Simple Door Open Alarm Circuit using Reed Switch

A Door Open Alarm Circuit with a Reed Switch is a basic electrical device that sounds an alarm each time a door is opened .

This simple door opening alarm circuit with a reed switch is used to secure doors against unauthorized access.

Typically, this circuit consists of a transistor, resistors and a battery depending on the condition of the reed switch the circuit regulates the supply of power to the alarm.

Circuit Working:

Parts List:

In this article the reed switch acts as an Inductive magnetic door sensor, it contains two metal reeds inside that function as a switch.

A magnetic pull is created when a magnet like the one on your door gets close.

The electrical circuit is completed when this pull causes the metal reeds inside the switch to come into contact.

The switch is “ON” when the door is closed and the magnet is close by.

The magnetic attraction vanishes when the door opens and the magnet moves out of the way.

The switch flips “OFF,” breaking the circuit, and the metal reeds separate.

Two transistors Q1 and Q2 are used in the circuit to regulate the alarm, which is typically a buzzer.

The Reed Switch is “ON” when the door is closed permitting a tiny amount of electricity to pass through transistor Q2.

By doing this, Q2 is turned “ON,” thus grounding the circuit and stopping the buzzer from going off.

The reed switch flips “OFF,” stopping the current to Q2, when the door opens.

Since Q2 is now “OFF,” transistor Q1 may now receive additional current that is managed by a resistor turning it “ON.”

When Q1 is set to “ON,” electricity can enter the buzzer and cause the alarm to sound.

Transistors are used in this circuit to function as electronic switches regulating the alarms electrical supply in response to the reed switches signal.

Formulas:

Above circuit that activates an LED and buzzer when the door is opened by utilizing a Reed Switch and the parts you’ve mentioned.

When the door is opened or closed, the Reed switch will function as a magnetic sensor to detect it.

Here are some design guidelines and pertinent formulas for this circuit, along with how to create it:

BC547 Transistor Base Resistor Calculation:

Base Resistor R_B:

Determine the amount of the base resistor to guarantee appropriate transistor saturation and current flow for operating the LED and buzzer:

R_B​ = V_in​−V_BE​​ / I_B​

where,

• The input voltage 5V is denoted by Vin
• The base emitter voltage drop (usually around 0.7V for BC547) is represented by VBE
• The base current is represented by IB.

Calculating an LED Current Limiting Resistor:

Resistor LED RLED:

Determine the resistor value required to restrict the current flowing through the LED in order to guard against damage and guarantee correct operation:

R_LED ​= V_supply​−V_LED​​ / I_LED​

where,

• The supply voltage 5V is the Vsupply,
• The forward voltage drop of the VLED (usually around 2V for a red LED),
• And the intended LED current (20mA) are represented by ILED.

This simple yet efficient door open alarm circuit uses a reed switch to deliver a visual and auditory alert when a door is opened.

Adaptations and changes can be done in accordance with particular needs and component availability.

How to Build:

To build a Simple Door Open Alarm Circuit using Reed Switch follow the below mentioned connections steps:

• Assemble all the components as shown in above diagram.
• Connect collector of transistor Q1 between the negative pin of 5V buzzer an a cathode of red LED, connect base of transistor Q1 between resistor R2 and collector of transistor Q2, connect emitter of transistor Q1 to ground.
• Connect collector of transistor Q2 to resistor R2 to positive supply, connect base of transistor Q2 to reed switch and resistor R1 to positive supply, connect emitter of transistor Q2 to ground.
• Connect a Red LED and a series resistor R1 from the collector of Q1 transistor to positive supply.
• Connect the anode of diode D1 with the collector of Q1 transistor and cathode of diode to positive supply.
• Connect a positive of 5V buzzer to positive supply and negative of buzzer to collector of Q1 transistor.

Conclusion:

A door open alarm circuit using reed switch employs a magnet to sense when a door is opened.

The magnet closes the circuit by producing a magnetic field that completes it, which silences the alert.

The buzzer sounds when the door opens because the magnet travels away, cutting the circuit.

While creating and utilizing this basic system, make sure to take all necessary safety precautions.

References:

Design of a Home Door Security System Based on NodeMCU ESP32 Using a Magnetic Reed Switch Sensor and Telegram Bot Application