# Constant current led driving

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#### theasus

##### Member level 3
I want to use IR led and photodiode in my project as a sensor. And for stable working I have to assure constant current for IR Led.How can i do this operation?Could you post a simple circuit and its usage?

Could you propose more sensitive circuit ?
I think this is not a sensitive.And this one more sensitive but i couldn't calculate the compenent values(transistor,resistor..)
Could you help about this circuit?And also for this operation we can prefer fet to transistor.But as i said i didn't calculate the right value for one led driving.

How constant does it really have to be? With a high enough
and tight enough voltage supply, the diode forward variation
can be small relative to headroom and a resistor will be fine.

So how good, and why?

I want to use an IR led for a sensor application.And for this application IR led's current must be constant for stable working. And I think,to end of my voltage regulation circuit which is consists of 7805, connecting a constant current source for IR led is neccessary. And My problem is searching a good circuit for this operation.

How much current do you need to run through your LED?

18 mA

18mA is a relatively small current. An output of common OpAmp can source or sink 18mA.

Here's an example of an OpAmp constant current source: http://www.faqs.org/docs/electric/Semi/03039.png. This is just one of many topologies. LED can be hooked up instead of a load resistor.
Here's a small collection of OpAmp current source and sink circuits: **broken link removed**

- Nick

This is how you calculate:

The input+ has a voltage set by the zener diode=Vz. Input- = Input+ due to feedback. So Vsense=Vz. The current through the resistors=Vz/Rsense

This is a good current sink, better than other designs in this thread.

__________________________________________
Low-price resistors, capacitor, and LEDs: **broken link removed**

theasus said:
Could you propose more sensitive circuit ?
I think this is not a sensitive.And this one more sensitive but i couldn't calculate the compenent values(transistor,resistor..)
Could you help about this circuit?And also for this operation we can prefer fet to transistor.But as i said i didn't calculate the right value for one led driving.

or current source, depending where you put the load

__________________________________________
Low-price resistors, capacitor, and LEDs: **broken link removed**

vistapoint said:
This is how you calculate:

The input+ has a voltage set by the zener diode=Vz. Input- = Input+ due to feedback. So Vsense=Vz. The current through the resistors=Vz/Rsense

This is a good current sink, better than other designs in this thread.

__________________________________________
Low-price resistors, capacitor, and LEDs: **broken link removed**

theasus said:
Could you propose more sensitive circuit ?
I think this is not a sensitive.And this one more sensitive but i couldn't calculate the compenent values(transistor,resistor..)
Could you help about this circuit?And also for this operation we can prefer fet to transistor.But as i said i didn't calculate the right value for one led driving.