PROJECT 4 - LIGHT Sensor report
From Physical Programming
Sensor Technology Report
LIGHT SENSOR
Contents |
How does it operate?
Light sensors detect variance in their field of light sensitivity and convert this into electrical signals which then produce a specific electrical output. Light sensors comprise of two Light Dependent Resistors (LDR) which have resistances that vary according to the amount of light that fall on them. Depending on what kind of LDR they are, their resistance will either increase or decrease with an increase in light.
examples:
1. In a porch light or burglar alarm, once a person has walked passed and the light sensor has detected the variance and the predetermined variance threshold has been reached, an electrical impulse is sent to activate the bulb or alarm.
2. In an SLR camera, different thresholds can be set for different conditions. Once the threshold has been set, the light sensor reports the amount of light as a number within this threshold, eg. 0 would be total darkness. This information is then used to turn the flash “on” or “off”, and sets the camera gain and shutter speed for the best image quality.
There are many types of light sensors, including:
- photodiodes
- phototransistors
- photoresistors
- photomultipliers
- solar cells
However the most common and least expensive light sensors are:
- Silicon phototransistors
- Cadmium Sulfide (CdS) photocells
Photocells are easy to work with but have a slow response time compared to the phototransistor. Photocells are therefore suitable for detecting levels of ambient light. They detect ALL visible light and hence are best at detecting presence of light, such as in light and dark-activated circuits.
Phototransistors are light-sensitive transistors, they have a higher responsivity for light than the photocell and do not react much to ambient light. The more light which reaches the phototransistor, the more current passes through it, which is then amplified by the transistor. Phototransistors are better to use for a very rapid response time.
Applications
Light sensors are used to provide information on:
- distance
- shape
- speed
- dimensions
Light sensors are used in:
- SLR cameras: for calculating camera exposures
- LCDs for portable electronic devices: for controlling dimming in order to extend the life
- Robotics: for motion sensing, proximity and light seeking
- Room occupancy detection and light adjustment
- Manufacturing industry: An LED coupled with a light detector can count the number of products made. The light sensor detects a marking on the product and each time the marking passes the sensor it registers on the sensor, counting each marker as one product.
Use within interactive projects
- Detection: A photocell can be used to determine when a box is open or closed.
- Target Sensing: Photocells can be used as target sensors for laser pointers.
- Touch Keypad: Photocells can be used in place of keys on a keyboard, whereby a person's finger will block the light enough to trigger a readable change in current.
Interfacing with the Arduino microcontroller
The emitter pins of phototransistors and photocells can be connected quite simply to the Arduino microcontroller and a code must be written for the light sensor to work in a specific way.
phototransistor connected to Arduino
photocell on left, sensing light as shown by light up led
References
PHOTOCELL Phototransistors Photocells WHAT IS A LIGHT SENSOR?
