Types of Photocells

Photovoltaic1. A photovoltaic cell converts solar energy into electrical power. Photons knock electrons on the cell itself into a higher state of

energy, causing a usable current.

Charge-Coupled Devices2. Charge-coupled devices are used by the scientific community as an extremely reliable and accurate photosensor. Charges

produced by photosensitive sensors are used to analyze a range of things from galaxies to single molecules.

Photoresistor3. Light-dependent resistors are devices whose resistivity to electrical currents decreases with the amount of light they're

exposed to. Many alarms and camera light meters use cheap photoresistors for their applications.

Golay Cell4. Golay cells are used to detect infrared radiation. A tube with a blackened metal plate on one end is filled with xenon gas.

Infrared energy that falls on the blackened plate heats up the gas and distorts the flexible diaphragm on the other end, the motion of which is used to determine the output of the energy source.

Photomultiplier5. Photomultipliers are extremely sensitive detectors. The faintest wave of light is multiplied by as much as 100 million times.

A photoresistor or light dependent resistor or cadmium sulfide (CdS) cell is a resistor whose resistance decreases with increasing incident light intensity. It can also be referenced as a photoconductor.

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 hole partner) conduct electricity, thereby lowering resistance.

A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, e.g. 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 (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.

Applications

Photoresistors come in many different types. Inexpensive cadmium sulfide cells can be found in many consumer items such as camera light meters, street lights, clock radios, alarms, 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 sulfide 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.

Circuit symbol

Introduction To Light Dependent Resistor (LDR)

In the dark, the resistance of the LDR is very high, typically around 1M ohm. In bright light it is low, typically 1K ohm. An example of the peak spectral response of the LDR (VT936G from EG&G) is 550nm. The continuous power dissipation is 80mW and the maximum voltage which can be applied to it is 100V.

The snake like track on the face of the LDR is a cadmium sulphide (CdS) film. On each side is a metal film which is connected to the terminal leads. Use a multimeter to measure its resistance when light is shine on it and when it is placed in a dark place. This will help to enhance your knowledge of the concept as to how the LDR works

What is a photocell?

Photocells are sensors that allow you to detect light. They are small, inexpensive, low-power, easy to use and don't wear out. For that reason they often appear in toys, gadgets and appliances. Theys are are often refered to a CdS cells (they are made of Cadmium-Sulfide), light-dependent resistors (LDR), and photoresistors.

Photocells are basically a resistor that changes its resistive value (in ohms Ω) depending on how much light is shining onto the squiggly face. They are very low cost, easy to get in many sizes and specifications, but are very innacurate. Each photocell sensor will

act a little differently than the other, even if they are from the same batch. The variations can be really large, 50% or higher! For this reason, they shouldn't be used to try to determine precise light levels in lux or millicandela. Instead, you can expect to only be able to determine basic light changes

For most light-sentsitive applications like "is it light or dark out", "is there something in front of the sensor (that would block light)", "is there something interrupting a laser beam" (break-beam sensors), or "which of multiple sensors has the most light hitting it", photocells can be a good choice!

Some basic statsHow to measure light using a photocell

As we've said, a photocell's resistance changes as the face is exposed to more light. When its dark, the sensor looks like an large resistor up to 10MΩ, as the light level increases, the resistance goes down. This graph indicates approximately the resistance of the sensor at different light levels. Remember each photocell will be a little different so use this as a guide only!

Note that the graph is not linear, its a log-log graph!

Photocells, particularly the common CdS cells that you're likely to find, are not sensitive to all light. In particular they tend to be sensitive to light between 700nm (red) and 500nm (green) light.

What the heck is lux?

Most datasheets use lux to indicate the resistance at certain light levels. But what is lux? Its not a method we tend to use to describe brightness so its tough to gauge. Here is a table adapted from a Wikipedia article on the topic!

Illuminance Example

0.002 lux Moonless clear night sky

0.2 lux Design minimum for emergency lighting (AS2293).

0.27 - 1 lux Full moon on a clear night

3.4 lux Dark limit of civil twilight under a clear sky

50 lux Family living room

80 lux Hallway/toilet

100 lux Very dark overcast day

300 - 500 lux Sunrise or sunset on a clear day. Well-lit office area.

1,000 lux Overcast day; typical TV studio lighting

10,000–25,000 lux Full daylight (not direct sun)

32,000–130,000 lux Direct sunlight