Infrared Alarm System. Design Note. 19

IR-LED-PairThis design note explains how to design an invisible alarm system easily. Infrared Transmitter ( Tx) and Receiver ( Rx) LED pair is easily available which can be used for making this alarm system. Infrared transmitter LED emits infrared rays in the wavelength around 950 nm. This LED is a high current device that can handle more than 100 mA current. The IR receiver LED is a kind of Photodiode that generates a Photo current when it receives IR rays. So the IR LED pair is ideal to make an alarm system that operates both day and night.
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PIR Light Switch

PIR-1You can easily make a Burglar Alarm system using the PIR Sensor module. The PIR sensor module is readily available now and costs less than Rs.100. You can make the complete unit at the cost of less than Rs.200 but its use is worth. The PIR Sensor module has a circuit board with a timer. The timer circuit gives a time delay between 30 seconds to 3 minutes that can be adjusted using the present. It also has a sensitivity adjustment for adjusting the sensitivity from few centimeters to 10 meters. In some modules, an LDR is also provided for night time operation only.
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Invisible Burglar Alarm. Beginners Trial 2

You can easily make this Invisible Burglar alarm using Infrared LED and a Phototransistor. When there is no obstruction in the path of IR rays, alarm will be silent. When somebody crosses the IR beam, alarm sounds. If the IR LED and Phototransistor are enclosed in black tubes and aligned perfectly, the range of the circuit is around 1 meter.
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Simple Alarm Generator. Design Trick 8

You can make a simple alarm generator based on an oscillator to use in burglar alarm circuits. It uses only two transistors and a few other components. Oscillator is a circuit that generates AC pulses from DC. The AC pulses thus generated can be converted into audio frequency using a speaker. Let us see how it works.
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PIR Sensor based Alarm

This Passive InfraRed Alarm system can switch on a Lamp or Alarm when it detects the heat from an object like moving person. The name PIR (Passive Infrared) is given to the sensor because it receives the infrared rays passively and do not emit any infrared ray. The lamp or alarm remains on few minutes depending on the mono time setting of the PIR Sensor used. Ready to fix PIR Sensors are available with Fresnel lens to increase sensitivity and to filter ambient light. Its range is around 5 meters at 45 degree cone angle.

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