This circuit solved a problem encountered in developing a portable device with an ARM Cortex-M0 processor. The power supply consisted of two AA batteries (2 × 1.5 V), and I needed to signal the user about the battery's state. The Cortex-M0 operates from 3.3 V down to 2.0 V, and an AA battery is considered discharged when its voltage drops to 1.1 V, so the available supply is 1.1 V × 2 = 2.2 V. This was a very low-power application that could work down to 2.2 V, so it was not necessary to warn the user to change the batteries until that voltage level was reached...
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