PIR Sensor Heat Detector

1. Objective

The objective of this project is to sense heat with the help of infrared radiations emitted by a warm body. Thus it aims at contactless detection of heat

2. Approach

A. Testing the PIR Transducer

The PIR sensor [1] was tested for the nature of output that it gave when a motion was detected. It was observed that the sensor gave a constant voltage at its source terminal when there was no motion in its field of vision. The constant output voltage of the sensor was dependent on the surrounding room temperature.
When a motion of heat source such as the human body occurs in the field of vision of the sensor, the output of sensor decreases. The magnitude of the decrease in the voltage depends on the amount of heat radiated by the body. The output voltage of the sensor does not return to its constant value until the body has moved out of the field of vision of the sensor.

B. Signal Conditioning (Amplifier)

Since the output voltage of the PIR sensor is in millivolts, it should be amplified before processing it. An Op-Amp as an inverting amplifier [2] cascaded with an Op-Amp as an inverting buffer is used to amplify the output signal of PIR.

C. Output Logic (Comparator)

The instantaneous value of the signal is compared with the steady value of the signal to know if heat has been detected. If the instantaneous value is lower than the steady value the output becomes high, indicating the presence of a heat source. If the instantaneous value is higher than the steady value, which occurs in the presence of cold objects the output remains low indicating that there is no potential heat source around.

D. Output (LED)

The output is indicated with the help of a led, which glows when the heat is detected.

E. Calibration

Since the steady value of the sensor system depends on the surrounding temperature, the value with which the instantaneous signal is compared needs to be calibrated. The calibration is done with the help of a potentiometer.

3. EDA Tools Used

The EDA tools used by us were KiCad and FreeRouting.
Kicad is an open-source EDA tool which was used by us to design the schematic and layout of the PCB.
Freerouting is the tool which was used by us to auto-route the PCB tracks.

4. Schematics

i. PIR Sensor

ii. Amplification stage

iii. Comparator stage and final output

Complete Schematic

5. PCB Layout

6. Conclusions

The implemented circuit thus detects a heat source in its field of vision successfully, without establishing any physical contact with the source.
Objects that are at room temperature or colder do not affect the output.
The main component of this project is a PIR sensor, the sensor is said to be a passive sensor because it does not emit energy of any type but merely accepts incoming radiations.
The system of the project is dependent on the surrounding temperature, and it needs to be calibrated before using. Thus there lies a scope of improvement in this project to make it independent of the surrounding conditions.

7. Applications

A. Human Motion Detection

The circuit can successfully detect the presence of a human body around it due to the heat emitted by the human body. Thus it can act as an intrusion alarm. The advantage of this circuit is that it can even function in a darker environment.
Another application of human detection using this circuit is its implementation in home automation such as light switching.
An advanced application of this circuit can be a human tracking system based on PIR sensor network and video [3]

B. Overheating detection of a system

This circuit can be used as a safety mechanism inside a system to detect if it overheats, by calibrating the detector to the normal operating temperature of the system. The overheating would cause the detector’s output to become high which can in interrupt the normal processing of the system and initialize a predefined procedure to cool down the system. Thus the PIR based heat detector can be used as a part of a feedback network of a system. It can also be used as an alarm for protection of a heat-sensitive system to indicate the presence of an unwanted heat source nearby.

C. Remote temperature measurement

The circuit can be modified to indicate the temperature difference of a remote object related to its surrounding, by measuring the change in amplitude that it causes in the steady value of the PIR sensor.

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