4.1 Introduction to circuit devices and how to investigate their characteristics and potential uses

Some components are designed to change resistance in response to changes in the environment e.g.

the resistance of an LDR varies with light intensity,

the resistance of a thermistor varies with temperature,

 and these properties used in sensing systems to monitor changes in the environment.

These kind of circuit components can be used to turn systems on and off, increase or decrease power to control the output depending on the ambient (surrounding) conditions.

Circuit 31 (left) is the sort of circuit you can use to test a device in terms of its current - voltage behaviour.

By varying the voltage from the power supply using the variable resistor you can readily get lots of pairs of readings of p.d. (V) and current (A).

Then use Ohm's Law equation (R = V/I) to calculate the value of the resistance of the device for any pair of p.d. and current readings.

 

To test some devices you also need to be able to vary the temperature (thermistor) or light intensity (LDR)

Practical work to help develop your skills and understanding may have included the following:

investigating potential difference/current characteristics for LDRs and thermistors,

planning and carrying out an investigation to find the relationship between the resistance of thermistors and their temperature,

investigating the change of resistance of LDRs with light intensity.