if light < DARK:
    # something is in the way

bright room : 41200
covered     :  3800
                    -> pick something in between

speaker.freq(880)         # 880 times a second = the pitch
speaker.duty_u16(32768)   # half on, half off = sound
speaker.duty_u16(0)       # never on = silence

'F' → Fullscreen

Build

Leave everything from 2.8 and 2.9 wired. Create a new MicroPython program named 2-10-alarm.

Task 1: Read the Light

• Wire the photocell and a 10K resistor in series between 3V3 and GND, with A2 connected to the point between them.
• Print the raw reading and wave your hand over the sensor.

1
from machine import Pin, ADC
2
import time
3

4
light_sensor = ADC(Pin(42))
5

6
while True:
7
    print(light_sensor.read_u16())
8
    time.sleep(0.2)

Task 2: Measure Both Ends

• Reuse read_average() from 2.9 so the reading holds still.
• Write down two numbers: the reading in normal room light, and the reading with the sensor fully covered.
• Pick a threshold somewhere between them and write that down too.

Put your threshold nearer the covered value than the bright one. An alarm that fires when someone walks past the window is worse than one that needs a proper blocking.

1
from machine import Pin, ADC
2
import time
3

4
light_sensor = ADC(Pin(42))
5

6

7
def read_average(sensor, samples):
8
    total = 0
9
    for i in range(samples):
10
        total = total + sensor.read_u16()
11
        time.sleep(0.01)
12
    return total // samples
13

14

15
while True:
16
    print("light:", read_average(light_sensor, 10))
17
    time.sleep(0.3)

Task 3: Make a Noise

• Wire the speaker through a 330Ω resistor to GPIO 23.
• Write beep(speaker, hz, seconds) that sets a pitch, turns the sound on, waits, then silences it.
• Play three different pitches to prove it works.

The resistor matters. A speaker connected straight to a pin will try to draw far more current than the pin can give.

1
from machine import Pin, ADC, PWM
2
import time
3

4
light_sensor = ADC(Pin(42))
5
speaker = PWM(Pin(23))
6

7

8
def read_average(sensor, samples):
9
    total = 0
10
    for i in range(samples):
11
        total = total + sensor.read_u16()
12
        time.sleep(0.01)
13
    return total // samples
14

15

16
def beep(speaker, hz, seconds):
17
    speaker.freq(hz)
18
    speaker.duty_u16(32768)
19
    time.sleep(seconds)
20
    speaker.duty_u16(0)
21

22

23
beep(speaker, 440, 0.3)
24
beep(speaker, 660, 0.3)
25
beep(speaker, 880, 0.3)

Task 4: Sound the Alarm

• Put your measured threshold in a named constant, DARK.
• Beep whenever the reading falls below it.
• Test it by covering the sensor.

1
from machine import Pin, ADC, PWM
2
import time
3

4
light_sensor = ADC(Pin(42))
5
speaker = PWM(Pin(23))
6

7
DARK = 12000
8
ALARM_HZ = 880
9

10

11
def read_average(sensor, samples):
12
    total = 0
13
    for i in range(samples):
14
        total = total + sensor.read_u16()
15
        time.sleep(0.01)
16
    return total // samples
17

18

19
def beep(speaker, hz, seconds):
20
    speaker.freq(hz)
21
    speaker.duty_u16(32768)
22
    time.sleep(seconds)
23
    speaker.duty_u16(0)
24

25

26
while True:
27
    light = read_average(light_sensor, 10)
28
    if light < DARK:
29
        print("Beam broken!")
30
        beep(speaker, ALARM_HZ, 0.2)
31
    time.sleep(0.05)

Task 5: Latch It, Then Reset It

• An alarm that stops the moment the intruder moves on is useless. Make it keep sounding once triggered.
• Flash the LED array along with the beep.
• Press the button to re-arm.

The trick is a variable that remembers. armed starts True, becomes False when the beam breaks, and only the button puts it back.

1
from machine import Pin, ADC, PWM
2
import time
3

4
leds = [
5
    Pin(28, Pin.OUT),
6
    Pin(29, Pin.OUT),
7
    Pin(30, Pin.OUT),
8
    Pin(31, Pin.OUT),
9
    Pin(32, Pin.OUT),
10
]
11
light_sensor = ADC(Pin(42))
12
speaker = PWM(Pin(23))
13
button = Pin(33, Pin.IN, Pin.PULL_DOWN)
14

15
DARK = 12000
16
ALARM_HZ = 880
17

18

19
def read_average(sensor, samples):
20
    total = 0
21
    for i in range(samples):
22
        total = total + sensor.read_u16()
23
        time.sleep(0.01)
24
    return total // samples
25

26

27
def beep(speaker, hz, seconds):
28
    speaker.freq(hz)
29
    speaker.duty_u16(32768)
30
    time.sleep(seconds)
31
    speaker.duty_u16(0)
32

33

34
def all_on(leds):
35
    for led in leds:
36
        led.value(1)
37

38

39
def all_off(leds):
40
    for led in leds:
41
        led.value(0)
42

43

44
armed = True
45
all_off(leds)
46
print("Armed.")
47

48
while True:
49
    light = read_average(light_sensor, 10)
50
    if armed:
51
        if light < DARK:
52
            print("Beam broken!")
53
            armed = False
54
    else:
55
        all_on(leds)
56
        beep(speaker, ALARM_HZ, 0.2)
57
        all_off(leds)
58
        time.sleep(0.1)
59
        if button.value() == 1:
60
            armed = True
61
            print("Re-armed.")
62
    time.sleep(0.05)

Challenge (Optional): A Real Trip-Wire

• Aim a laser diode across the room at the photocell instead of relying on room light.
• Re-measure both ends — a laser on the sensor reads far brighter than a lit room, so your threshold will need to change.
• Never aim the laser at anyone's eyes. Point it at the wall, then bring the sensor to the beam.

Checkpoint

Your finished program should behave like this:

Armed.
Beam broken!
Re-armed.
Beam broken!

• Nothing happens in normal room light, however long you leave it running
• Covering the sensor triggers the alarm within about a second
• The alarm keeps sounding after you uncover the sensor
• Pressing the button silences it and re-arms
• Walking past the sensor does not set it off

Reflection

Answer the following questions before submitting your work.

1. Activity 2.9 worked hard to get an accurate temperature. This activity throws most of that precision away. Explain what a threshold does to a reading, and why an alarm does not need the exact value.
2. Nobody could tell you what number to use for DARK. Describe how you actually chose yours, and why the same number would not work in a different room.
3. Without the armed variable, the alarm stops as soon as the sensor is uncovered. Explain how one True/False variable changes the behavior, and why that is what an alarm should do.

Submit

Submit the required files to the appropriate dropbox.

Activity Complete