Battery LED Supply

This circuit uses a simple approach to create a 91 kHz oscillator. It uses a 2N3904 NPN transistor and a 2N3906 PNP transistor, which are configured as a high-gain amplifier. A 1-megaohm resistor provides bias current, and a 1-kilohm resistor helps to linearize the oscillator waveform into a shape that is close to a square wave with a 50% duty cycle.

A capacitor provides positive feedback from the output of the amplifier to the non-inverting input. The frequency of oscillation is determined primarily by the RC constant of the feedback capacitor and the input-stage impedance. In this circuit, the oscillator operates at 91 kHz with a 48% duty cycle.

You can use almost any common NPN or PNP transistors in this circuit, as long as they have a moderate forward-current gain of 50 or more and can handle 100-mA collector currents. The LED is connected across the output transistor because this approach allows the inductive kickback voltage to add to the battery-supply voltage, making the LED brighter.

This circuit operates well from approximately 0.8 to 1.6 volts, which is the useful range of an alkaline battery. The LED-light output decreases as the supply voltage decreases from 1.6 to 0.8 volts.