White LED shines from piezoelectric-oscillator supply
A piezoelectric ceramic buzzer powers a white LED from a single cell.
TA Babu, Chennai, India; Edited by Charles H Small and Fran Granville -- EDN, January 10, 2008
LED drivers that receive their power from a single cell are receiving a great deal of attention. To generate the high voltage for illuminating a white LED from a low-voltage power supply basically requires some form of an electronic oscillator, and one of the simplest is a piezoelectric buzzer. An unusual application of a piezoelectric transducer serves as an oscillator and drives a white LED (Figure 1). The piezoelectric diaphragm, or bender plate, comprises a piezoelectric ceramic plate, with electrodes on both sides, attached to a metal plate made of brass, stainless steel, or a similar material with conductive adhesive. The circuit uses a three-terminal piezoelectric transducer. In this transducer, the diaphragm has a feedback tab on one of its electrodes. The oscillation is a result of the resonance between the inductor and the element, which is capacitive. The frequency of operation is: fOSC=1/(2π
), where L is the value of the inductor and C is the capacitance of the piezoelectric element.
With the initial application of potential to the circuit in Figure 1, transistor Q1 turns on. When the transistor conducts, the current through inductor L1 increases gradually, and the potential across the plates flexes the piezoelectric ceramic. This flexing generates a negative potential at the feedback tab, which feeds back to the base of the transistor. The transistor then switches off. When turn-off occurs, the stored energy in the inductor dumps into the LED. This flyback voltage is sufficient to light the LED.
