A capacitor can be used in a sensor circuit to switch on a transistor after a time interval.
Such a voltage divider circuit will act as a kind of timer.
The time the voltage across the capacitor reaches 0.7 v, to switch on the transistor, can be short or long depending on the size of the capacitance of the capacitor or the size of the resistor its being charged through.
To lengthen the time interval the resistance can be made bigger or the vakue of the capacitor’s capacitance can be made bigger.
A thermistor is a resistor whose resistance changes with temperature.
When it becomes cold its resistance gets bigger. When it becomes hot its resistance becomes smaller.
Check the voltages across the voltage divider.
The mosfet acts like a switch. When the voltage at its gate becomes bigger than 2 volts it will switch on the relay which in turn switches the fan on.
Click the picture to go to the sim.
- What do you notice about the variable resistor’s voltage as its resistance increases?
- What do you notice about the sum of the voltages across the resistors?
- For each resistor in turn, take the ratio of voltage across it and divide by the resistance. What do you notice?