The reed switches have an unbelievably high mechanical switch reliability and can operate for billions of cycles before they fail. They will also work in explosive environments where a spark may potentially cause disastrous results because of their sealed construction.
Reed switches can be older but are far from obsolete. You can use this simple Open Alarm Circuit with the Reed Switch if you want to protect any doors from unwanted access.
Hardware Component
The following components are required to make Door Open Alarm Circuit
| S.no | Component | Value | Qty |
|---|---|---|---|
| 1. | Reed switch | – | 1 |
| 2. | Transistors | BC547, SL100 | 1, 1 |
| 3. | Diode | 1N4007 | 1 |
| 4. | Led | – | 1 |
| 5. | Resistor | 1K, 2.2K, 330Ω | 1, 1, 1 |
| 6. | Buzzer | – | 1 |
| 7. | Battery | 5V | 1 |
BC547 Pinout
For a detailed description of pinout, dimension features, and specifications download the datasheet of BC547
Door Open Alarm Circuit
Working Explanation
Reed switch acts as a sensor for door position detection (close/open). The Reed switch is a small device with two ferrous materials in it.
If it is exposed to the magnetic field, ferrous materials pull together in the reed switch, and the switch closes. If the magnetic field is absent, ferrous materials can be separated and turned on. This element reacts in the manner of a non-contact circuit switch, and can also carry up to 1.2A current.
This alarm circuit was designed with two familiar transistors BC547 and SL100. Here SL100 NPN behaves as an ON and OFF switching system; the 5Volt DC supply is supplied directly to the Reed switch through R3, another reed switch terminal to the Q2 transistor base. The Q1 base terminal is connected directly to the Q2 transistor.
Applications and Uses
A permanent magnet fixed on the door, placed parallel to the reed switch, is used to apply the magnet field. The sensor circuit will determine the door position and control the alarm.
