This guide provides the wiring diagram and description of the Low Battery Warning Board.
The board gives a low battery indication for your 2.7v-5.5v device. It is designed with the Raspberry Pi in mind, but will work with almost any project. The technical function is detailed here. The board is available for purchase here.
The connections labeled GND (or -IN in the upcoming Revision 2) and +IN are for the power input. This can be any voltage from 4v up to 18v, as long as the voltage is higher than the voltage of your battery (which is normally 3.0-4.2v for a lithium battery). Both the front and back of the board are shown here for clarity only.
The BAT+ connection goes to the positive terminal of your battery. There is no need to connect the negative terminal of the battery to the board as long as the battery is being used to power your project.
The connection labeled LED is for the signal output. When the battery is operating at a normal voltage, the LED connection is in a floating state, and is not connected to anything. When the battery voltage drops though, the LED pin connects to GND. This GND connection can be used to provide a notification of the low battery, and typically this is done using an LED.
The negative side of your LED can be soldered to the LED connection on the board. The positive side of your LED is soldered to a power source (using a resistor as usual). When the battery voltage drops, the LED turns on. When the battery voltage is increased by charging, the LED turns off.
This board has only one adjustable feature, and that’s the reference voltage. Turning the board’s variable resistor will change the voltage at which the LED connection will switch to GND. A voltmeter can be connected between GND (-IN) and the center pin of the variable resistor, and you can set the voltage to your desired level by rotating the resistor.
Revision 2 of this board will have easily accessible pads for checking the reference voltage.
Although this board was designed primarily for the Raspberry Pi, it is very useful for many projects. I look forward to seeing other uses you guys come up with.