Photoresistor

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. The photoresistor is facing rightwards, and controls whether current flows through the heater which opens the main power contacts. At night, the heater cools, closing the power contacts, energizing the street light. The heater/bimetal mechanism provides a built-in time-delay.

A photoresistor or LDR is an electronic component whose electrical resistance decreases with increasing incident light intensity. It can also be referred to as a light-dependent resistor (LDR), photoconductor, or photocell.

A photoresistor is made of a high-resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its electron hole partner) conduct electricity, thereby lowering Electrical_resistance.

A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, eg. silicon. In intrinsic devices, the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities added, which have a ground state energy closer to the conduction band — since the electrons don't have as far to jump, lower energy photons (i.e. longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms(impurities), there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.

Cadmium sulphide cells Cadmium sulphide (CdS) cells rely on the material's ability to vary its resistance according to the amount of light striking the cell. The more light that strikes the cell, the lower the resistance.Although not accurate, even a simple CdS cell can have a wide range of resistance from less than 100 Ω in bright light to in excess of 10 MΩ in darkness. Many commercially available CdS cells have a peak sensitivity in the region of 500nm - 600nm (green light). The cells are also capable of reacting to a broad range of frequencies, including infrared (IR), visible light, and ultraviolet (UV). They are often found on street lights as automatic on/off switches. They were once even used in heat-seeking AIM-9 Sidewinder#Architecture to sense for targets.

Applications Photoresistors come in many different types. Inexpensive cadmium sulphide cells can be found in many consumer items such as camera light meters, clock radios, security alarm, street lights and outdoor clocks.

They are also used in some dynamic compressors together with a small incandescent lamp or light emitting diode to control gain reduction.

Lead sulphide- and indium antimonide-LDR are used for the mid infrared spectral region. At the other end of the scale, germanium:copper photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy.

Circuit symbol Below is a symbol for a photoresistor as used in some circuit diagrams.

See also

Optoelectronics
Phototransistors react much quicker to light change. As such, they're preferred for receiving data.

External links

Using a photoresistor to track light

Photoresistor - Wikipedia, the free encyclopedia
A photoresistor or Light Dependent Resistor or CDS Cell is an electronic component whose resistance decreases with increasing incident light intensity.

Photodetector - Wikipedia, the free encyclopedia
Phototubes containing a photocathode which emits electrons when illuminated and in general behaves as a photoresistor. Phototransistors incorporating one of the above sensing methods

Photoresistor - Hutchinson encyclopedia article about Photoresistor
Component of electronic circuits whose resistance varies with the level of illumination on its surface. Usually resistance decreases as illumination increases, as the nature of the ...

Examples
Photoresistor by BARRAGAN Reads values from a photoresistor connected to the analog input pin 0. The value read from the sensor is proportional ...

How To: Connecting a Photoresistor
How To: Connecting a Photoresistor ... A photoresistor is a sensor whose resistance varies with light intensity.

Robot Room - Photoresistor Grab Bag
Testing a cadmium-sulfide photoresistor component grab bag for variety and quality revealed some shorts.

Compressor Using Photoresistor
From: gstopp@fibermux.com. Date: Mon, 31 Jul 95 15:09:10 PST. Encoding: 53 Text. Subject: Re[2]: Effects . Here's a compressor circuit from my early days of screwing around:

Photoresistor
Photoresistor ... Item contains a CdS cell photoresistor. The RoHS exemption request status for CdS cells is currently unclear; there are no RoHS-compliant exact replacement ...

photoresistor definition of photoresistor in the Free Online ...
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photoresistor - Wiktionary
Definition from Wiktionary, a free dictionary