Is your QT Py all alone, lacking a friend to travel the wide world with? When you were a kid you may have learned about the "buddy" system. Well, this product is kinda like that! A board that will watch your QT Py's back and give it the power and support to wander out past your USB port.

This little lipo charger is designed to fit onto the back of any QT Py board, and make it easy for portable powering. Simply plug any standard 3.7V/4.2V lithium polymer or lithium-ion rechargeable battery into the JST plug on the end. When powered by the USB port on the QT Py, the charger kicks in and will charge up the battery at 200mA max rate. You'll see the yellow LED light up when that happens.

When the USB is removed, the battery will reverse charge into the 5V port via a Schottky diode. You can turn it on or off with the built in slide switch as well.

A simple voltage divider on the cathode side of the diode and connected to pin A2 will let your firmware detect basic modes: if the voltage is over 4.3V, it is probably plugged into USB. If the voltage is 4.2V or below its probably running on the Lipoly battery and you can roughly determine the battery state.

We include some header that you can solder to your QT Py. You can also pick up an Itsy Bitsy short female header kit to make it removable but compact, you'll just need to trim down the headers to 7 pins long.

For use with Adafruit LiPoly/LiIon batteries only! Other batteries may have different voltage, chemistry, polarity, or pinout.

• Comes as an assembled and tested PCB
• Uses the 5V input via USB Type-C connector on QT Py
• For charging single Lithium Ion/Lithium Polymer 3.7/4.2v batteries (not for older 3.6/4.1v cells)
• 200mA charge current

Batteries and QT Py are not included.

This QT Py BFF has a bunch of features packed into a cute little board. This page will explain the hardware!

Power Slide Switch

Battery JST Connector

Charge LED

Battery Monitor Pin and Jumper

There are a pair of jumper pads, located to the right of center, below the pins along the top edge. These pads are connected to pin A2, between the pin and the voltage divider. In the event that you want to use pin A2 for something other than battery monitoring, you can cut the trace between these pads to disable monitoring on pin A2. Solder the pads back together to enable monitoring again.

Charge Chip and Voltage Divider

The voltage divider, located below the A3 pin label on the BFF silk, is connected between A2 and 5V via the cathode side of a diode. It allows for simple monitoring of voltage levels, which can indicate whether the QT Py is running off USB, or being powered by a battery plugged into the BFF.

To obtain the voltage level from A2 using CircuitPython, run the following code.

import time
import board
import analogio

analog_pin = analogio.AnalogIn(board.A2)


def get_voltage(pin):
    return (pin.value / 65535) * 3.3 * 2.0 // voltage divider reduces
                                           // voltage by 1/2 do must
                                           // double to get the voltage
while True:
    print(get_voltage(analog_pin))
    time.sleep(0.1)

import time
import board
import analogio

analog_pin = analogio.AnalogIn(board.A2)


def get_voltage(pin):
    return (pin.value / 65535) * 3.3 * 2.0 // voltage divider reduces
                                           // voltage by 1/2 do must
                                           // double to get the voltage
while True:
    print(get_voltage(analog_pin))
    time.sleep(0.1)

Files

• MCP73831 Datasheet
• EagleCAD PCB files on GitHub
• Fritzing object in the Adafruit Fritzing Library
• 3D models on GitHub

Schematic and Fab Print

3D Model