For most electronic circuits and equipments, regulated power supplies are a must. For voltage references required, the usual procedure is to adopt a zener diode in the circuit. But there is one problem with this. When we need regulated voltages like 3V or 2V, suitable zeners are not available.
A red LED usually has a constant drop of approx. 1.6V across it. This can be used as a voltage reference. Also, the problem of indication is overcome simultaneously. A lamp indicator unnecessarily wastes power, whereas a neon and a zener in the circuit means extra cost. Both purposes, that of voltage reference and indication, are solved using a single LED. Also, one can get as low as 2V regulated DC using an LED.
The circuit shown can readily be used as a power supply for any commercially available electronic calculator. By adjusting the preset, one can set the exact voltage required. A 250mA transformer will do. For instance, Fx-31 Casio calculator consumes about 85mA; this circuit readily meets the requirement.
A point to note: Commercial adaptors available for use with calculators are not regulated, but it is better to have regulation.