted: 12:26pm 23 Feb 2015
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Hi viscomjim;

Generally this is acceptable. At least for slow speed signals.

But it also "depends".
Some optocouplers have more leakage current than others especially at higher temperatures. If this leakage current is to high the input may look like a low to the input pin. So a smaller pullup would be needed.

Why do you not want to use a lower pullup resister than the internal one?

redrok
 
viscomjim
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Joined: 08/01/2014
Location: United States
Posts: 925
Posted: 12:45pm 23 Feb 2015
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Hi Redrok, appreciate your input!!! I am laying out a thru hole part pc board and have 5 opto couplers. So with the current limitiing resistor on the led side and a pull up on the transistor side that adds up to a lot of pcb real estate. I need to get my mind wrapped around surface mount for sure. This particular board might be a kit so surface mount is not really an option.

Anyway, I am trying to get away with eliminating 5 resistors.

Where in US are you?

Thanks!!!
 
Grogster

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Joined: 31/12/2012
Location: New Zealand
Posts: 9589
Posted: 12:51pm 23 Feb 2015
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I always use an external 10k pull-up for opto's, but that is just me.

I don't see any obvious reason why your idea would not work, but just as redrok says, I would be cautious of the internal pull-up, as it being 100k or so, might result in a slightly touchy sensor input, to paraphrase him for a moment....

Hook it up like that on a breadboard first, and run some tests - would only take a couple of minutes.
Smoke makes things work. When the smoke gets out, it stops!
 
redrok

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Joined: 15/09/2014
Location: United States
Posts: 209
Posted: 01:27pm 23 Feb 2015
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Hi viscomjim;
  viscomjim said  Anyway, I am trying to get away with eliminating 5 resistors.

You can greatly reduce board area by using 1/10th or 1/8th watt resisters. Then install them "vertically" by bending one lead 180 degrees. Some call this the "Hair Pin" or "Bobby Pin" technique.

Ok, that image is of a 1/4th watt resistor but 1/10th is much smaller.
  Quote  Where in US are you?

I'm from White Bear Lake, Minnesota.
45.081286,-93.023837
  Quote  Thanks!!!

redrokEdited by redrok 2015-02-24
 
viscomjim
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Joined: 08/01/2014
Location: United States
Posts: 925
Posted: 01:35pm 23 Feb 2015
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Thanks for the info Grogster and Redrok! I just stumbled upon SIP resistor array. Talk about saving space. I will pull up the opto instead of using internal. Just for peace of mind.

Stay warm Red! It was 76 here today in Clearwater Florida.
 
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