It won't stay at 100%. Better designs will design for a lower CTR so that the degradation will take longer to impact the circuit.

As the other member said, an optocoupler isn't required. All you need is an inverter and a transistor to drive the LED high side.

If you're able to drive the LED low side, you don't need the inverter and can use either an NPN transistor or N channel MOSFET.

Thank you. How exactly does the CTR of the optocoupler affect the output? Also, is the R1 the led limiting resistor? In the previous message you calculated R3 to be 10K, however I am required to use a 1k resistor after the signal. Should I just add a 9k resistor after? Thank you.

 

however I am required to use a 1k resistor after the signal.

If this is for a personal project, why are you "required" to use certain components in certain places? Why not just use whatever circuit actually makes sense for your specific application?

It certainly sounds like this is schoolwork, but that's not my place to judge. Assuming it's not schoolwork, why do you have such rigid component requirements?

 

How exactly does the CTR of the optocoupler affect the output?

You're counting on a CTR of 100%.

Also, is the R1 the led limiting resistor?

In my schematic it is. There are no component designators in yours. BTW, it's LED because it's an acronym for Light Emitting Diode. It's pronounced L-E-D, not led.

In the previous message you calculated R3 to be 10K, however I am required to use a 1k resistor after the signal.

You say this isn't schoolwork, yet you seem to be required to use specific components that aren't very appropriate for the task you have. Why is that?

 

Using an opto this way is not good practice (as a few previous posts have rightly pointed out), and its not recommended, but if you really want to use one, you can. See below.
When the input is low, the LED is ON. If you want to use an output supply of 5v, change RLed to 300 ohms.



Edit: fixed opto symbol.

 

This is the circuit proposed in post #3. It is the simplest circuit I can think of that will do what you want.

 

In this circuit LED current is almost independent from V(V2):

 

Thank you. How exactly does the CTR of the optocoupler affect the output?

CTR is the ratio of input current versus output current specified as a percent.
The datasheet specifies a minimum of 100% transfer at the test conditions shown on the datasheet.
So...if the VCE = 10 V and the input current is IF = 10 mA, the supported output load current will be a minimum of 10mA.

 

If this is for a personal project, why are you "required" to use certain components in certain places? Why not just use whatever circuit actually makes sense for your specific application?

It certainly sounds like this is schoolwork, but that's not my place to judge. Assuming it's not schoolwork, why do you have such rigid component requirements?

That is because this personal project has limitations that another engineer has assigned to me in the program

 

You're counting on a CTR of 100%.
In my schematic it is. There are no component designators in yours. BTW, it's LED because it's an acronym for Light Emitting Diode. It's pronounced L-E-D, not led.
You say this isn't schoolwork, yet you seem to be required to use specific components that aren't very appropriate for the task you have. Why is that?

Because I am building off another previously made board that has that specific resistor

 

how do i find the mimumum amount of current needed to turn on the optocoupler?

 

Using an opto this way is not good practice (as a few previous posts have rightly pointed out), and its not recommended, but if you really want to use one, you can. See below.
When the input is low, the LED is ON. If you want to use an output supply of 5v, change RLed to 300 ohms.

View attachment 272153

Thank you. If i replaced the Rin with a 1k would this circuit still work? Also what is U2 in your schematic?

 

In this circuit LED current is almost independent from V(V2):
View attachment 272157

Thank you. Why is the transistor needed?

 

Thank you. If i replaced the Rin with a 1k would this circuit still work?

If you mean "will it still light the external LED", yes, but so faintly it will appear OFF (depending on the type of LED).
Because you will be reducing the CTR, that in turn, reduces the drive current to the internal transistor.

Also what is U2 in your schematic?

The Vishay 4N36 optocoupler, except I left the base pin off the symbol (its not used), but I will fix.

 

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

Can’t you also switch the low side?

UPDATE: Never mind… it just got weird when he said “another engineer” told him what to do with his “personal project”.

 

Thank you. Why is the transistor needed?

Because transistor Q1 and transistor of optocoupler together
work as stabilizer of LED current.

 

If you mean "will it still light the external LED", yes, but so faintly it will appear OFF (depending on the type of LED).
Because you will be reducing the CTR, that in turn, reduces the drive current to the internal transistor.



The Vishay 4N36 optocoupler, except I left the base pin off the symbol (its not used), but I will fix.

So the current that comes out of the transistor is the current that will power the LED? I though the optocoupler was U1? Thanks.

 

So the current that comes out of the transistor is the current that will power the LED?

Current doesn't come out of the transistor. It is limited the resistor "RLed".
Simply put, the conductivity of the transistor's C-E junction is controlled by the internal diode current.

I though the optocoupler was U1? Thanks.

There only one U1 and thats the optocoupler.

 

Current doesn't come out of the transistor. It is limited the resistor "RLed".
Simply put, the conductivity of the transistor's C-E junction is controlled by the internal diode current.



There only one U1 and thats the optocoupler.

Thank you. To confirm, on the datasheet it says that there is a minimum CTR of 100% and does not list the max. Does this mean that using a 1k resistor is instead fine because the end LED will still be bright? For example, if I were to replace the 220 ohm resistor with a 1k resistor, there would only be 2mA current flowing through the LED in to optoisolater. Then can I just make the current limiting resistor in front of the LED smaller to make the LED brighter? Thanks.

 

Thank you. To confirm, on the datasheet it says that there is a minimum CTR of 100% and does not list the max. Does this mean that using a 1k resistor is instead fine because the end LED will still be bright? For example, if I were to replace the 220 ohm resistor with a 1k resistor, there would only be 2mA current flowing through the LED in to optoisolater. Then can I just make the current limiting resistor in front of the LED smaller to make the LED brighter? Thanks.

You can change anything you want.
But if you change the diode current, you are changing the CTR and conductivity of the internal transistor. Reducing the diode current causes a decrease in the conduction of the C-E junction (or, in increase in the C-E" junction resistance").

 

You can change anything you want.
But if you change the diode current, you are changing the CTR and conductivity of the internal transistor. Reducing the diode current causes a decrease in the conduction of the C-E junction (or, in increase in the C-E" junction resistance").

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