I am trying to build an optocoupler-based fast switching circuit. But I am continuously getting 24V as output. 5V is the input to the optocoupler and I want output as a 24V signal with 20mA current. If anyone can help with how to calculate resistor value at the output please help. I tried level shifter circuit but it is limiting my voltage to 18V. need help to solve this puzzle!! Thanks in advance (Interchange pinout 3 and 4 from schematic)

 

This is the way it should be wired for 24 volt out with the switched closed.
You didn't have series resistor (R1) in your schematic so you probably damaged the opto.
In this application I don't see the need for the buck converter. You can switch the opto directly from the 24 volt supply, just need to change R1 to 2.2K ohm.

 

Thanks for the updated schematic. I made changes as you said and the circuit worked perfectly.

 

In this application I don't see the need for the buck converter.

I don't see the need for the opto-isolator. Both sides are connected to the same supply - what's being isolated?
You could replace it with a BC547.

 

I don't see the need for the opto-isolator. Both sides are connected to the same supply - what's being isolated?
You could replace it with a BC547.

the switch I've shown in my schematic is a mechanical design that needs to be operated at low current and voltage, that's why I used buck and optocoupler for switching.

 

The opto-isolator is running at 10mA. A BC547 would only need 240uA.

 

The opto-isolator is running at 10mA. A BC547 would only need 240uA.