Optocoupler Circuit

Discussion in 'The Projects Forum' started by Paul Braham, Dec 5, 2014.

  1. Paul Braham

    Thread Starter New Member

    Dec 5, 2014
    Hi all

    First post on here. A bit of a newbie so hoping someone can offer a little friendly advice.

    I am currently in the process of working on a monitoring system to attach to the intruder alarm at home. The alarm has multiple outputs for different conditions that are held at 13v in normal operation, and drop to 0v under an alarm condition.

    I am attempting to interface this to a Raspberry Pi, using the 3.3v high/0v low. As i'd like both devices isolated from eachother, I have chosen to use an optocoupler rather than a simple resistor voltage divider.

    Many other use cases online seem to use an ILQ74 - (http://www.ret.hu/DataSheets/48_OPTOCSATOLO/VISI_360/ild74.pdf). My concern is, reading the datasheet, the input side seems to have a forward voltage of 1.5v max.

    The following website (http://www.dreamgreenhouse.com/features/rpi/io/index.php) shows an ILQ74 being used to interface with a raspberry pi as below:
    I'm hoping one of you guys can explain if this is correct? The 2.4kohm resisistor is being used to limit the current in to the LED if I am not mistaken. Does this mean that the LED is being driven with 12v? If so, is this outside the specs?

    Another thread on the Raspberry Pi forums seems to mention using the ILQ74 for the same purpose (

    Thanks in advance

  2. ScottWang


    Aug 23, 2012
    The LED is work for current not voltage, so no matter how high the voltage, even 110Vdc also ok, you just remember to using resistor to limiting the current less than 20mA.

    The Vmax=1.5V was tested on 20mA.

    For a continuing current 5mA to 16 mA is just fine for the LED brightness.
  3. Paul Braham

    Thread Starter New Member

    Dec 5, 2014
    Thanks for that :)