Alarm System for Entrance/Exit Gate

Overview

The circuit employs the functions of reed switches to produce a gate alarm that is powered by a universal AC/DC power supply

Terminology

• Oscillator – an electronic circuit that produces a time varying or repetitive electronic signal, sine wave or square wave, without an external input signal using positive feedback
• Light Emitting Diode (LED) – a semiconductor diode that is commonly a source of light when electric current pass through it
• Reed Switch – an electronic switching device that operates magnetically by opening or closing a circuit with the presence of a nearby magnet
• CMOS 4093 – a quad 2-input NAND with Schmitt trigger inputs integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption

Circuit Explanation

Logic functions are acquired in this circuit with the use of 4093 integrated circuit which consists of four Schmitt trigger circuits. Each circuit operates as a two-input NAND gate having Schmitt trigger action on both inputs. During the operation, the gate switches at different points for positive and negative going signals. A hysteresis voltage is produced as the difference between the positive and the negative voltage.

The first NAND gate IC1a functions as a fast oscillator while the second functions as slow oscillator. A warning sound will be produced upon opening the gate or window with the combination of these oscillators. The sound being created by the sound is not as loud as a siren but needs to be noticeable in such a way that it will provide an acknowledgment upon opening the gate or window. To make a not so loud sound, diode D1 and resistor R1 can be removed while the value of resistor R2 can be reduced. The frequency of the can will be controlled by the variable resistor VR1.

The timing in which the alarm will produce a warning sound is modified by the IC1d which produces 20 to 30 times of pip sound until the gate is closed or becomes silent. A piezo disk is used as switch for the purpose of changing the voltage upon deformation or it can be substituted with a LED with a 1K ohm resistor in series.

The second figure shows how a normally closed switch may be operated using an ordinary reed switch with a mounted magnet near the reed switch and a mounted magnet on the gate, during the opening of the gate or window. Wires that are soldered near the reed switch should not be flexed near the switch because they are very vulnerable. Switches such as microswitches may be used as well.

Application

Reed switches are commonly used in applications involving relays. These reed relays have high operating speed, operates in low currents, have long life and high reliability, and provides good performance even with minimal current. In creating the reed relay, the reed switch is placed inside a coil where it gets energized to produce a magnet. In the field of communications, reed switches are particularly utilized for electrical control circuit while in mechanical systems, they are being utilized as proximity switches which are triggered by magnets like the method of tamper proofing and the system of burglar alarm employed in window and door sensors. On the other hand, employing a strong magnetic field might disable these proximity switches.

Another application of reed switches is a bicycle computer where the frequency of a beam passing is counted while the distance and speed are identified by the mounted magnet. In measuring the liquid levels, reed switches are used as sensors where the magnet is on a float moving up and down and the reed switch is fixed at the wall.

The use of reed switches brings several advantages like easily mountable and requires less operating force because of their small size. Since they are widely available and cheap, they are appropriately used as magnets. Behind these advantages lie some disadvantages while utilizing reed switches like being limited in controlling high currents and voltages that may cause spark on the reeds. The reed switch is known for overheating when heavy current is applied. During soldering against thick wires, reed switches are fragile which may break the seals and glass because it is delicate and small.

Source:www.zen22142.zen.co.uk/Circuits/Alarm/Galarm.html