Here's an upgrade to my last circuit. Now, the current level can be changed by using the potentiometer. The current range is around 1.25mA ~ 20mA. Play with the value of the load resistor (lower one) and watch how the current stays at the set level.
I'm using my LM317 IC approximation. It is all the parts that are inside the switches with three pins: Input, Output, and Adjust. More details can be found here if you're interested: https://everycircuit.com/circuit/6138847104335872
A constant current source maintains a constant current through a load (resistor). It will change the voltage if the resistance changes according to Ohms Law (I = V/R) to keep the current constant.
An adjustable current source (like the one I have here) works the same way as a constant current source, EXCEPT that the current level to be maintained through the load is adjustable. It kinda like an adjustable voltage regulator, but with current instead of voltage.
The formula for this circuit can be simplified to an OK approximation if Rp >> R
I(min) ≈ 1.25 / Rp, 1.25 / 1k ≈ 1.25mA
I(max) ≈ 1.25 / R, 1.25 / 56 ≈ 22mA
Just as in my last circuit (https://everycircuit.com/circuit/5424946364547072 ), this one is not perfect. If the load resistor gets too high, the LM317 will not have sufficient output voltage to drive it at the designated current level, and the regulation breaks down.
One interesting thing to note about this circuit is that the current adjustment is non-linear. This means that a change in potentiometer resistance will not result in the same change in output current. (Calculus Warning: slope ΔR/ΔI or dR / dI ≠ M [constant]). I don't know if that is desirable or not. If you have good info on it, leave a comment.
LM317 Series:
Adjustable Voltage Regulator: https://everycircuit.com/circuit/6138847104335872
Constant Current Source: https://everycircuit.com/circuit/5424946364547072
Soft Power On: https://everycircuit.com/circuit/6070007032971264
Simple Battery Charger: https://everycircuit.com/circuit/4594156412076032
CCCV Li-ion Battery Charger: https://everycircuit.com/circuit/5336266329292800
Linear Power Supply: https://everycircuit.com/circuit/6413550326906880
