Replaces 1.5...3v batteries (1x or 2x 1.5v AA/AAA batteries) by a Li-Ion/Li-Po battery with an efficient voltage regulator based on MOSFET for low power applications. Notes: 1) Select MOSFET with a low Vgs threshold voltage of 1.8...2v 2) Vout: ~2.7v-3.0v depends on the LED1 direct voltage ~1.4v if replace LED1 by 4148 diode ~1.9v if replace LED1 by two 4148 diodes in sequence 3) Power depends on the MOSFET which works in a linear mode. Developed for a low power applications of up to 100mA. Still delivers 1A with, for example, IRFZ44. 4) Developed for Li-Ion/Li-Po batteries powered applications. Ensures extremely low quiescent current with no load (in uA range).