A photoresistor or light-dependent resistor or photocell is a light-controlled variable
resistor.
The resistance of a photoresistor decreases with increasing incident light intensity;
in other words, it exhibits photoconductivity.
A photoresistor can be applied in light-sensitive detector circuits, and light- and dark-activated
switching circuits.
A photoresistor is made of a high resistance semiconductor.
In the dark, a photoresistor can have a resistance as high as a few megaohms, while in the light,
a photoresistor can have a resistance as low as a few hundred ohms.
If incident light on a photoresistor exceeds a certain frequency, photons absorbed by the
semiconductor give bound electrons enough energy to jump into the conduction band.
The resulting free electrons conduct electricity, thereby lowering 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 charge carriers and is not an efficient semiconductor,
for example, 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, also called 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
are sufficient to trigger the device.
If a sample of silicon has some of its atoms replaced by phosphorus atoms, there will be
extra electrons available for conduction.
This is an example of an extrinsic semiconductor.
Design considerations
Photoresistors are less light-sensitive devices than photodiodes or phototransistors: the
two latter components are true semiconductor devices, while a photoresistor is passive
component and 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.
Photoresistors also exhibit a certain degree of latency between the moment they are hit
by light and the subsequent moment that their resistance actually drops.
This drop is usually about 10 milliseconds.
Also, they exhibit a much longer latency when going from lighted to dark environments, often
of as long as one second.
This property makes them unsuitable for sensing rapidly flashing lights.
Applications
Photoresistors come in many types.
Inexpensive cadmium sulphide cells can be found in many consumer items such as camera
light meters, street lights, clock radios, alarm devices, night lights, outdoor clocks,
solar street lamps and solar road studs, etc.
They are also used in some dynamic compressors together with a small incandescent or neon
lamp, or light-emitting diode to control gain reduction.
A common usage of this application can be found in many guitar amplifiers that incorporate
an onboard tremolo effect, as the oscillating light patterns control the level of signal
running through the amp circuit.
The use of CdS and CdSe photoresistors is severely restricted in Europe due to the RoHS
ban on cadmium.
Lead sulphide and indium antimonide LDRs 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.
References
See also Optoelectronics
Photodiode Phototransistor
External links Using a photoresistor to measure nocturnal
light levels - The Moonlight Sensor Using a photoresistor to track light
Connecting a photoresistor to a circuit Photoresistor overview - detailing operation,
structure and circuit information