With TJ Byers
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?
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.