Door Opening Alarm using IC LM358

Circuit Design:

1. LM358 Connections:
   • Connect the VCC pin (pin 8) of the LM358 IC to the positive supply voltage (e.g., 9V).
   • Connect the GND pin (pin 4) of the LM358 IC to the ground (0V) of the circuit.
2. Voltage Divider:
   • Create a voltage divider using two resistors (e.g., 10kΩ and 100kΩ).
   • Connect one end of the 10kΩ resistor to the positive supply voltage.
   • Connect the other end of the 10kΩ resistor to the non-inverting input (pin 3) of the LM358.
   • Connect the junction between the two resistors to the inverting input (pin 2) of the LM358.
   • Connect the end of the 100kΩ resistor to the ground.
3. Reed Switch and Magnet:
   • Connect one end of the reed switch to the inverting input (pin 2) of the LM358.
   • Connect the other end of the reed switch to the ground.
   • Place a magnet near the reed switch so that when the door or window is closed, the magnet holds the reed switch in a closed position.
4. Alarm Output:
   • Connect the output (pin 1) of the LM358 to the positive terminal of the buzzer or speaker.
   • Connect the negative terminal of the buzzer or speaker to the ground.
   • Connect a diode (e.g., 1N4148) in parallel with the buzzer or speaker to protect against voltage spikes.

Working Principle:

• The LM358 is configured as a comparator with a voltage divider setting a reference voltage.
• When the door or window is closed, the reed switch is in a closed position, and the voltage at the inverting input is higher than the reference voltage.
• When the door or window is opened, the reed switch opens, and the voltage at the inverting input drops below the reference voltage.
• The LM358 output goes high, triggering the buzzer or speaker, alerting that the door or window has been opened.