The amazing and endless world of electronics sometimes brings unexpected surprises to its fans. To do this, you just need to look more often at the Internet sections devoted to electronic crafts (photo below).
There you can always find such original solutions as a security system operating on just one bipolar transistor. After assembling and installing it at the front door of the working workshop, for example, any user can know in advance about the arrival of invited or uninvited guests.
Homemade burglar alarm scheme
Before proceeding with the assembly of the security system circuit, you will need to understand in detail the principle of its operation and prepare the main electronic components.
As you can see from the above photo, the circuit includes only 4 elements, namely:
- Any medium power NPN junction transistor.
- A resistor with a resistance of the order of 100 ohms, which sets the operating point of the transistor.
- 12 volt power supply or car battery.
- Acoustic head with a resistance of more than 1 kohm (the more, the better).
Note: Instead of a buzzer, a 12.0 Volt light bulb or a small piece of LED strip for the same voltage can be used.
The option of using both the buzzer and the incandescent light bulb at the same time is not excluded, but in this case the transistor and the power source must be designed for high power.
Principle of operation
The principle of operation of the circuit is extremely simple: until the security circuit is broken (the wire is not broken), current will flow through it, and the transistor will remain closed. The latter is explained by the fact that to open a bipolar semiconductor element between its base and the emitter, a voltage of about 0.5-0.7 Volts must act.
While it is close to zero, the transistor is closed, and the current does not flow through the buzzer and it does not "issue" any signals. The circuit can remain in this mode for a very long time (until the battery is discharged or the power supply is turned off).
When the guard wire breaks, the current has no choice but to flow through the base-emitter junction, which leads to its opening and switching the bipolar transistor to active mode. Due to this, a relatively large current begins to flow through the collector junction towards the emitter, which causes the buzzer to sound or the LED lamp to glow.
Assembling the circuit
You can assemble the security alarm circuit on a small scarf purchased from Ali Express (the kit is shown in the photo below).
It has enough room to accommodate all parts, including the transistor and other electronic components. They are connected to each other by means of MGTF wire or any other type. The buzzer capsule can be fixed to the board with cold glue. The same brand of wired product is used as a guard or signal wire (as it is convenient for anyone), as for connecting elements on the board (MGTF photo is given below).