Laser Tripwire with Raspberry Pi

Create a shielded light sensor using the LDR and straw.

1. Place the top of the LDR inside one end of the drinking straw to shield it from ambient light.
2. On the breadboard, connect one leg of the LDR to a 3.3V pin on your Raspberry Pi.
3. Connect the other leg of the LDR in series with the 1µF capacitor.
4. Connect the capacitor's positive leg to GPIO 4 on your Raspberry Pi.
5. Connect the capacitor's negative leg to a GND pin on your Raspberry Pi.

Verify the sensor can detect the laser beam.

1. Create a Python test script on your Raspberry Pi.
2. Run the script to see the light value (between 0 and 1).
3. Shine the laser pointer into the straw and note the value increase.
4. If the ambient light value is too high, wrap the straw in electrical tape to improve shielding.

from gpiozero import LightSensor

ldr = LightSensor(4)
print(ldr.value)

Write a Python script that triggers when the beam is broken.

1. Create a new Python script that imports the LightSensor class and pygame.
2. Initialize pygame for sound playback.
3. Create a sound object pointing to your alarm sound file (.wav format).
4. Use the when_dark method to trigger the alarm when the beam is broken.
5. Save the script.

from gpiozero import LightSensor
import pygame

pygame.init()

ldr = LightSensor(4)
my_sound = pygame.mixer.Sound('path/to/my/soundfile.wav')

ldr.when_dark = lambda: my_sound.play()

# Keep the script running
import signal
signal.pause()

Position the laser and sensor across a doorway.

1. Connect the components directly to your Raspberry Pi using jumper leads (tape the connections for strain relief).
2. Optionally, place the sensor assembly in a small box with a hole for the straw.
3. Position the sensor on one side of a doorway.
4. Mount the laser pointer on the opposite side so the beam enters the straw and hits the LDR.
5. Run your alarm script and test by walking through the doorway.