This circuit helps you to build simple Exhaust Fan Controller in your kitchen using temperature IC LM35 and LED driver LM3914. It will monitor the ambient temperature and turn on Exhaust fan when the temperature above the preset temperature levels. User can switch three preset temperature levels using a jumper/slide switch (JP1), which determines the heat level to activate the relay and hence the electric exhaust fan wired through the relay contacts. It needs 12V DC power supply to work.
Only one adjustment is required in this kitchen exhaust fan controller circuit. After construction, set jumper point in its first position, i.e. base terminal of T1 is connected to pin 13 of IC2 and adjust the preset P1 carefully so that relay RL1 is energized when ambient temperature level reaches near 29oC. However this is not very critical as you can select any threshold level by connecting the jumper points to other unused output pins of IC2 (here only 3 outputs are used).