PIR Sensor based Security Alarm System

Generally, in security systems, infrared or laser transmitters and receivers are used. But these methods are cost effective. This article explains about PIR based security system in which PIR sensor is used instead of transmitter or receiver. This saves the power consumption and is low cost. PIR sensor is the short form of Passive Infrared Sensor.

PIR Sensor based Security Alarm Circuit Principle:

The main principle of the circuit is to provide security. This is based on PIR sensor with an IC that produces siren. The PIR sensor detects the IR radiations emitted from the humans and it produces a digital output. This digital output is applied to the UM 3561 siren IC. Thus it produces the sound when any human is detected.

The UM3561 is a ROM IC. It generates multi siren tones like ambulance siren, fire engine siren, police siren, machine gun sound.

PIR Sensor based Security Alarm Circuit Diagram:

PIR Sensor based Security Alarm Circuit Diagram

Circuit Components:

• PIR sensor
• UM3561 siren IC
• Transistor (NPN)
• Resistors R1, R2, R3, R4.
• Speaker 8 Ohms
• Battery

PIR Sensor based Security Alarm Circuit Design:

The proposed system mainly consists of PIR sensor, UM3561 IC, Speaker, transistors. The UM3516 IC is a Siren generator IC. It has 8 pins. Fifth and sixth pins are shorted and are connected to the PIR sensor output through a resistor. One end of the 220K resistor is connected to the seventh pin of the IC and the other end is connected to the eighth pin of the IC. The first pin and the sixth pin are tone selection pins and these pins decide the tone to be produced. Output is taken from the third pin of the IC. It is connected to a speaker through a resistor and transistor. The base of the transistor is connected to the output of the IC through a resistor of 470 ohms. Emitter pin is connected to the ground while transistor is connected to the collector. A battery is connected to the speaker and 470 ohm resistor.

Working of PIR Sensor based Security Alarm System:

1. Switch on the circuit initially.
2. The PIR sensor is powered and it detects the IR rays emitted from any human.
3. This PIR sensor has a range of 5meters. One can adjust the pot provided for the sensor to vary this distance.
4. When any human is detected, this PIR sensor outputs a logic high value i.e. voltage of 3.5v to 5v.
5. This voltage is applied as input to the siren generator IC.
6. The siren generator has an oscillator internally, to produce the sound.
7. The oscillator circuit is tuned to a certain frequency and using a 220K resistor externally.
8. Then it is passed to the control circuit, which depends on tone selection pins.
9. These tone selection pins decide one tone from different tones produced by the IC.
10. Thus oscillations along with selected tone are sent to the address counter. The address counter then sends the data to the ROM.
11. ROM then sends the tone on the output pin 3.
12. The output is given to the NPN transistor to amplify the siren.
13. The base of the transistor gets voltage from output pin of the siren generator.
14. Transistor starts conducting when it gets the cutoff voltage at the base and the speaker is negative pin and is connected to the ground.
15. Thus sound produced can be heard from the speaker when human is detected.
16. In the present circuit, it produces fire engine sound.

PIR Sensor Based Security Alarm System Applications:

• This can be used in the museums to protect the valuable things.
• This can also be used as an automatic door bell circuit that rings the bell when human is detected.
• This can be used in defense applications to detect the humans in war field.
• This can be used in toy applications that produce sound.

Limitations of this Circuit:

• PIR sensor pot should be adjusted in such a way to detect the humans only.
• This can detect the human only within its range of 5 meters.