LED flasher indicates low-battery condition

David Johnson, Consultant, Littleton, CO

Many portable electronic devices could benefit from a visual indication when their battery requires replacement. Typical low-battery indicators simply turn on an LED when the battery voltage drops below a certain value. Such circuits actually accelerate the battery’s extinction by drawing excessive current. The circuit in Figure 1 draws a mere 1 m A in its standby mode and averages only 20 m A when activated. The low current imposes a minimal burden on any battery circuit. An off-the-shelf, undervoltage sensor IC (IC₁) monitors the battery voltage. The IC changes state when the voltage drops below approximately 2.4V. This voltage is the typical end-of-life level for a two-cell zinc battery. The monitor IC connects to a two-transistor oscillator that produces an efficient LED-flashing circuit. When the undervoltage IC changes state, the flasher circuit comes alive. Because the LED receives short, 2-msec pulses every two seconds, the average current drain is lower than 20 m A. (DI #1972)

Read More Source :
http://www.edn.com/archives/1997/010297/01di_04.htm

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