Photoconductive. It is the one that changes its electrical resistance due to exposure to radiant energy.


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  • 1 Light resistance
  • 2 Photogeneration of carriers
  • 3 Driving state without photogeneration
  • 4 Variation of resistance as a function of radiation wavelength
  • 5 Applications
    • 1 Application example
  • 6 Sources


An example is the LDR (Light Dependent Resistor) or light dependent resistance , photoconductors or photoconductive cells ; As its name suggests, it is a resistance that varies its value depending on the light that falls on its surface. The more the light intensity that hits the LDR surface, the lower its resistance and the less light it hits, the greater the resistance. The external shape may vary from that shown in this photo since this specific model is not very common but the function is the same. When light falls on the photoconductive material, electron-hole pairs are generated. As there are a greater number of carriers, the value of the resistance decreases. Thus, the illuminated photoresistor has a resistance value low.

Carriers photogeneration

If it is no longer illuminated, the photogenerated carriers will recombine until they return to their initial values. Therefore the number of carriers will decrease and the value of the resistance will be higher.

Driving state without photogeneration

The photoresist material will respond to certain wavelengths. That is, the resistance variation will be maximum for a given wavelength . This wavelength depends on the material and the doping, and must be supplied by the supplier. In general, the resistance variation as a function of the wavelength present curves.

Resistance variation as a function of radiation wavelength

The most used material as a sensor is CdS, although Silicon, GaAsP and GaP can also be used.


LDRs are used to detect ambient light levels or to track lights or flashlights, so you can create a light tracker with several LDRs arranged around the robot and make it follow direct light to focus on it, they can also be used to turn on the spotlights or beacon lights of the robot in the absence of light.

Application example

To make an ambient light meter or a photocell that turns on a certain process in the absence or presence of light, it can be done in two ways, using an operational amplifier to detect and adjust the sensitivity and point at which the output is triggered as in the case of the CNY70.

The circuit consists of a voltage divider formed by the LDR, a resistor and a model 74LS14 inverter schmitt trigger trigger. As the LDR varies depending on the light , the output signal of the splitter will also do so and when it passes the trigger threshold of the schmitt trigger it will change the state of its output accordingly.


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