Economical circuit of an electronic alarm, notifies you when your door or window is open, is presented here. This circuit wired around the Quad 2-input NAND Schmitt trigger IC 4093 (IC1) can be powered from a standard 9V battery. Here, a normally-closed (N/C) push switch (S1) is used to monitor the door/window activity. For satisfactory operation,it is crucial to fix S1 in the door/window frame such that when the door/window is closed, S1 remains in open state.
Schematic of the Door Alarm
When the door/window is opened, capacitor C1 charges through R1 (and S1) until it reaches the point where output of IC1A switches from high to low. After this slight delay, output of IC1B then goes high to enable the gated astable realized using IC1C. Components C2 and R3 sets the astablefrequency. Note that here IC1D is used as inverting buffer/amplifier that gives single-ended drive to the 30mm piezo-ceramic transducer (electric-to-acoustic converter) BZ1. The monotone when IC1C is in “on” mode is, gated by a high input at pin 12 of IC1D. The signal reaching the piezo-ceramic element BZ1 is a square wave with peak-to peak amplitude equal to the supply voltage.
Ambient light sensor mechanism, built around the 150K preset pot (VR1) and the 5mm LDR, ensures that the alarm sounds only when ambient light is very low (ie night time). VR1 can be used to fine-tune the detection senstivity of this light sensor. In case of no interest you can remove this option by replacing the LDR with a 100K Resistor,and connecting pin 12 of IC1 to the positive rail via a 10K resistor.
This simple circuit works perfect with multiple switches. Just add desired number of N/C push switches in parallel with S1 (through low-voltage cabling) to cover all the entry and exit points!