Step 1: The Circuit - Overview
Like the USB-Musicator design, this circuit makes use of the LM431 "Programmable Zener" circuit. Although packaged like a transistor in a familiar TO92 form, there is a half-dozen transistors inside which forms a powerful op-amp and driver, which we will make use of here.
We have added a 2N4401 transistor as an output stage, which will allow us to drive LEDs up to 600mA. So it is not advisible to substitute any other device for it.
The basic design is for a 9-volt battery, but 12-volt operation is also possible with a few value changes.
A point-to-point diagram is supplied below, a larger / clearer version is also avilable here.