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Thermistors
It contains semiconducting metallic oxides whose resistance decreases markedly when the temperature rises either due to heating the thermistor directly or to passing the current through it.
One is shown in the fig. below, with its symbol and the circuit to demonstrate its action.
Heating the thermistor with match lights the bulb. A thermistor in series with a meter marked in oC can measure temperature in, for example, the range -5 to 70 oC.
Light dependent resistor – (L.D.R.)
Its action depends upon the fact that the resistance of certain semiconductors such as cadmium sulphide , decreases as the intensity of light falling on them increases. For example, it might be several mega ohms in the dark but only a few kilo ohms in the daylight.
The symbol of LDR is shown below.
LDRs can be used in light meters and automatic switches, which turn a light on when it gets dark.
LDRs and thermistors are often reffered to as sensors. They sense a change in the light level or the temperature and convert it into electric information.
Input transducers
A transducer is a device that transfers energy from one form to another. An input transducer may be needed to transfer, for example, heat or light energy to eletrical energy. Many machines have to take action if exposed to light or heat or some other input. The controls of the boiler will light the flame if teh water is too cold. The computer controlling the house will close the curtains when it gets dark. The LDR and thermistors are suitable eletrical inducers for giving an electrical signal for this purpose.
LDR and thermistor are best used as part of a potentiometer.
Light sensitive switch
Consider the following circuit.
The relay is not energized so long as the resistance of the LDR stays low. If the illumination reduces, the resistance of LDR increases and the potential difference across the coil goes up. When the illumination is sufficiently low, the relay will close the contact.
Note that the circuit controlled by the relay does not need to have any connection whatsoever to the relay coil circuit. This can be an important safety features of the circuit.
Temperature operated alarms
Consider the following circuit.
In this circuit, the transistor will start to conduct electricity, and the bell will sound if the potential difference accross resistor R2 goes to more than 0.6 V. The value of R2 must be set to make this occur at the correct temperature. If R2 was made a variable resistor , then the temperature of the alaram could be adjusted.