Because I will use the 1k resistor but I still want my led to be bright, I have created the following schematic. Is my pull up resistor correct? Or should it be a pull down resistor. Can you please give me recommendations on which mosfet to use and what I should look out for when choosing a mosfet for this scenario, could I use a bjt instead? Thanks. The optoisolater is still 4n36

If you connect an external Mosfet, then the LED current is no longer dependent on the current capacity of the opto transistor.
You can use a PMOS like you've shown. But you'll need to reverse the connections to the diode so that diode current flows only when the input signal is at ground. Using a 1k resistor is dicey because it reduces the current to about 4mA but it should work. What helps this configuration is the voltage driven mosfet. It only requires an extremely small amount of current to turn on. Use a small 20V mosfet but change the supply to 5V and external LED resistor to 300 ohms (for 10mA).

 

If you connect an external Mosfet, then the LED current is no longer dependent on the current capacity of the opto transistor.
You can use a PMOS like you've shown. But you'll need to reverse the connections to the diode so that diode current flows only when the input signal is at ground. Using a 1k resistor is dicey because it reduces the current to about 4mA but it should work. What helps this configuration is the voltage driven mosfet. It only requires an extremely small amount of current to turn on. Use a small 20V mosfet but change the supply to 5V and external LED resistor to 300 ohms (for 10mA).

What do you mean that I have to reverse the connection to the diode? Is my schematic not correct? Thanks

 

What do you mean that I have to reverse the connection to the diode? Is my schematic not correct? Thanks

If you want the LED to turn on when the input is low then your schematic is not correct. You need to connect the diode anode to 3.3v and the cathode to the input signal.

 

What do you mean that I have to reverse the connection to the diode? Is my schematic not correct? Thanks

BTW- you can also do this with two mosfets as explained way back in post #8.
R1 may or may not be needed depending on the type of signal source.

 

You can also do it with 1 transistor or MOSFET as soon suggested in post #3 and shown it n post #25, but the TS must use a specific redundant optocoupler and 1K resistor because “another engineer” sugeested it for his “personal project.”

 

You just need an inverter driving a high side switch.
View attachment 272147
Or with just MOSFETs:
View attachment 272148

I beleive that because you are using a p channel enhancement mosfet, you would need to have a pull down resistor instead of a pull up resistor for R2. The enhancement mosfet is not on when the gate and source voltage is 0

 

I beleive that because you are using a p channel enhancement mosfet, you would need to have a pull down resistor instead of a pull up resistor for R2. The enhancement mosfet is not on when the gate and source voltage is 0

R2 insures that Q1 is off (gate source voltage = 0V) when Q2 is off.

FYI, MOSFET is an acronym (Metal Oxide Semiconductor Field Effect Transistor) and is capitalized. We also capitalize the type of silicon.

 

R2 insures that Q1 is off (gate source voltage = 0V) when Q2 is off.

The TS wants the LED ON when the input is low (off)