This simple circuit uses only a handful of discrete components to generate a relatively constant current through a load. The main components are a NPN transistor, a handful of diodes, and a couple resistors. Additionally you only need a power supply and a load. The diodes are connected in series to create a stable voltage reference which is connected in parallel with the transistor's Base-Emitter junction and a set resistor. To calculate your set resistor take the diode reference voltage and subtract your Vbe from it, the resulting voltage is therefore constant across the resistor. Divide that voltage by your desired current and you get the resistor value. In order to change the current level all you need to do is either change the diode reference or the emitter resistor.
This circuit is technically a current sink, not a current source since the transistor and emitter resistor are between your load and ground. The Vce rating of this transistor limits the maximum voltage of the supply to about 30 or 40V for this particular transistor. The minimum limit for supply voltage is determined by your diode reference plus a little safety margin.
Generally silicon transistors and diodes have a 0.5-0.8V junction voltage. Therefore you will always want at least two diodes in series to give you a voltage above your Vbe such that you are left with voltage across your emitter resistor. There needs to be a resistor in series with your diodes to prevent a short circuit between your supply and ground, this resistor is a function of your transistor beta value, a sample calculation is shown at the end of the video, but the value isn't very critical.
