5V to 24V & 24V to 5V Optocoupler Circuit

Thread Starter


Joined Apr 20, 2004
While searching for the optocoupler circuit in our forum I coudn't find one.Can someone help me?

But I read the post by a member asking query about the same.More details please.

What exactly you want to do?

I wish to drive an output say 24V relay from my 5V controller output.In another case my reed switch signal ( 24V) needs to fed to the controller having 5V input.I need led indication on bothe 5V and 24V side for confirmation.

See the attached.
The LED in a 4N25 optocoupler has a max 1.5vF @ 10mA; max continuous is 80mA. I chose resistors for around 30mA current.
Rlimit = (Vsupply - VfLED)/LEDcurrent
You can use that same formula for calculating your LED resistors. Use a resistor whos' value is equal to or greater than the result of the formula above.

Thanks Wookie.I will build this.

SgtWookie can you post the circuit from circuitmaker? I'm having problems on mesuring.

You're lucky I still have it. This thread is rather old.

Had to .zip it up; the .ckt file is inside the .zip.

Just the standard library models were used.

I know I am digging up an old thread here but I have a quick question. How big does R4 (750 Ohm) need to be. I mean watts. Will a 1/4W do or does it need to be bigger?

Thanks in advance

Welcome to the forums.

This topic has been hijacked in the past, but enough is enough. This is the original thread - http://forum.allaboutcircuits.com/showthread.php?t=14812

Current in a resistor is : I = E/R. E is 24 volts, but the LED will need about 1.5 volts to conduct, so use 22.5 volts. I = 30 ma. Power is given by I2R, or 675 mw. Use a 1 watt resistor.


About what schematic in what thread are you talking?

Ah, I see that beenthere already gave an answer.



Last edited:

Thread Starter


Joined Apr 20, 2004
Well, the above mess is the result of a merge gone wrong. The original OP and the later ones is obscured, but the circuit and calculations somehow survived.

Apologies for the mess.