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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 serves a dual purpose, functioning not only as an intruder alarm but also finding applications in areas such as home automation systems and energy efficiency systems. It operates by detecting the motion of individuals or objects and then responding accordingly based on the circuit’s configuration.

Typically, motion detectors utilize a range of sensors, including the passive infrared sensor (which detects motion by sensing body heat), the microwave sensor (which detects motion by measuring frequency changes in the emitted beam), the ultrasonic sensor (which relies on acoustic signals to detect motion), among others. Some advanced motion detectors combine multiple technologies and incorporate various sensors, such as PIR, microwave, and ultrasonic sensors, to minimize false triggers and enhance motion detection accuracy.

This circuit is both straightforward and reliable, employing an infrared sensor to emit an IR beam and a phototransistor to receive it. Any interruption or interference between the transmitted and received beams signifies a breach and triggers an alarm. Compared to conventional motion detectors, this circuit is easy to construct and cost-effective.

Block Diagram of Motion Detector:

Motion Detector Block Diagram

The IR sensor generates a high-frequency beam, which is emitted onto the phototransistor via a 555 timer in the transmitter. In the event of an interruption in this high-frequency beam, the photo transistor activates the 555 timer in the receiver section, leading to an alert being sounded through the alarm.

Motion Detector Circuit Diagram:

Motion Detector

Motion Detector Circuit Explanation:

  • Using a 555 timer configured as an astable multivibrator in the transmitter section, the IR sensor generates a high-frequency beam at 5 kHz.
  • The receiver section employs a photo resistor to capture this high-frequency beam produced by the IR sensor. When there’s an unobstructed path between the IR sensor and the photo transistor, the frequency remains in one phase, resulting in no output from the entire circuit. Conversely, if the connection between the IR sensor and the photo transistor is interrupted, the IR sensor’s beam shifts out of phase. The photo resistor promptly detects this phase change, prompting the 555 timer to activate an alert through the speaker.
  • In cases without intrusion, the photo transistor keeps pin 2 of the 555 timer, which is configured in monostable mode, in a high state, leading to no output in this configuration. In the presence of intrusion, the photo transistor’s action lowers pin 2 of the monostable timer, triggering the alarm. The duration of the alarm is contingent upon the values of capacitor C1 and the variable resistor POT.

Main Components in Motion Detector Circuit:

The fundamental concept behind an infrared sensor is to generate an infrared light beam. Infrared light falls within a wavelength range that is longer than visible light but shorter than microwaves, typically having a wavelength greater than 6 micrometers (µm). IR sensors are rooted in three fundamental laws: Planck’s radiation law, Stefan-Boltzmann law, and Wien’s displacement law.

  • 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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