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Design IdeasJanuary 2, 1997 |
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)
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