Connecting optocoupler to GPIO

Projects General Guidance
1

Hello,
I have a question about how to connect optocoupler.

opto

I am bringing 12V to PIN 1 on optocoupler. PIN 2 and pin 3 are connected to the ground and PIN4 is connected to pull up (5V) and to the arduino GPIO.
My question is how big of a resistor (in series with pin 1) do I need to not destroy the LED inside the optocoupler.

Optocoupler: QTM357 QT-Brightek

I cant seem to understand nor find the max current allowed through the diode. Could someone help me out ?
Currently I am using 1.2kOhm resistor but I dont think it's enough since I destroyed the led inside of opto.

2

Hello,

Please choose the forum category carefully when you post ➜ Could you take a few moments to Learn How To Use The Forum. It will help you get the best out of the forum in the future.

Thank you.

PS/ I moved your post out of the Uncategorized category

3

which optocoupler do you have ? 12V might destroy the "LED"

4

I'm a bit foggy atm so I ask, can you measure the forward voltage between pins 1 and 2? Otherwise go with Vf for an IR or red led.

Isn't resistance == (V - Vf) / current? Current < 5mA is desireable.

I've done this once before but that was almost 30 years ago, wired a Hall switch on 12V line to a parallel port.

5

I have provided the info of optocoupler in my question
QTM357 QT-Brightek

6

Typical Vf is 1.24V so I need voltage drop of almost 11V. With my calculations (If=10mA datasheet) that would mean I need a resistor of about 1.1 kOhm so I used 1.2kOhm (just in case), Does that make sense?

7

10mA is likely guaranteed to work and I get 1076 Ohms, 1.1k looks good to me.

1 Like
8

I would think about 3mA (3.3k resistor) would be plenty. What value pullup resistor on the UNO side?

9

I put 100kOhm ... Should have been 10k, I have to fix that. I still get 4.86V on the GPIO, which is fine

10

It's not the voltage you need to limit but the current.
The less you need, the better. Arduino pins have a total max source + sink capacity of 200mA. You don't really want to use half of that.

11

This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.