Question

Question: What are the Passive InfraRed Sensors (PIR) technical requirements?

Answer 1

The term PIR is the short form ofthe PassiveInfra Red. The term “passive”indicates that the sensor does not actively take part in the process, whichmeans, it does not emit the referred IR signals itself, rather passively detects the infrared radiationscoming from thehuman body in the surrounding area.

->The detected radiations are converted into an electrical charge, which is proportional to the detected level of the radiation.

->Then this charge is further improved by a built in FET and fed to the output pin of the device which becomes applicable to an external circuit for further triggering and amplification of the alarm stages.

->The PIR sensor range is up to 10 meters at an angle of +15o or -15o.

PIR Sensor’sWorking Principle

->The PIR sensors are more complicated than the other sensors as they consists of two slots. These slots are made of a special material which is sensitive to IR.

->The Fresnel lens is used to see that the two slots of the PIR can see out past some distance.

->When the sensor is inactive, then the two slots sense the same amount of IR.The ambient amount radiates from the outdoors, walls or room,etc.

->When a human body or any animal passes by, then it intercepts the first slot of the PIR sensor.

->This causes a positive differential change between the two bisects.When a human body leaves the sensing area,the sensor generates a negative differential change between the two bisects.

->The infrared sensor itself is housed in a hermetically sealed metal to improve humidity/temperature/noise/immunity. There is a window which is made of typically coated silicon material to protect the sensing element.

Main Specifications

The main technical specifications and dimension parameters of a PIR sensor can be learned from the following details:

Now let’s investigate the technical details of the PIR sensor HC-SR501.

Operating Voltage:

The HC-SR501 is from 5 V to 20 V, which makes great flexibility for circuit designers.

Current consumption:

The HC-SR501 is a battery friendly device; its current consumption is 65 mA when it detects any change in IR light.

Output Voltage:

When the module detects a motion of infrared the output goes HIGH at 3.3 V, if the module detects no motion it goes LOW or 0 V after a fixed period.

Delay time:

A knob is provided to adjust the time for the output to stay HIGH after detecting the IR. This is time period can be adjusted from 5 seconds to 5 minute.

Sensitivity Range:

The angle of the detection area is around 110 degree cone. A knob is given to adjust the sensitivity; we can vary from 3 meter to 7 meters perpendicular to senor. The sensitivity reduces as we move either sides of the sensor.

Operating Temperature:

The HC-SR501 has an impressive operating temperature ranging from -15 to +70 degree Celsius.

Quiescent current:

The Quiescent current is the current consumed from the supply, when the sensor is not detecting any motion or when it is in idle. It consumes less than 50 uA, which makes the sensor battery friendly.

Trigger modes:

The PIR module has two trigger modes: Single trigger / non-repeat mode and repeat trigger. These two modes can be access by changing the jumper position given in the module.

Single Trigger Mode / Non-repeat Mode:

When the PIR sensor is set in single trigger mode (and the timer knob / delay time is set for 5 seconds (say)), when a human is detected the output turns HIGH for 5 seconds and turns LOW.

Repeat Trigger Mode:

When the PIR sensor is set in repeat trigger mode, when a human is detected the output turns HIGH the timer counts for 5 seconds, but when another human is detected with in those 5 seconds the timer reset to zero and counts another 5 seconds after 2nd human is detected.

Block Time:

The block time is the time interval where the sensor is disabled or will not detect motion. The block time for HC-

SR501 is 3 seconds by default.

This occurs after the delay time (which was set by timer knob) the output goes LOW for 3 seconds; during this interval no motion will be detected. After the 3 seconds (LOW) the sensor will be ready to detect motion again.
In other words, when the sensor detects motion the output goes HIGH, the output remains HIGH as per the timer knob (say 5 seconds), after 5 seconds the PIR sensor goes LOW, the LOW signal will remain for 3 seconds regardless of the new motion if any.

Dimensions of the module:

The sensor is compact enough to hide from people’s view so that it won’t affect decorates etc. It measures 32 mm x 24 mm.

Lens Size:

The white dome structure which encloses the pyroelectric sensor is called Fresnel lenses, which increase the detection range and it looks opaque. It measures 23 mm in diameter.