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7.3 Building an Alarm Siren Lamp

In this project, we’ll create an Alarm Siren Lamp using the Raspberry Pi Pico 2. This device simulates the flashing lights and siren sound of a police car or emergency vehicle. It’s a fun way to learn about PWM (Pulse Width Modulation), interrupts, and controlling multiple components like LEDs and buzzers.

What You’ll Need

In this project, we need the following components.

It’s definitely convenient to buy a whole kit, here’s the link:

Name	ITEMS IN THIS KIT	LINK
Newton Lab Kit	450+	Newton Lab Kit

You can also buy them separately from the links below.

SN	COMPONENT	QUANTITY	LINK
1	Raspberry Pi Pico 2	1	BUY
2	Micro USB Cable	1
3	Breadboard	1	BUY
4	Jumper Wires	Several	BUY
5	LED	1	BUY
6	Transistor	1(S8050)	BUY
7	Resistor	3(1KΩ, 220Ω, 10KΩ)	BUY
8	Passive Buzzer	1	BUY
9	Capacitor	1(104)	BUY
10	Slide Switch	1

Understanding the Components

• Passive Buzzer: Requires an external signal to produce sound. We’ll use PWM to generate varying frequencies, creating a siren effect.

• LED: Will simulate the flashing light of a siren by changing brightness.

• Slide Switch: Acts as an on/off switch to control the alarm.

• NPN Transistor (S8050): Used to drive the buzzer, as the Pico’s GPIO pins cannot supply enough current directly.

• Resistor and Capacitor: Used to debounce the slide switch, ensuring stable readings.

Circuit Diagram

• GP17 is connected to the middle pin of the slider, along with a 10K resistor and a capacitor (filter) in parallel to GND, which allows the slider to output a steady high or low level when toggled to the left or right.

• As soon as GP15 is high, the NPN transistor conducts, causing the passive buzzer to start sounding. This passive buzzer is programmed to gradually increase in frequency to produce a siren sound.

• An LED is connected to GP16 and is programmed to periodically change its brightness in order to simulate a siren.

Wiring Diagram

Writing the Code

We’ll write a MicroPython script to control the buzzer and LED based on the position of the slide switch.

Note

• Open the 7.3_alarm_siren_lamp.py from newton-lab-kit/micropython or copy the code into Thonny, then click “Run” or press F5.

• Ensure the correct interpreter is selected: MicroPython (Raspberry Pi Pico).COMxx.

import machine
import utime

# Initialize PWM for buzzer and LED
buzzer = machine.PWM(machine.Pin(15))
led = machine.PWM(machine.Pin(16))
led.freq(1000)  # Set LED PWM frequency

# Initialize the slide switch
switch = machine.Pin(17, machine.Pin.IN, machine.Pin.PULL_DOWN)

# Function to map values from one range to another
def interval_mapping(x, in_min, in_max, out_min, out_max):
    # Ensure in_min != in_max to avoid division by zero
    if in_max - in_min == 0:
        return out_min
    return int((x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min)

# Main loop
try:
    while True:
        if switch.value() == 1:
            # Alarm is ON
            # Increase frequency and brightness
            for i in range(0, 100, 2):
                # Map 'i' to LED brightness and buzzer frequency
                brightness = interval_mapping(i, 0, 100, 0, 65535)
                frequency = interval_mapping(i, 0, 100, 500, 2000)

                # Set LED brightness
                led.duty_u16(brightness)

                # Set buzzer frequency and duty cycle
                buzzer.freq(frequency)
                buzzer.duty_u16(32768)  # 50% duty cycle

                utime.sleep(0.01)

            # Decrease frequency and brightness
            for i in range(100, 0, -2):
                brightness = interval_mapping(i, 0, 100, 0, 65535)
                frequency = interval_mapping(i, 0, 100, 500, 2000)

                led.duty_u16(brightness)
                buzzer.freq(frequency)
                buzzer.duty_u16(32768)

                utime.sleep(0.01)
        else:
            # Alarm is OFF
            # Turn off LED and buzzer
            led.duty_u16(0)
            buzzer.duty_u16(0)
            utime.sleep(0.1)
except KeyboardInterrupt:
    # Clean up
    buzzer.deinit()
    led.deinit()
    print("Program stopped.")

Once the code is running, toggle the slide switch to the ON position. The buzzer should emit a siren sound, and the LED should flash accordingly. Toggle the switch to OFF to stop the alarm.

Understanding the Code

1. Initialization:

   • buzzer: PWM object on GP15.

   • led: PWM object on GP16, frequency set to 1kHz for smooth brightness control.

   • switch: Input pin on GP17 with an internal pull-down resistor.

2. Interval Mapping Function:

   Maps a value from one range to another, useful for scaling the loop variable to desired frequency and brightness ranges.

   # Function to map values from one range to another
   def interval_mapping(x, in_min, in_max, out_min, out_max):
       # Ensure in_min != in_max to avoid division by zero
       if in_max - in_min == 0:
           return out_min
       return int((x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min)
   

3. Main Loop:

   • Checks the state of the switch.

   • If the switch is ON (switch.value() == 1):

     • Runs two loops to simulate the siren effect:

     • Increasing frequency and brightness.

     • Decreasing frequency and brightness.

     • The buzzer frequency varies between 500 Hz and 2000 Hz.

     • The LED brightness varies from off to full brightness and back.

     if switch.value() == 1:
         # Alarm is ON
         # Increase frequency and brightness
         for i in range(0, 100, 2):
             # Map 'i' to LED brightness and buzzer frequency
             brightness = interval_mapping(i, 0, 100, 0, 65535)
         ...
     
             utime.sleep(0.01)
     

   • If the switch is OFF: Turns off the LED and buzzer.

     else:
         # Alarm is OFF
         # Turn off LED and buzzer
         led.duty_u16(0)
         buzzer.duty_u16(0)
         utime.sleep(0.1)
     

   • Exception Handling: Captures a keyboard interrupt (Ctrl+C) to cleanly deinitialize the PWM objects.

     except KeyboardInterrupt:
         # Clean up
         buzzer.deinit()
         led.deinit()
         print("Program stopped.")
     

Experimenting Further

• Adjusting the Siren Effect:

  • Modify the frequency range in the interval_mapping function to change the pitch.

  • Adjust the delay in the loops (utime.sleep(0.01)) to speed up or slow down the siren cycle.

• Add More LEDs:

  • Incorporate additional LEDs of different colors to create a more dynamic light show.

  • Use multiple GPIO pins and PWM channels.

• Motion Activation:

  Replace the slide switch with a motion sensor (e.g., PIR sensor) to trigger the alarm when movement is detected.

• Remote Control:

  Integrate an IR receiver to control the alarm using a remote control.

Conclusion

You’ve successfully built an Alarm Siren Lamp using the Raspberry Pi Pico 2! This project demonstrates how to control multiple components and create interactive effects. It’s a great foundation for more complex projects like security systems, emergency signals, or creative art installations.