In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Beau Schwabe wrote:

Kramer,

"Is that really 5volts output from a 7805?  I am a bit confused how the diode between the ground and the regulator might affect the output voltage."

Yes,  going from left to right, the first diode and capacitor combination off of the INPUT terminal to the regulator form a first stage "diode/capacitor filter". The second diode connected to the regulators GND terminal shifts the Ground reference by 1 diode (.6V) in preparation for the third diode.  The third diode connected to the regulators OUTPUT forms a second stage "diode/capacitor filter".  Since the third diode creates a diode voltage drop of .6V it is necessary to compensate that with the use of the second diode to shift the voltage up by .6V so that the net output is 5V.

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