Circuit diagram of Home security alarm
- Battery 6V
- Resistors ¼ watt (150kΩ)
- LDR (Light Depended Resistor)
- Transistor BC547
- 6V Buzzer
- This circuit is based on LDR (Light Depended Resistor), a variable resistor in which the resistance varies according to the light intensity falling on it.
- The LDR and resistor R1 forms a potential divider network, which is the main part of our security alarm circuit.
- We have already discussed about how transistor acts as a switch, the same principle is used here.
- The voltage drop across the LDR is used to drive the transistor switch. When the voltage drop is above cut in voltage (0.6V), the transistor is turned ON.
- LDR has low resistance (mΩ range) in the presence of light and high resistance (MΩ range) in the absence of light.
- In our security alarm, a LASER light is allowed to fall on the LDR continuously using 3 mirrors (see the project arrangement figure).
- Light from other sources should not be allowed to fall on the LDR, so place the LDR in a box with a single hole to pass LASER.
- In this situation, the resistance offered by LDR is too low, since the LASER light is continuously allowed to fall on the LDR surface.
- Thus the voltage drop across the LDR is also low [V=IR (Ohm’s law)] which is insufficient to turn ON the transistor, so the transistor remains in OFF state.
- When a person (eg: thief) makes a block to the continuous flow of LASER beam, then the light falling on the LDR gets blocked. Thus its resistance increases to a high value in the order of MΩ range (According to Ohm’s law V=IR).
- While resistance increases the voltage drop also increases, when this voltage drop exceeds the cut in voltage of the silicon NPN transistor (0.6V; BC547), it will turn ON.
- Then current from Vcc starts flowing to ground via the buzzer and transistor, which makes the beep sound.
- The beep sound from the security alarm gives the indication of some security failures.
Simulation (animation) of the project
To realize the concept we are providing animation/ simulation of home security alarm circuit below. For animated demonstration, LED is used instead of buzzer.
The arrangement of LASER path is shown in the above image.
Video Demonstration of Home security Alarm
Here is the video from Circuits Gallery video lab