Simple Door Open Alarm Circuit using Reed Switch

Simple Door Open Alarm Circuit using Reed Switch uses reed switch which makes alarm sound when door opens; this simple circuit help protect door from people not allowed.

Also, it have reed switch, transistors, resistors, buzzer, diode and LED and here, reed switch control power to alarm.

Circuit Working:

Parts List:

Components	Values	Quantity
Resistors	1k 1/4 watt	1
	2.2k 1/4 watt	1
Semiconductors	Transistors BC547	2
	Diode 1N4007	1
	Red LED 5mm 20mA	1
	Buzzer 5V	1
	Reed switch	1

Reed switch work like magnetic door sensor and inside it have two metal pieces like switch.

When the door closes, the magnet approaches the reed switch, pulling the metal contacts together and completing the circuit, as a result, the switch turns ON.

When door is open the magnet move away and pull goes away and the metal pieces separate and now switch is OFF and the circuit break.

Circuit use two transistors Q1 and Q2 to control buzzer alarm.

When the door closes, the reed switch turns ON and allows a small current to flow to Q2 and as a result, Q2 turns ON, grounds the circuit and keeps the buzzer quiet.

When door opens the reed switch goes OFF with no current goes to Q2 and then Q2 turn OFF and now Q1 get more current from resistor and turn ON.

Hence, Q1 let power go to buzzer and buzzer makes sound and then transistors act like switch and controls alarm based on reed switch signal.

Formulas:

This circuit uses Reed switch, LED, buzzer and transistor and when door opens it turn ON LED and buzzer.

Reed switch act like magnetic sensor which detects door open or close, below are some design tips and formulas:

1. Base Resistor RB for BC547:

Need base resistor to turn ON transistor properly.

Formula:

RB = (Vin − VBE) / IB

where:

• Vin the 5V input
• VBE is 0.7V for BC547
• IB is the base current

2. LED Resistor RLED:

Need resistor to protect LED from too much current.

Formula:

RLED = (Vsupply − VLED) / ILED

where:

• Vsupply is the 5V
• VLED is the 2V for red LED
• ILED is 20mA for LED current

Therefore, this simple circuit give sound and light when door opens and we can change design based on our needs.

How to Build:

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

• First, put all parts same like in diagram.
• Next, connect Q1 collector between buzzer negative pin and LED cathode, connect Q1 base between resistor R2 and Q2 collector and connect Q1 emitter to ground.
• After that, connect Q2 collector to resistor R2 and then to +5V, connect Q2 base to reed switch and resistor R1 and then to +5V and then connect Q2 emitter to ground.
• Then connect red LED and resistor R1 from Q1 collector to +5V and connect diode D1 anode to Q1 collector and cathode to +5V.
• Finally, connect buzzer positive to +5V and buzzer negative to Q1 collector.

Conclusion:

Overall, this Simple Door Open Alarm Circuit uses a reed switch and a magnet to detect when the door opens and when the door closes, the magnet activates the reed switch and keeps the alarm silent.

Hence, when door open the magnet moves and circuit break and buzzer makes sound, so be careful and safe when building this circuit.