PIR Sensor Based Security Alarm This project will guide you to build a simple and cost effective security system using a PIR (Passive Infrared) sensor. You can hide this circuit at the entrance of the house to detect any human body. Whenever someone passes from in front of the PIR sensor attached with this circuit, it detects the infrared radiations emitted from human body and generates an alarm siren. This circuit is basically and extension of this simple PIR sensor circuit where I have added the siren generator IC UM3561. Along with a PIR sensor, siren generator IC UM 3561 is used here to generate the alarm sound. This IC can generate various types of sounds like fire brigade, ambulance siren, police siren, gun sound etc. Check here the UM 3561 pinout. Some other common electronics components are used in this PIR sensor based alarm circuit, which are listed below. Components Required: - IC UM 3561 PIR Sensor BC547 Transistor Resistors (10k x 2, 220k, 470) 12v battery Speaker 8 ohm Circuit Diagram Circuit Description When someone passes from in front of PIR sensor, it detects the IR rays emitted from the human body and gives a logic high value output of 3.5v to 5v. This output is applied to the tone generator IC UM3561. Depending on the configuration, this IC sends the tone signal to its output pin 3 which is amplified by an NPN transistor and supplied to the speaker. About PIR Sensor PIR sensor is an electronic device which can detect the infrared radiations in its surrounding. It can be used to activate an alarm when it detects the infrared in form of body heat from human body. As shown in the below image, a PIR sensor has 3 pins namely DC voltage pin, output pin and ground pin. Complete assembly of the PIR sensor is mounted on a PCB with various other electronic parts and covered with a Fresnel lens. If you see closely, there is a small window behind the lens to allow infrared to enter in the sensor. This window is covered with a transparent plastic layer which only allows IR rays to enter and blocks visible lights from entering in the sensor. About IC UM3561 IC UM3561 is a siren generator IC generally used in alarms and toy applications. Depending on the selection, it can generate 4 different types of sirens which are – ambulance siren, fire brigade sire, police siren and gun sound. Here we will see how we can make these different siren sounds using different sele tio s i the IC, ut efore that let’s have a look to its pi diagra a d des riptio . Pin Diagram: Pin Description: Pin No. Symbol Description 1 SEL2 Sound Effect Selection Pin No.2 2 Vss Negative of Power Supply 3 OUTPUT Mono-tone Output 4 NC Internal Testing Pin : Leave Open for Normal Operations 5 Vdd Positive of Power Supply 6 SEL1 Sound Effect Selection Pin No.1 7 OSC1 External Oscillator Terminal 1 8 OSC2 External Oscillator Terminal 2 How to Generate Different Types of Sirens using IC UM3561? As shown in the above pin diagram, pin 1 (SEL2) and pin 6 (SEL1) are the selection pins. We can use these two pins to select the siren sound as per given in the below table. SEL1 SEL2 Sound Effect NC NC Police Siren VDD NC Fire Engine Siren VSS NC Ambulance Siren X VDD Machine Gun NC – No Connection X – Don’t Care