LOW POWER UNDER-VOLTAGE LED FLASHER

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Many portable electronic devices could benefit from a visual indication when their battery requires replacement. Typical low battery indicators simply turn a LED on when the battery voltage drops below

a certain point. Such circuits actually accelerate the battery extinction by drawing excessive current. The circuit shown (AdobePDF file) draws a mere 1ua in its standby mode and averages only 20uA when activated. The low current places a minimal burden on any battery circuit. An off-the-shelf under-voltage sensor IC (A1) is used to monitor the battery voltage. The IC changes state when the voltage drops below about 2.4v. Such a voltage is the typical "end of life" voltage for a two cell zinc battery. The monitor IC is linked to a two transistor oscillator circuit that produces an efficient LED flashing circuit. When the under-voltage sensor IC changes state, the flasher circuit is activated. Since the LED is driven with short 2ms pulses every two seconds, the average current drain is less than 20uA.