It's my first instructable so please leave feedback how to improve the explanation and make it more readable.
And here's the circuit:
Components i used:
R1: 1M Ohm (depends on your currente needs)
Pot1: 10k variable resistor (depends on your currente needs)
R2: 1 Ohm (depends on your currente needs)
R3: 10k Ohm
R4: 10k Ohm
Q1: large N-channel MOSFET.
U2: Just a voltage regulator to power the op-amp and give constant current to R1+Pot1.
How it works:
R1 and Pot 1 (current set resistors) values set the maximum current of the LEDs by changing the voltage across Pot 1.
This sets the maxiumum voltage across R2, the current sense resistor.
It's realy important the voltage across R1 and Pot1 is constat so we can calculate the voltage across Pot1.
If connected has shown, everytime the voltage in R2 is over the voltage in Pot1 the op-amp will turn off and turn back on again.
It doesn't actualy fully turn off, just enough to keep the current at what we want
At 5V, the voltage in pot 1 is equal to ( 5/(R1+pot1)) *pot1. This gives you the voltage across pot 1.
At the maximum of 10k the voltage is (5/1 010 000)*10 000)=0.1V.
By reducing the resistance of pot 1 you can reduce the LEDs current and their luminosity.
To know how much current there is in the LEDs we need to do VPot1*R2. In max luminosity and with R2=1Ohm, 0.1V*1=0.1A or 100mA.
The MAX921 only works from 2.5V to 11V so you need a voltage regulator to power it above those values. The minimum isn't realy a problem since you need at least 3V for 1 LED.
Altough the MAX921 works at 2.5V most MOSFETs require at least 5V to be turn on. Logic level MOSFETs should work at 3V.
Because of this you should power your op-amp with at 3 or 5 volts depending on your mosfet
Be careful with the max input voltage of the voltage regulator you use.
You have to watch the max rated current and voltage trough Q1 so chose well your MOSFET.
The op-amp never fully turns off the LEDs even when the "+" is conected to ground. You realy have to cut off the power or connect "-" to 5V (remember to keep R4).
Pros and cons over dan's original circuit:
Input supply only needs to be 0.1V over the LEDs foward voltage.
Easily dimmable luminosity.
Uses more components
Needs a constant voltage trough R1+Pot1.
Needs regulated voltage for the op-amp.