A transistor is also wired up, switching a 2.2 K resistor in parallel with the LEDs. When turned on by the PIC, this transistor causes roughly a 10 mA current to flow through the Zener diode and ...

[James Lewis] does an experiment with a regulator circuit ... You might also add a capacitor in parallel with the Zener to smooth out some of its noise. Zener diodes are wonderful little devices ...

C2 and C4 are decoupling capacitors; their action further reduce ripple factor. The regulator 78xxr, U1 utilizes the action of zener diode ZD1 which is in parallel with the potentiometer, R1. The ...