We'll start with a basic photocell. This is a resistor that changes resistance based on how bright the light is. You can read tons more about photocells in our tutorial but basically we'll be able to measure how bright or dark the room is using the photocell. Note that photocells are not precision measurement devices, and this technique is also not very precise so its only good for basic measurements. For precision sensing, you'd want a digital lux sensor like this one - we don't have a tutorial on connecting that to the Pi but we do have example code for Arduino.
The following code can be downloaded to your raspberry pi
# SPDX-FileCopyrightText: 2019 Mikey Sklar for Adafruit Industries
#
# SPDX-License-Identifier: MIT
# Example for RC timing reading for Raspberry Pi
# using CircuitPython Libraries
import time
import board
from digitalio import DigitalInOut, Direction
RCpin = board.D18
while True:
with DigitalInOut(RCpin) as rc:
reading = 0
# setup pin as output and direction low value
rc.direction = Direction.OUTPUT
rc.value = False
time.sleep(0.1)
# setup pin as input and wait for low value
rc.direction = Direction.INPUT
# This takes about 1 millisecond per loop cycle
while rc.value is False:
reading += 1
print(reading)
Let's put this file right in your home directory for simplicity. The wget command makes things easy.
$ wget https://raw.githubusercontent.com/adafruit/Adafruit_Learning_System_Guides/master/Basic_Resistor_Sensor_Reading_on_Raspberry_Pi/Basic_Resistor_Sensor_Reading_on_Raspberry_Pi.py
With the Pi connected to the Cobbler, run the script and shade your hand over the sensor to test it out!
The following command will start the program and you should see the ADC output on your screen.
$ sudo python3 ./Basic_Resistor_Sensor_Reading_on_Raspberry_Pi.py
Once you know it works you can change what pin you are using by changing the
RCpin = board.D18
to
RCpin = board.D(yourpinnumberhere)
any pin will work.
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