Get our free book (in Spanish or English) on rainwater now - To Catch the Rain.
Most vending machine sensors use a type of technology known as passive infrared sensors (PIR), which unlike optical sensors that use an LED transmitted and infrared receiver, emits nothing. Rather than radiating, the PIR responds to infrared energy being emitted by any nearby objects. Any object with a temperature above zero degrees Celsius emits infrared energy, through black-body radiation. Invisible to the human eye, the magnitude of this radiation varies with temperature - which is exactly what makes the PIR function the way it does. <br />
Although invisible to the human eye, the magnitude of infrared energy can be quantified by a pyroelectric sensor. This sensor is placed behind an infrared-transparent cover, so that it may monitor objects with varying infrared energy. Similarly to the way an electric charge is created when visible light strikes a solar cell, these sensors generate a small charge when subjected to infrared energy. As an object with a more intense infrared energy, such as a person, is detected by the sensor, it overlaps a section on the chip that had previously been subjected to some much cooler object, such as a wall in the background. The pyroelectric sensor is connected to an energized relay, which acts as a "switch," completing the load circuit. As this warmer, or more energized object, moves along the lens of the sensor, the relay becomes de-energized, the contacts of the relay become operational, activating the detection switch of the control panel. Similarly, if an object colder than the background wall is presented into the field of view of the sensor, the difference in infrared energy will still cause the relay to be de-energized, activating the detection switch.
<ref> Machine Design (2008), ''Sensor Sense: Passive Infrared Motion Sensors'',http://machinedesign.com/article/sensor-sense-passive-infrared-motion-sensors-0710</ref> <ref> Wikipedia, ''Passive Infrared Sensor'',http://en.wikipedia.org/wiki/Passive_infrared_sensor#PIR-based_motion_detector</ref>
Many other objects tend to emit an undesirable amount of infrared energy - at least to the vending machine sensor's point of view. Something as simple as flicking on a light switch can produce a sudden change in infrared energy that the sensor will undoubtedly pick up. In a room where lights are constantly turned on and off, this would prove to be a major inconvenience, turning the machine on and off repeatedly without the intent to actually vend.
The unit's comparator solves this problem by ignoring objects that emit a constant source of infrared. Therefore, if a new "heat spot" shows up on the sensor and stays constant in its place and level of emittance, the comparator 'decides' not to send a signal, triggering the machines start-up.
The model below outlines a simple diagram of the inner workings in a passive infrared sensor.