LDRs : LDRs Slide 2: LDR Photoresistor or light dependent resistor or cadmium sulfide (CdS) cell and sometimes referrenced as photoconductor .
Is a type of photocell specifically a resistor whose resistance decreases with increasing incident light intensity. It can also be referenced as a photoconductor. Slide 3: LDR Circuit Symbols: 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. Slide 4: LDR Advantages:
small, inexpensive, low-power
easy to use and don't wear out
easy to get in many sizes and specifications
batch variations can be really large, 50% or higher Slide 5: LDR Construction Slide 6: LDR 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.
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. Slide 7: In the dark, the resistance of the LDR is very high, typically around 1M ohm. In bright light it is low, typically 1K ohm. When its dark, the sensor looks like an large resistor up to 10MΩ, as the light level increases, the resistance goes down. LDR Slide 8: LDR Slide 9: 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. LDR Slide 10: Application LDR