The circuit described here can be used as burglar alarm or can be used as an indicator that door or window is open.
Now the summer time is coming and we all are going to use AC in our home and offices. AC functions efficiently, only when doors and windows are closed properly. If any of the door or window is left open then effect of AC cannot be felt. So we have described a circuit here that will give an alarm when door or window is left open.
Another application when you can use this circuit, in your cupboard or lockers. When light fall on the sensor it will give an alarm indicating that somebody is trying to steal your valuable thing. And will also give an alarm when any unwanted person is trying to open door and window.
Fig. 1: Prototype of Window and Door Guard with Light Sensor Circuit on Breadboard
This circuit is based on 4093 IC, Reed switch and LDR as a sensor with few more components.
4093 is a Quad 2 input NAND schmitt triggers IC. It comes in 14 pin Dual inline package. Important feature of IC is when there is no external component schmitt action will be provided to each input pin. It can be used in astable and monostable multivibrator application and in high noise environment system.
NAND gate will give output as high when any of the input logic signal goes low or both the input signal become low. And if both the logic signals applied are high then we will receive low output.
CD4093 contain 4 individual gate in a single IC. In this pin1 and 2 is for input and pin 3 is for output. Similarly pin 5 and 6 is input and pin 4 is for output. In this circuit we are utilizing all four gates of CD4093 IC.
Working of circuit is very simple when all doors and windows are close then pin 1 and 2 of IC1 is at high state therefore out pin 3 will be low, therefore no sound will be heard. As soon as when somebody left the door or window open than capacitor C1 charges through resistor R1 and after some delay input pin 1 and 2 of N1 become low. This will trigger the pin 5 and 6 of N2 and output pin 4 becomes high. Which in turn enable the gate N3 and buzzer starts sounding. In this circuit N4 is used as an inverting buffer. And capacitor C2 and resistor R3 is used to set the astable frequency.
Now when there is no light falls on LDR its resistance is high therefore pin 12 and 13 both are high and pin 11 of N4 is low so no buzzer will sound. But as soon as light falls on LDR its resistance become low and pin 12 of N4 goes low to enable the buzzer. You can set the sensitivity of LDR with the help of variable resistor VR1. If you want to remove this section you can do so by just removing the LDR and replace it with 10K resistor.
If you want to protect more than one door or window then just add more number of switches in parallel with switch S1.