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From the Q&A

With TJ Byers


Low-Battery Indicator

Question:

I have an electric guitar that uses active pickups powered by a nine-volt battery. I'm looking for a simple circuit that would live inside the guitar and tell me at a glance if the battery is still good. I imagine an LED that turns yellow when the voltage drops to five or six volts, otherwise the LED does not light. Do you have any suggestions?

David Walker
via Internet


Answer:

How about a blinking LED? The circuit below monitors the voltage of the nine-volt battery, and flashes the LED every two seconds when the voltage falls below 7.2 volts (the voltage most manufacturers consider the end of battery life).

The circuit draws a mere 120 microamps, which is just slightly more than the self-discharge current of the same battery sitting on a shelf. To calibrate the monitor, connect it to a 7.2-volt source (see insert) and slowly adjust R1 until the LED starts to blink.


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