Exhaust fan is an important component in kitchens. Here is a simple circuit to control kitchen fans by monitoring the ambient temperature. It is built around the renowned precision integrated temperature sensor chip LM35 (IC1). Rest of the circuit is a non-traditional electromagnetic relay driver wired around the popular LED driver LM3914 (IC2). User can switch three presetted 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 works off 12V DC power supply.
Only one adjustment is required in this kitchen exhaust fan controller circuit. After construction, set jumper point in its first position, ie base terminal of T1 is connected to pin 13 of IC2 and adjust the preset P1 carefully so that relay RL1 is energised 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). More details about LM3914 and/or LM35 are available as pdf datasheet from NSC website.