Sensing Circuits

• Sensing circuits can be used to turn on or increase the power to components
• 1. This is a sensing circuit for a fan..
  2. The fixed resistor and the fan have the same potential difference because they are connected in parallel.
  3. The potential difference of the power supply is shared out between the thermistor and the loop made out of the fan and fixed resistor (in parallel), according to their resistances. The more resistance a component has the more potential difference it will take up.The potential difference of the power supply is shared between the thermistor and the fan (in parallel).

  
• Sensing circuit for a fan

  1. Fixed resistor and fan have same potential difference (connected in parallel)

  2. Potential difference of power supply shared between thermistor and loop of fan and fixed resistor (according to their resistances)

  3. More resistance, more potential difference taken up

  4. Potential difference of power supply shared between thermistor and fan (in parallel)

  5. As room gets hotter, thermistor resistance decreases, takes up smaller share of potential difference, potential difference across fixed resistor and fan increases, making fan go faster
• 1. You can use a variable resistor instead of a fixed resistor
  2. The potential difference between the LDR and the lamp will be high when it is dark and the resistance of the LDR will be high
  3. More potential difference = more energy
  4. Therefore, the bulb connected across the LDR will get brighter when it is darker