Schematic Reference

Light From Flat Batteries

Button or coin cells that appear to be flat in their normal function may yet be discharged further. This is because in many cases, for instance, a quartz watch stops to function correctly when the battery voltage drops to 1.2 V, although it can be discharged to 0.8 V. Normally, however, not much can be done with a single cell. In the present circuit, a super-bright LED is made to work from voltages between 1 V and 1.2 V. This may be used for map-reading lights, a keyhole light, or warning light when jogging in the dark. When a yellow, superbright LED is used with a fresh battery, it may be used as an emergency reading light or to read a front door nameplate in the dark or to find an non-illuminated doorbell.

Normally, LEDs light at voltages under 1.5 V (red) or 1.6–2.2 V (other colours) only dimly or not at all. The present circuit uses a multivibrator of discrete design that oscillates at about 14 kHz. The collector resistor of one of the transistors has been replaced by a fixed inductor, which is shunted by the LED. Because of the self-inductance, the voltage across the LED is raised, so that the diode lights dimly at voltages as low as 0.6 V and becomes bright at voltages from about 0.8 V up. The circuit requires a supply voltage of 0.6–3 V and draws a current of about 18 mA at 1 V.