Low-battery indicator uses fleapower

Posted on Feb 14, 2013

It is always desirable to use a low-battery indicator that consumes as little power as possible. For a 9V, 450-mAhr alkaline battery, a 50-µA low-battery indicator can by itself run the battery down in a little more than a year. Battery-powered devices that need to run continuously for a long time require battery indicators that consume minimal power. The circuit in Figure 1, designed for a 9V battery, uses extra-low power. When the battery is at full charge (9V), the circuit draws 1.4-µA current. At the indication-threshold voltage, 6.5V, the circuit draws 1 µA. Assume that the average operating current is 1.2 µA.

The circuit uses 42 mAhr in a four-year period, less than 10% of the battery`s rated energy. A red LED, D2, flashes periodically when the battery voltage drops below 6.5V. IC1, an LTC1540, is a nanopower comparator with a built-in 1.18V reference. A

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