Circuit Project: 3V Low Battery Voltage Flasher

Many battery powered
devices use two AA alkaline cells. Often you will not know when it is
time to replace the batteries until the device powered by them actually
stops operating. The hobby circuit below can be connected to a 3v
battery, to give you some warning when the battery is nearing its end of


Circuit diagram

It will flash a LED when the battery
voltage drops to about 2.4 volts. The electronic circuit draws only 1ua
of current in standby mode and jumps to only 20ua when flashing, so it
can safely be included without depleting the battery energy. A voltage
detector IC from Panasonic (Microchip also makes similar devices) is
used to monitor the battery voltage. The device’s open drain output
swings low, when the battery voltage is below 2.4 to 2.5 volts. This
action turns on the two transistor oscillator circuit, which drives the LED with short current pulses lasting only 2ms.

Source: DiscoverCircuits

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