Potential divider circuits with thermistors and LDRs

You could replace the variable resistor in the circuit above with a thermistor or light dependent resistor (LDR), in order to form a temperature or light sensor.

A light dependent resistor’s resistance decreases as light intensity increases.

These types of sensors can be used to trigger certain events, for example in the circuit above, a light dependent resistor is used. If the light intensity falls, resistance across R₁ will increase. This will cause the total circuit resistance to increase and so the circuit current will decrease. Using Ohm’s law (V = IR), you can see that this means that the voltage across R2 decreases, so the p.d out decreases. If you want this effect to be reversed, you can switch the position of the LDR and resistor, meaning that the p.d out would increase as light intensity decreases and so this circuit could be used to cause a light bulb to be switched on, once a certain threshold voltage has been met.