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Motion Detector Circuit


  • Introduction to Motion Detector:
  • Block Diagram of Motion Detector:
  • Motion Detector Circuit Diagram:
    • Motion Detector Circuit Explanation:
    • Applications of Motion Detection:

Introduction to Motion Detector:

The Motion Detector is not only used as intruder alarm but also used in many applications like home automation system, energy efficiency system, etc. The motion detector will detect the motion of the people or objects and give the appropriate output according to the circuit.

In general, motion detectors use various types of sensors, such as passive infrared sensor (which detects motion of a person using the person’s body heat), microwave sensor (which detects motion of a person by measuring the change in frequency from the produced beam), ultrasonic sensor (which detects motion of a person using acoustic signals), and so on. Some motion detectors will employ various technologies and contain a variety of sensors (PIR, microwave sensor, ultrasonic sensor, and so on) to reduce false triggering and improve motion detecting accuracy.

This is a simple and dependable circuit that employs an infrared sensor to transmit the IR beam and a photo transistor to receive it. If there is any disruption or interference between the transmitting and receiving of the beam, it will justify that there has been an incursion and will sound an alarm. When compared to a standard motion detector, this circuit is simple to build and has a cheap cost.

Block Diagram of Motion Detector:

Motion Detector Block Diagram

IR sensor will produce the high frequency beam which is projected on the photo transistor with the help of 555timer at the transmitter. When this high frequency beam has got any interruption, the photo transistor will trigger the 555 timer of receiver section and gives alert through the alarm.

Motion Detector Circuit Diagram:

Motion Detector

Motion Detector Circuit Explanation:

  • With the help of a 555timer set to astable multivibrator mode in the transmitter portion, the IR sensor will create a 5 kHz high frequency beam.
  • The photo resistor in the receiver part receives the high frequency beam produced by the IR sensor. When there is no interruption between the IR sensor and the photo transistor, this frequency will be in one phase. During this period, the entire circuit will not provide any output. When the connection between the IR sensor and the photo transistor is broken, the IR sensor’s beam will be out of phase. The photo resistor will detect the change in phase immediately, causing the 555 timer to sound an alert through the speaker.
  • When there is no intrusion, the photo transistor will make the pin2 high of 555timer which is set in monostable mode, and there will be no output given in this configuration. When there is intrusion, the pin 2 of monostable timer is made low which will make the alarm to alert. The alarm time depends on the capacitor C1 and variable resistor POT.

Main Components in Motion Detector Circuit:

The primary notion of an infrared sensor is to produce a beam of infrared light (whose wave wavelength is longer than visible rays but shorter than microwaves, with a standard infrared wave length of greater than 6m). Planks radiation law, Stephan Boltzmann law, and Wien’s displacement law are the three laws that IR sensors are built on.

  • Planks Radiation Law states that the energy of electromagnetic radiation is confined to indivisible packets (quanta), each of which has energy equal to the product of the Planck constant and the frequency of the radiation (planks constant = 6.62606957 × 10-34 m2 kg /s).
  • Stephan Boltzmann Law states that total energy radiated per unit on a black body using  all wavelengths per unit time J* is directly proportional to the fourth power of the black body’s thermodynamic temperature T:
Formula for J in Motion Detector
  • Wien’s Displacement Law: The wavelength of a body’s maximum emission is proportional to its absolute temperature (measured in Kelvin). As a result, the maximum (peak) of radiant energy shifts toward the shorter wavelength (higher frequency and energy) end of the spectrum as the temperature rises.

Peak intensity occurs at this wavelength ƛ =(0.0029 meter.K)/Temperature in Kelvin

The infrared source and transmission of infrared are two critical components of an IR sensor. There are various infrared sources, such as black body radiators, tungsten lights, and silicon carbide in IR sensors. As an infrared source, they will employ LEDs with infrared wavelengths. Different transmission media will be used, such as air, optical fibre, and so on.

Photo Transistor: Photo transistors are the detectors of IR radiation or any photo radiation. They will convert this IR radiation into current or voltage.

Applications of Motion Detection:

  • Motion detectors can be used as an intruder alarm in home, offices, banks, shopping malls etc.
  • They can be used as counting machines, automatic light control etc.
  • They can be used in energy efficient systems, home automation system and control systems.

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