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Engineering LibreTexts

2.1: Theory Overview

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  • One typical LDR is the CdS (Cadmium Sulfide) cell. The resistance of the CdS cell is inversely proportional to light levels. In darkness, it may exhibit a resistance of tens or even hundreds of kilo ohms. Under high brightness, the resistance may be as little as a few hundred ohms. Thermistors come in two types: PTC or Positive Temperature Coefficient whose resistive value increases with temperature, and NTC or Negative Temperature Coefficient whose resistance decreases with increasing temperature. In contrast, ordinary resistors are designed to be immune to temperature change as much as possible.

    One way of using these devices is by placing them in a voltage divider. The resulting voltage will reflect the light levels or temperature. Depending on the position of the device, the voltage can be made to either increase or decrease as the environmental factor increases. For example, the voltage could rise as temperature rises but it could also be designed to have the voltage decrease as temperature rises. Both functions have their uses. Finally, it is worth noting that these devices do not necessarily respond immediately to environmental changes. For example, a thermistor might be used to sense air temperature. If the air temperature were to suddenly rise, there would be some time lag in the response of the thermistor. This is due to the fact that the thermistor itself has mass and requires some time to either heat up or cool down.

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