P3.2.7 - Sensing circuits

Question 1

What is a sensor circuit?

Answer 1

A circuit which turns on a component on/off automatically, in response to external stimuli

Question 2

How are thermistors used in thermostats?

Answer 2

They are used in sensor circuits in thermostats to turn central heating systems on/off at desired temperature

Question 3

How are LDRs used for street lamps?

Answer 3

LDRs are used in sensor circuits for street lamps to turn the lights on/off with changing light levels

Question 4

How does a sensor circuit work?

Answer 4

• A fixed resistor R1 is connected in series with a thermistor of resistance R2
• The p.d. of the battery is shared out amongst the fixed resistor and thermistor
• The thermistor is connected in parallel with a component (e.g. heater), the p.d. across both is V out.
• As the temperature in house decreases, resistance of thermistor R2 increases
• The p.d. dropped across the thermistor, V out, increases.
• Once V out increases to a high enough level, the ‘’working potential’, of the heater is reached and it switches on

Question 5

Suggest and explain why you need to choose a ‘suitable’ resistance for the fixed resistor.

Answer 5

If it is the value of the resistance is too large/small compared to the resistance of the thermistor/LDR, changes in thermistor/LDR resistance will not vary up the p.d. across them significantly, and therefore any device connected in parallel with them will either remain on the entire time, or off