Introduction

A CircuitPython helper class to detect a shadow cast over an analog light sensor such as the ALS-PT19 phototransistor.

ShadowWatcher is a CircuitPython helper class to detect a shadow cast over an analog light sensor such as the ALS-PT19 phototransistor used in the Adafruit PyPortal, PyGamer, PyBadge, CircuitPlayground Express, CircuitPlayground Bluefruit, and the ALS-PT19 breakout board. Incorporates a low-pass filter to reduce sensitivity to flickering light levels which may be caused by power line frequency or light dimmer PWM passthrough. Useful as a simple gesture detector.

Dependencies

This driver depends on:

• Adafruit CircuitPython
• Analog light sensor hardware such as the ALS-PT19 phototransistor with an output value directly in proportion to the light intensity.

The ShadowWatcher was primarily built for and tested on the PyPortal, but should be able to function reliably on other microcontrollers with similar sensors. The automatic samples mode will test the microcontroller's analog acquision latency and adjust the internal low-pass filter's sample size to maintain the ~25 Hz cutoff frequency.

Please ensure all dependencies are available on the CircuitPython filesystem. This is easily achieved by downloading the Adafruit library and driver bundle or individual libraries can be installed using circup.

Installing to a Connected CircuitPython Device with Circup

Make sure that you have circup installed in your Python environment. Install it with the following command if necessary:

pip3 install circup

With circup installed and your CircuitPython device connected use the following command to install:

circup install cedargrove_shadowwatcher

Or the following command to update an existing version:

circup update

Usage Example

import board
import time
from shadowwatcher import ShadowWatcher
# Instantiate detector class and establish background level
gesture = ShadowWatcher(pin=board.LIGHT, auto=True)
while True:
    if gesture.detect():
        print(f"SHADOW DETECTED")
        while gesture.detect():
            # Wait until the shadow is gone
            time.sleep(1)
        # Rebaseline the background level
        gesture.refresh_background()
        print(f"background: {gesture.background:6.0f}")

Documentation

API documentation for this library can be found on Read the Docs.

The ShadowWatcher helper class was tested on the PyPortal, but should be able to function reliably on other microcontrollers with similar sensors. The automatic samples mode will test the microcontroller's analog acquisition latency and adjust the internal low-pass filter's sample size to maintain the ~25 Hz cutoff frequency.

The left oscilloscope image shows the sampled sensor light level value in a room with a dimmed LED light source (yellow trace). Note that the primary frequency of the signal is 120Hz (purple frequency spectrum graph). The right image shows the sampled sensor light level value after filtering.

For information on building library documentation, please check out this guide.

Contributing

Contributions are welcome! Please read our Code of Conduct before contributing to help this project stay welcoming.