The circuit shown on the left combines a potential divider, circuit using a thermistor, with a transistor circuit featuring a buzzer.

The graph shows how the resistance of the thermistor varies with temperature.

The transistor allows current to flow fully from collector to emitter when the voltage at X reaches 0.7V.

1. What value must the resistor R be if the circuit sounds an alarm when the temperature reaches 25 degrees?

2. What would the colour code of that resistor be to give the closest match?

3. What value must the resistor R be if the circuit sounds an alarm when the temperature reaches 75 degrees?

4. What would the colour code of that resistor be to give the closest match?

5. What component could you use in place of the resistor which could be used to cater for both temperatures?

6. If you wanted the circuit to switch on a mains powered cooling fan instead of a buzzer, what component would you use and why?

7. Redraw the circuit diagram to show the component.