A Solar-powered Embedded System for Ecological Audio Signal Processing

In this step we test for I2C communication between the LC709203F battery monitor and the Pi. In particular, we want to be able to get the current battery voltage value from the LC709203F, and to set the INT pin on the LC709203F to go LOW at a specific threshold (here 3.9V). As shown in picture:

• Connect board VIN (red wire) to Pi 5V
• Connect board GND (black wire) to PiGND
• Connect board SCL (yellow wire) to PiSCL
• Connect board SDA (blue wire) to PiSDA
• Plug battery into either of the JST battery ports.

Detailed instructions here. Once everything is connected, power on the Pi, then:

Install the LC709203 library.

pip3 install adafruit-circuitpython-lc709203f

Slow down the I2C clock as described here:

sudo nano /boot/config.txt

At the end of file, add:

# Clock stretching by slowing down to 10KHz
dtparam=i2c_arm=on
dtparam=i2c_arm_baudrate=10000

Test whether the LC709203 is there:

i2cdetect -y 1

You should get something like this:

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         -- -- -- 0b -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

Test battery monitor:

sudo nano battery.py

Copy and save the following code. This reads battery voltage every second and sets INT pin to go LOW at 3.9V (I2C register 0x14).

import time
import board
from adafruit_lc709203f import LC709203F

sensor = LC709203F(board.I2C())
sensor._write_word(0x14, 3900)

while True:
    print(sensor.cell_voltage)
    time.sleep(1)

Run:

python battery.py

Terminal should now print the battery voltage.

Step 8: Test Solar Charger

Here we want to test whether we can read the state of the solar charger PGOOD pin through the Raspberry Pi GPIO pin 13. The PGOOD pin indicates whether sunlight reaches the solar panel or not. Connect PGOOD to GPIO 13, as shown in picture. Then create the file sun.py:

sudo nano sun.py

Copy and save the following code:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)

sun = 13
GPIO.setup(sun, GPIO.IN)

while True:
    if GPIO.input(sun):
        print("no sun")
    else:
        print("sun")
    time.sleep(1)

Run:

python sun.py

Try to hover a light on the solar panel, then move it away. Terminal should print “sun” and “no sun” accordingly.

Step 9: Enable Poweroff Overlay

Enable poweroff overlay. GPIO pin 26 will go LOW at pi shutdown, will be HI after boot. This will be useful to make a graceful automatic shutdown.

sudo nano /boot/config.txt

At the end of file, add:

dtoverlay=gpio-poweroff,active_low

If all tests were successful, we can now make the power cycling circuit.

Step 10: Schematics

• The first QuadNOR (QuadNOR1) is used as an inverter. It inverts the value of PGOOD and INT. All the inputs of the two unused gates are connected to ground.
• The NE555 is set in monostable mode (a.k.a. single shot mode) to de-noise the value of INT at the 3.9V threshold. Specifically, when INT switches from LO to HI at 3.9V, the booster turns on, causing the Pi to boot. This sudden load causes the battery monitor to detect a voltage lower than 3.9V. The INT value therefore goes LO again. Since this happens in a fraction of a second, the Pi finds itself still in a very early boot phase – in particular, GPIO26 will still be LO. With both Pi and INT LO the booster turns off again, causing an infinite boot loop. To avoid this the NE555 IC is set in monostable mode to keep the INT value HI for at least 10 seconds, which is largely enough for the Pi GPIO pin 26 to go HI and complete the boot. The value of R1 and C1 determines how long the output of the NE555 timer will stay HI. Click here to open an online simulation.
• QuadNOR2 performs the logic desribed in step 3.

Step 11: Prepare the Perf Board

The circuit is built on a small piece of perforated board. Take your enclosure and measure two mounting points, drill holes.

Step 12: Solder Sockets

Step 13: Solder All Caps and Power Rails

Step 14: Complete the Logic

Step 15: Solder Connectors

Step 16: Solder Pi and Booster

Step 17: Put It All Together

Insert the solar charger and battery monitor in the sockets. Connect cables to Pi SDA – SCL and to GPIO 13 – 26. Connect booster ENABLE to the power cycling circuit.

Step 18: Test

Test the power cycling circuit. Start by providing a current of 4.0V and hover a light relatively close to the solar panel. Once Pi boots, run sun.py. Move the lamp away from the solar panel and check that terminal prints “no sun”. Move the lamp back on the panel and check that terminal prints “sun”. Now run battery.py and check that you get correct voltage printed (~4V). Now turn down the voltage from your power supplier to a value below 3.9V, and shut down your Pi manually. Pi should shutdown, followed by booster turning off. Now turn up your power supply to exactly 3.9V. Booster should turn on and Pi should boot. Move away the lamp from the solar panel and shut down Pi manually. Pi should shutdown, followed by booster turning off.

If test succeded, the circuit is working properly. We can now mount everything in the waterproof enclosure.

Step 19: Battery Holder

Step 20: DC Plug and Power Switch

Step 21: Jack Plug

Step 22: MEMS Microphone

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