Photoresistor

Source: Wikipedia, the free encyclopedia.
Not to be confused with Photoresist.
Photoresistor
TypePassive
Working principle‍Photoconductivity
Electronic symbol

The symbol for a photoresistor

A photoresistor (also known as a light-dependent resistor, LDR, or photo-conductive cell) is a passive component that decreases in resistance as a result of increasing luminosity (light) on its sensitive surface, in other words, it exhibits

resistance
. The resistance range and sensitivity of a photoresistor can substantially differ among dissimilar devices. Moreover, unique photoresistors may react substantially differently to photons within certain wavelength bands.

A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own

dopants, added whose ground state energy is closer to the conduction band; since the electrons do not have as far to jump, lower energy photons (that is, 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.[1]

Design considerations

Three photoresistors with scale in mm
Large CdS photocell from a street light

A photoresistor is less light-sensitive than a

phototransistor. The latter two components are true semiconductor devices, while a photoresistor is a passive component that does not have a PN-junction
. The photoresistivity of any photoresistor may vary widely depending on ambient temperature, making them unsuitable for applications requiring precise measurement of or sensitivity to light photons.

Photoresistors also exhibit a certain degree of latency between exposure to light and the subsequent decrease in resistance, usually around 10 milliseconds. The lag time when going from lit to dark environments is even greater, often as long as one second. This property makes them unsuitable for sensing rapidly flashing lights, but is sometimes used to smooth the response of audio signal compression.[2]

Applications

streetlight
. 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.

Photoresistors come in many types. Inexpensive

alarm devices (as the detector for a light beam), nightlights
, outdoor clocks, solar street lamps, and solar road studs, etc.

Photoresistors can be placed in streetlights to control when the light is on. Ambient light falling on the photoresistor causes the streetlight to turn off. Thus energy is saved by ensuring the light is only on during hours of darkness.

Photoresistors are also used in laser-based security systems to detect the change in the light intensity when a person or object passes through the laser beam.

They are also used in some

guitar amplifiers that incorporate an onboard tremolo
effect, as the oscillating light patterns control the level of signal running through the amplifier circuit.

The use of CdS and CdSe[3] photoresistors is severely restricted in Europe due to the RoHS ban on cadmium.

Lead sulfide (PbS) and indium antimonide (InSb) LDRs (light-dependent resistors) are used for the mid-infrared spectral region. Ge:Cu photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy.

See also

References

  1. ISBN 978-1401835149
    .
  2. ^ "Photo resistor - Light Dependent Resistor (LDR) » Resistor Guide". resistorguide.com. Retrieved 19 April 2018.
  3. ^ "Silonex: TO-18 photocells on ceramic substrate" (PDF). Archived from the original (PDF) on 1 April 2013. Retrieved 17 October 2013.

External links

Wikimedia Commons has media related to Photoresistors.
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