This alarm system utilizes a PIR sensor (Passive Infrared) to detect motion and a 2N2222 NPN transistor to drive an active buzzer. The 7.4V Li-ion battery provides power to both the sensor and the load.
1. Circuit Components
•
PIR Sensor (HC-SR501): Detects infrared radiation from moving objects.
• 2N2222 Transistor: Acts as a switch to control the high-current buzzer.
•
Active Buzzer: Produces sound when DC voltage is applied.
• Resistor (Rb): A resistor to limit the base current
•
Power Source: 7.4V Li-ion battery pack.
2. Schematic Diagram
The following shows the circuit schematics:
3. Connection Details
The circuit is wired such that the transistor operates in the
saturation region when motion is detected.
•
PIR Sensor Connections:
- Vcc pin to Battery Positive(+)
- GND pin to Battery Negative(-)
- OUT pin to One end of the \(1k\Ohm\) resistor.
•
2N2222 Transistor Connections:
– Base (B): Connected to the other end of the \(1k\Ohm\) resistor.
–
Collector (C): Connected to the Negative (-) terminal of the Active Buzzer.
–
Emitter (E): Connected to the Battery Negative (+).
•
Active Buzzer Connections:
–
Positive (+) terminal to Battery Positive (+).
–
Negative (-) terminal to Transistor Collector (C).
4. Working Principle
1.
Detection: When the PIR sensor detects motion, its OUT pin
transitions from 0V to
approximately 3.3V.
2.
Biasing: This voltage creates a current \(I_B\) flowing
into the base of the 2N2222 transistor. The base current is calculated as:
\(I_B = \frac{V_{cc} - V_{BE}}{R_B} \)
Assuming \(V_{BE} = 0.7V \) for silicon:
\(I_B = \frac{3.3V - 0.7V}{1k\Ohm} \)
3. Switching: This current is sufficient to drive the transistor into saturation, allowing current to flow from the Collector to the Emitter.
4. Activation: The circuit for the buzzer is completed to ground, and the 7.4V potential difference across the buzzer causes it to sound. When motion stops, the PIR output returns to 0V, the transistor enters cutoff, and the buzzer turns off.
5. Component Considerations
•
Voltage Rating: The HC-SR501 PIR sensor typically accepts an input voltage range of 4.5V to 20V, making the 7.4V Li-ion battery ideal.
•
Current Limit: The 2N2222 can handle a continuous collector current (\(I_C\)) of up to 600mA or 800mA depending on the manufacturer, which is well
above the 20mA to 50mA typically drawn by an active buzzer.
•
Sensitivity: Use the potentiometers on the PIR module to adjust the sensitivity and
the "Time Delay" (how long the buzzer stays on after motion is
detected).
