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This circuit is very simple, the reference voltage is given by the two resistors wired as a divider bridge (on the "+" of the AOP).
On the "-" you can see an other divider bridge and the voltage between the resistor and the potentiometer is given by the formula : Vo=Vi[R2/(R1+R2)] (As for the first bridge).
All of this for one thing : the AOP have the particularity that when the voltage between "+" and GND is up to the voltage between "-" and GND, the out of the AOP is on, whereas when the voltage between "+" and GND is under the voltage between "-" and GND, the out is off.
So, the potentiometer can be replaced by an other variable resistor component (thermoresistor, photoresistor..) so at a certain resistance of your component, you can turn on and off what you want (not me thank you).
Then, the diode connected to the coil of the relay helps to delete peaks of current caused by the switching transistor.
This circuit can be used in many ways, so, let your imagination do the rest!
Hope this text helpt you, thank you reading all of this and don't hesite to comment!
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