- Build Your Inspiring 240VAC Mains Smoke Detector Alarm Circuit
- Build a Powerful 200mA Security Siren Circuit
- 3V Mini Alarm Circuit Diagram & Build Guide
- Laptop Alarm Circuit: Build Your Theft Protection Scheme
- DIY Motion-Activated Alarm Circuit with 100W Lamp
- 5-Digit Alarm Keypad: Build Your Secure Circuit Diagram
- Build Your 4-Digit Alarm Keypad Circuit Diagram
- Build Your 20-Minute Battery Burglar Alarm Schematic
- Enhanced Shed Alarm Circuit Diagram with 30-Second Delays
- Simple Fire Door Alarm Circuit Design & Timing Control
- Simple Shop Alarm: Build a 3-5 Second Beep Timer
- Four Transistor Alarm Circuits: Design & Build Your Own!
- 12 Small CMOS Alarm Circuits Diagrams – Build Your Own!
- Build Your 12V Burglar Alarm Circuit Diagram
- Build Your 30-Second Alarm: Circuit Diagram & Design
3V Mini Alarm Circuit Diagram & Build Guide
Description
This circuit, housed within a diminutive plastic container, can be carried within a purse or bag. A small magnet is positioned near the reed switch and linked to the wearer's hand or clothing via a slender cord. Should the bag be abruptly snatched, the magnet loses contact with the reed switch, SW1 opens, and the circuit commences oscillating, resulting in a loud alarm emanating from the loudspeaker. A complementary transistor pair is configured as a highly efficient oscillator, directly powering a small loudspeaker. Minimal component count and a 3V battery power enable a remarkably compact design.
Circuit Diagram:
Parts:
- R1 = 330K
- R2 = 100R
- C1 = 10nF-63V
- C2 = 100uF-25V
- Q1 = BC547
- Q2 = BC327
- B1 = 3V Battery or Two AA Cells in Series
- SW1 = Reed Switch & Small Magnet
- SPKR = 8R Loudspeaker (See Notes)
Notes:
The loudspeaker can be any type; its size is restricted only by the enclosure that will contain it. An auxiliary switch is superfluous as the standby current draw is less than 20µA. Current consumption when the alarm is sounding is approximately 100mA. If the circuit is intended for anti-bag-snatching, SW1 can be swapped with a 3.5mm mono Jack socket and the magnet with a 3.5mm Mono Jack plug with its internal leads shorted. The Jack plug will be connected to the tiny cord, and so on. It is not recommended to supply the circuit with voltages exceeding 4.5V; this will impede functionality and could potentially damage Q2. A 3V supply represents the optimal compromise in this instance.
Integrated Circuits - Detailed Description
The circuit employs two NPN bipolar junction transistors, BC547 and BC327, as the core of the oscillator. These transistors, commonly known as silicon power transistors, are fundamental building blocks in electronic circuits. The BC547 is a general-purpose transistor, suitable for a wide range of applications, characterized by its good switching speed and low collector resistance. It is a surface mount device (SMD) and is primarily used for amplification and switching. The BC327, similarly a general-purpose transistor, is another frequently used component due to its ease of use and reasonable performance characteristics. Both components facilitate the creation of a stable and efficient oscillating circuit, thereby driving the loudspeaker effectively. These transistors offer a trade-off between performance and cost, making them suitable for this particular application. Their selection is based on their reliable switching capabilities within the operational voltage and current limits specified for this design. Further specification of these components can be found on their respective datasheets, which provide detailed information about their electrical and thermal characteristics.