The AS726X and AS7265X spectral sensors give you the ability to read visible, UV, and IR over an I2C bus, and are relatively cheap ($27-$60 USD). The former focuses on just visible or IR, and the latter is a triad sensor that spans the entire spectrum. In this post, we’ll walk you through setting this up and reading the sensor values…

Github repositories for micropython libraries for interfacing with these sensors:

Demonstration with the AS7623 IR sensor

Bill of materials and hardware

Plug the gadgets together—It will be self-evident.

Load a Micro Python boot image

Connect the pico to your computer and load your favorite Micro python environment bootimage. Here we use rp2-pico-20220618-v1.19.1.uf2

Scan for I2C devices and their addresses

Load Thonny or your favorite IDE and make sure it finds the pico (be sure that you are in Micropython mode).

As we are using a Sparkfun MicroPro which comes with a QWIIC connector; the corresponding I2C pins are GP16 & GP17 (which are on controller I2C0). If you are using an ordinary Pico, you’ll need to wire it up accordingly; GP16 and GP17 are totally legitimate pins for this purpose, but you can choose others if you prefer.

import machine

I2C_SDA_PIN = 16
I2C_SCL_PIN = 17
i2c=machine.I2C(0,sda=machine.Pin(I2C_SDA_PIN), scl=machine.Pin(I2C_SCL_PIN), freq=400000)

print('Scanning I2C bus.')
devices = i2c.scan() # this returns a list of devices

device_count = len(devices)

if device_count == 0:
    print('No i2c device found.')
else:
    print(device_count, 'devices found.')

for device in devices:
    print('Decimal address:', device, ", Hex address: ", hex(device))

Our little AS7263 dutifully reports back:

Scanning I2C bus.
1 devices found.
Decimal address: 73 , Hex address:  0x49

(this is the default for the AS7263; it will be different for another sensor)

Set up the library and test the light

Let’s do this using jajberni/AS726X_LoPy’s library.

Clone the repository and copy the Device.py and AS726X.py files in lib/ to a new lib/ folder that you create on the Pico. Then create a new file to blink the lights.

from machine import Pin, I2C
from AS726X import AS726X #from https://github.com/jajberni/AS726X_LoPy
from time import sleep

# define the I2C pins
I2C_SDA_PIN = 16
I2C_SCL_PIN = 17

# setup I2C and connect to the sensor; AS726X library will automatically detect the device
i2c = I2C(0, sda=Pin(I2C_SDA_PIN), scl=Pin(I2C_SCL_PIN))
sensor = AS726X(i2c=i2c)
sensor_type = sensor.get_sensor_type()
#print(sensor_type)  #AS7263 

# blink the blue power indicator LED
sensor.enable_indicator_led()
sleep(1)
sensor.disable_indicator_led()
sleep(1)

# ramp up the indicator: integers 0..3 denote the current
sensor.enable_indicator_led()
for i in range(4):
    sensor.set_indicator_current(i)
    sleep(0.5)
sensor.disable_indicator_led()

# ramp up the white LED light source: integers 0..3 denote the current

sensor.enable_bulb()
for i in range(4):
    sensor.set_bulb_current(i)
    sleep(0.5)

# turn it off and go to sleep
sensor.disable_bulb()

This should do what the comments tell you. It will also print an acknowledgement about finding the correct sensor type

Reading a sensor value

Let’s demonstrate how to read a sensor value, turning on the light.
We’ll use default settings for the gain and integration time, but obviously you can change these as you see fit.

As described in the documentation for the library, reading from the sensors is a two-step process. First you use the take_measurements() or take_measurements_with_bulb() method to have the sensor acquire a value. Then you retrieve it using sensor_get_calibrated_values(), which returns a list of floats. The sensor light will be turned on to whatever value you set its current to (below we use the default).

from machine import Pin, I2C
from AS726X import AS726X # from https://github.com/jajberni/AS726X_LoPy

# define the I2C pins
I2C_SDA_PIN = 16
I2C_SCL_PIN = 17

# setup I2C and connect to the sensor; AS726X library will automatically detect the device
i2c = I2C(0, sda=Pin(I2C_SDA_PIN), scl=Pin(I2C_SCL_PIN))
sensor = AS726X(i2c=i2c, mode=3, gain=3, integration_time=50)

# return a list of the wavelengths being read
print("wavelengths (nm): ", sensor.get_wavelengths())

# read the sensor with the light on
sensor.take_measurements_with_bulb()
results = sensor.get_calibrated_values()
print("reading w/ light:", results) #same order as the wavelengths

# try it without the light on
sensor.take_measurements()
results = sensor.get_calibrated_values()
print("reading w/o light:", results)

For fun, I pointed the sensor at an orange Post-It note, and got the following result:

AS7263 online!
wavelengths (nm):  [610, 680, 730, 760, 810, 860]
reading w/ light: [25873.66, 4424.922, 1114.959, 774.937, 1693.204, 1241.406]
reading w/o light: [110.4013, 49.66749, 8.475845, 6.702158, 7.110569, 7.13452]

Observe how the reading with the light gives much larger values….we can fiddle with the integration time and the light intensity to modify these values

Next steps

Now that you know how to get readings porogrammatically, the next step is to read some values in. Some features might be:

  • Wait for a newline from sys.stdin.read() to trigger a reading (for example, once you’ve got a sample in place)
  • Read the values and print it in a desired format
  • Write a program for your laptop that reads these values and saves them…maybe with some type of annotation as to the sample you are considering?
  • Or…add a physical switch to the circuit and read from the switch to trigger the reading. Then design and print a housing to keep everything oriented the way you want.