LED indicator on 4N25 Opto

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ljbeng
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Joined: Mon Jan 13, 2003 1:01 am
Location: South Central NE
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LED indicator on 4N25 Opto

Post by ljbeng » Mon Oct 13, 2008 12:19 pm

I did kind of a dumb thing and had the boards made. I wanted to add led indication that the flow meter is sending pulses. The flow pulses (0 and 12vdc) go through a 3.3K resistor to pin 1 of the 4n25, pin 2 is grounded.

Pin 5 goes to a TTL input PIC micro. The LED flashes but pin 5 only drops to 1.6v and the processor does not see the pulse go low.

Code: Select all

6--NC

       33K
5----/\/\/\-----+5V
   |
   |-----|<---/\/\/\--- +5V
   |     LED    1K
   |---------- To Micro

4----Ground
For now I have taken out the 1K and dropped the led. Does anyone see an easy substitution fix? Thanks.

Bigglez
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Joined: Mon Oct 15, 2007 7:39 pm
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Re: LED indicator on 4N25 Opto

Post by Bigglez » Mon Oct 13, 2008 1:09 pm

ljbeng wrote: The LED flashes but pin 5 only drops to 1.6v and the processor does not see the pulse go low.
...
For now I have taken out the 1K and dropped the led. Does anyone see an easy substitution fix? Thanks.
The transistor output of the opto-coupler is
unable to drive this load. Options at this point
are to find a substitute opto-coupler with a
higher current transfer ratio, or boost the
opto-coupler's LED current by reducing the
3k3 ballast resistor. Other changes would
require a PCB change.

(1) Reduce the 3k3 to 1k0, to get 10mA of
LED current (you are at 3.18mA now).

(2) Try a 4N35 device and no other changes.

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