Single cell flashes white LED

摘要

MANY PORTABLE appliances and other products that must operate from a single cell are restricted to working at very low voltages. It is thus difficult to drive white LEDs that typically have a forward voltage of 3 to 5V. The ability to flash the LED with a supply voltage as low as IV presents additional complications. The circuit in the Figure 1 provides a discrete approach to these problems and al-lows a white LED to flash at a rate set by an RC time constant. Components Q{sub}1, Q{sub}2, R{sub}3, R{sub}4, and R{sub}5 form a simple Schmitt trigger that, together with R{sub}1, R{sub}2, and C{sub}1, controls the flashing of the LED. Q{sub}4, Q{sub}5, L{sub}1, and associated components form a voltage booster that steps up the single-cell voltage, V{sub}s, to a level high enough to drive the LED. Transistor Q{sub}3 functions as a switch that gates the booster on and off at a rate determined by the Schmitt-trigger section.