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9v Battery Voltage Monitor
Nine voltage batteries are used in a wide variety of electronic devices. But, you often do not know when it is time to change the battery until the device stops operating.
The hobby circuit below provides a visual indication when the battery needs to be replaced. It uses a neat IC from Linear Technology, which contains a very low power voltage reference and a voltage comparator. The circuit is designed to turn on a LED indicator light when the battery voltage drops below 7.2 volts.