Pico Power Switch

This is a simple breakout to switch the battery power for a Raspberry Pi Pico to minimize battery usage.

Functions:

Enable using a button, a RTC or an external signal
Disable by pulling a GPIO high from your program
keep time

The target application of this design is a program that runs in intervals. After startup the program executes it's tasks, sets the next wakup time and then pulls the "done"-GPIO high. The circuitry then cuts power, thus limiting the battery consumption to a very low level. Measured current draw in off-mode is about 0.35µA.

The design of the breakout allows it to be connected directly on the bottom side of the Pico.

Hardware Components

A D-type flip-flop drives the gate of a p-channel mosfet and switches power (source: battery, drain: VSYS). A button, the RTC or an external signal will reset the flip-flop. This will pull the gate low, thus enabling power.

The "done"-pin of the Pico is connected to the CLK-pin of the flip-flop. A rising edge will toggle the output and pull the gate high, which in turn cuts power.

A second p-channel mosfet will prevent back-powering from VBUS in case the Pico is connected using the USB-connector. The voltage-drop across this mosfet is about 0.1V.

PCB

PCB design files are in pico-power-switch.kicad.

Ready to use production files for JLCPCB are in production_files.

Software

A simple example program implemented in CircuitPython is provided in src/main.py. The software provides some boilerplate code for timer and alarm-based wake up.

In timer-mode, the program uses the countdown-timer of the rtc. It blinks the on-board LED for ten seconds, then goes to sleep for 15 seconds.

For longer intervals alarm-based wake up is more suitable. Since the rtc does not support seconds for alarms, it will always fire on "full minutes".