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Every battery powered device needs some way of notifying you when the battery is running low. The Pi doesn’t have this ability built-in, but it’s very easy to add using the LM393 Voltage Comparator. It’s an 8-pin device that compares two voltage sources and changes the output depending on the result of that comparison. The LM393 is actually a dual comparator, but for this purpose only one side is used.
The LM393 has one positive power input, V+ (red, Pin 8), and a ground power input, GND (black, Pin 4). The power for the LM393 can be supplied using the same +5v and ground that supply power to the Pi.
When the INVERTING INPUT (orange, Pin 6) voltage is higher than the NON-INVERTING INPUT (yellow, Pin 5) voltage, OUTPUT B (green, Pin 7) outputs the voltage from V+ (red, Pin 8). When the INVERTING INPUT (orange, Pin 6) voltage is lower than the NON-INVERTING INPUT (yellow, Pin 5) voltage, OUTPUT B (green, Pin 7) switches to GND (black, Pin 4).
For example, in my PSPi build the low battery LED is already connected to +5v, so a ground must be connected to the other terminal of the LED for it to light. In the PSPi, the output of Pin 7 is then used to switch the low battery LED on when Pin 7 is GND and off when Pin 7 is at +5v. As long as the battery voltage exceeds the reference voltage, the output on Pin 7 stays at +5v and no power is applied to the LED. When the voltage drops below the reference voltage, Pin 7 connects to GND and powers the LED.
The positive terminal from the battery connects to Pin 5 and the reference voltage connects to Pin 6. The 3.3v GPIO pin does a great job as the reference voltage for Pin 6, but be aware that this will not give a lot of time before the battery is fully depleted since the battery voltage drops very quickly when it is nearly depleted. I decided to use a Mini Buck Converter to generate a voltage of about 3.6v for my reference in the PSPi because that project has a high power drain and 3.3v only gave me about 10 minutes of low battery notice. Another very simple option for generating a reference voltage is to make a resistor voltage divider.
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