A Reed switch door alarm senses whether a fridge door has been left open and triggers an alarm. There are a number of other implied uses to this circuit as well. A refrigerator or freezer door that is left open or ajar may cause the food contents to spoil, which can lead to a pretty problematic situation in case someone accidentally consumes the said spoilt food. So, in today’s tutorial, we are going to go over a step by step procedure on How To Make a Reed Switch Magnetic Fridge Door Alarm.
Reed Switch
A reed switch comprises two ferromagnetic blades, separated by only a few microns. When another magnet comes close to these blades, the two blades pull towards one another. Once touching, the blades close the normally open (NO) contacts, allowing electricity to flow. Some reed switches also contain a non-ferromagnetic contact, which forms a normally closed (NC) output. Any approaching magnet will disconnect the contact and pull away from the switching contact.
Hardware Components
You will need the following parts to build tis project
| S.No | Component | Value | Qty |
|---|---|---|---|
| 1) | Reed Switch | NC switch | 1 |
| 2) | NPN Transistor | BC547 | 2 |
| 3) | LED | 3.5V/5mm | 1 |
| 4) | Buzzer | 5V | 1 |
| 5) | Resistor | 680 Ohm, 100K | 2 |
| 6) | Capacitor | 220uF/50V | 1 |
| 7) | Soldering Iron | 45W – 65W | 1 |
| 8) | Soldering wire with flux | – | 1 |
| 9) | DC Battery | 9V | 1 |
| 10) | Battery clip | – | 1 |
| 11) | Block Connectors | – | 1 |
| 12) | Jumper Wires | – | As per need |
BC547 Pinout
Useful Steps
1) Solder the two BC547 transistors on the board. Solder the emitter terminal of the Q1 transistor with the base terminal of the Q2 transistor. Also, Solder the collector terminal of the two transistors with each other.
2) Solder the +ve terminal of the 22uF capacitor with the base terminal of Q1 transistor and the -ve pin with the emitter terminal of Q2 transistor and the -ve of reed switch.
3) Solder the 100K resistor between the +ve terminal of the buzzer and the base terminal of the Q1 transistor. After that, solder a 680 Ohm resistor between the +ve pin of the buzzer and the +ve pin of the LED.
4) Solder the +ve pin of the LED with the 680 Ohm resistor and the -ve terminal with the -ve pin of the buzzer.
5) Solder the output block connectors for the magnetic reed switch.
6) After that, Solder the block connectors for the 9V supply. Also, solder the +ve and -ve terminals of the 9V buzzer with the board.
7) Power up and test the circuit.
Working Explanation
This circuit can be used with fridge doors, windows, gates, etc in order to indicate whether the gate has been opened or not. Since this is an NC reed switch. On moving the switches away from each other, the magnetic connection is made through mutual inductance of the coil & sends a control signal to the base of the first BC547 transistor.
The collector output of the first transistor triggers the 5V buzzer & the alarm goes off. In order to turn off the alarm, you simply displace the door back to its original position, moving the two reed switch terminals away from each other. This demagnetizes the two terminals & turns OFF the alarm.
Applications
- Used in appliances such as refrigerators and freezers to protect perishables from spoiling.
- Also used in high-risk areas such as security stations, police stations & checkpoints.
- An important security feature in sensitive workplaces such as military compounds.
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