Diode vs LED

The opto output is a transistor so you should be able to see a lower resistance across the collector and emitter connections (positive to collector) with an ohmmeter when the LED input is conducting.
But it's never a "short".
The maximum collector-emitter current will typically be some large fraction of the LED current.

Thank you very much I will try that.

Hello people
Please help me

My circuit is this diagran.
I always get 7.5 V in the vout.
I think pins 4 and 5 never get connected for some reason

Your new schematic does not show the polarity of the 20V.
If pin 1 is +20V and pin 2 is 0V then the transistor should turn on and Vout should be low. If pin 1 is -20V then the LED in the optocoupler might be destroyed.