LED lights that “drain” battery power

This LED lamp uses only three transistors and only requires a voltage of 0.71V to start the LED to emit light. One No. 5 alkaline battery can allow the lamp to light continuously for 170 hours, greatly maximizing the battery utilization rate.

The power supply voltage of the LED lamp is set according to the voltage range of 0.9V to 1.7V of the No. 5 alkaline battery. Figure 1 shows the actual battery power supply situation. This circuit can work up to a power supply voltage of 0.71V.

The circuit is shown in Figure 2. The forward current of the LED changes with the power supply voltage. Transistors Tr1 and Tr2 form a current mirror circuit to stabilize the bias (DC operating point). During the off period of Tr3, the bases of Tr1 and Tr2 are slightly reverse biased, and no current flows through R1. As a result, the circuit can oscillate stably over a wide temperature range, battery voltage, and component error range. Figure 3 is the collector voltage waveform of Tr1.

The DC resistance of inductor L1 in Figure 2 is 4Ω. Its larger resistance can limit the current when Tr3 or LED is short-circuited.