A low cost audio oscillator method of indicating battery condition in small pocket radios and the like where a meter is prohibited by cost and space considerations. In use, a button is pressed and if a -beep- is heard from the speaker, this indicates battery condition is good. If there is no -beep-, or if it is very weak, the listener knows he should soon replace the battery, even though performance of the set seems OK to him. The impedance value for the feedback circuit is chosen to yield oscillation failure at some desired point in the battery voltage curve appropriate to the circuit and transistor betas.
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https://aes2.org/publications/elibrary-page/?id=1033
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