power supply

This circuit will provide electric power from a backup battery when AC power is cut, always providing a clean 5V power supply for sensitive equipment like microcontrollers and logic circuits.
The value of C1 is to be higher than 220 uF. the bigger the capacitance of this capacitor, the more power you can deliver to your [...]

A stable variable power supply with an adjustable output voltage from 5 volts to 30 volts can be easily constructed with the regulator ICs UA78G or UA79G. These ICs differ from the common three-terminal regulator since their output voltages are adjustable by a voltage level at their control inputs. The maximum current delivered by these [...]

This simple circuit, consisting of two LEDs and a photo diode, generates a negative voltage with a current level of a couple milliamps. It is ideal for supplying a negative rail to low power “rail to rail” op amp circuits [...]
Source: http://www.discovercircuits.com/DJ-Circuits/negsupply2.htm