Sensor: The circuit utilizes a magnetic reed switch for the required sensing operations. Reed switches show continuity across their leads when brought very close to a magnetic field and vice versa.
The sensor, attached at one of the edges of the circuit enclosure, is positioned over the door or the window in such a way that it’s sensitive surface angle exactly coincides with a parallel magnet fixed over the frame of the entrance at a touching distance (see figure). Thus as long as the door remains shut the magnetic field from the magnet keeps the reed switch activated which in turn keeps the electronic alarm switched OFF, however when the door is opened, the reed relay senses the inhibition of the magnetic field and instantly triggers the preceding electronic circuit and the alarm.
A typical reed switch, also called reed relay as shown in the figure, basically consists of a couple of tin plated metallic (iron) strips sealed inside a tiny cylindrical glass mold. These strips are purposely attributed with a spring-like property and therefore are able to change their positions or orientations in the presence or absence of magnetic fields.
Normally, in the absence of a magnetic field the strips remain separated from each other, however when a magnetic field is brought very near to it, the plates or the strips inside the glass mold instantly switch and connect with each other making a continuity between the strip and their ends, which are terminated externally to form the leads of the reed switch.
Oscillator: The alarm section is primarily governed by the IC 4049. It basically works as the tone generator or the oscillator which is amplified in the next stage for the required output levels. The associated pots can be varied to get different sets of tone for the alarm outputs.
Tone Amplifier: The high frequency intermittent alarm note derived from the IC 4049 is applied to a transistor coil network for the required amplification so that the generated alarm is strong enough and may be heard up to longer distances.
Timer: The stage comprising two general purpose transistors BC547 and BC557 along with a few other passive components forms the timer section. The stage decides for how long the alarm stays activated irrespective of the door position i.e. the alarm continues to ring for the stipulated period even if the door is pulled back to its original closed position.