If you're familiar with the original Musicator-Jr
, you'll know it uses a dual op-amp to convert sound from an electret microphone to display on LEDs. This new design uses a much-simplified circuit and is able to support up to 40 LEDs.
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