Automatic exhaust Fan circuit for Kitchen

Having a poorly ventilated kitchen might make cooking a laborious affair. Even with sufficient ventilation, we occasionally need an exhaust fan to keep the kitchen cool when cooking. However, we frequently ignore the fan when cooking because we are so busy.

Using a circuit to automate the exhaust fan in your kitchen to turn on when you begin cooking and off when you are finished is a straightforward way to solve this issue. This automatic exhaust fan circuit tracks the ambient temperature using a temperature sensor and controls the fan’s ON/OFF status based on the temperature. Examine a comparable bathroom exhaust fan circuit that activates the fan when a person uses the toilet.

Working of Automatic exhaust fan circuit :


LM50 temperature sensor:

LM50 is where this circuit’s operation begins. Temperatures between -40 and 125 degrees can be measured by this sensor. Depending on where you live, our point of interest ranges from 35 to 40 degrees. When we begin cooking, this will roughly be the temperature range in a kitchen. This temperature range is well within the range that the sensor can detect. The output of this sensor changes by 10mV for every degree of temperature variation. The output of this sensor will decrease between 0.75 and 1 volts if we operate between 35 and 40 degrees Fahrenheit. This is depicted in the graph below.


This sensor’s output is sent into the non-inverting terminal of an opamp that serves as the circuit’s comparator.


The reference voltage is provided by a 50k potentiometer attached to the LM307’s inverting terminal. This pot’s adjustment will alter the input voltage supplied to the LM307’s inverting terminal. Keep in mind that you must change the resistance of this pot so that the input voltage can range from 0.75 to 1 volts. For greater precision, you can use a 100k resistor in place of a 50k resistor. The output voltage comparator responds appropriately after comparing the pot’s input voltage with the LM50.

Set the input voltage of the pot to the desired value. Keep in mind that while you aren’t cooking, this voltage should be a little lower than the LM50 output voltage. To acquire the correct voltage, you might wish to measure the LM50 voltage at room temperature and then adjust the pot resistance accordingly. As you start cooking, the temperature rises. As a result, the output voltage of the LM50 will rise. The output of the LM307 goes high and the transistor is activated when this voltage is higher than the input voltage from the pot.

Transistor and Relay:

The transistor is turned on by the LM307’s output. This enables current to pass through the collector, turning on the relay. This relay turns on the fan and completes the circuit. The relay in question, model number SL12VDCSLA, runs on 12 volts and can handle 20 amps of load. Since the exhaust fan often uses less than 2A, this relay is a good option. If in doubt, verify the maximum ratings for your exhaust fan and choose your relay accordingly. Another thing to keep in mind is to make sure the coil current of the relay you choose does not exceed the collector current of the transistor you employ.

By doing this, the Fan will come ON automatically when you start cooking or when the temperature rises. Up till the temperature lowers, it stays on.


Calibration is highly critical to this circuit. To determine the preset voltage you need to set using a pot resistor, and measure the LM50 output voltage both before and during cooking.

Pay attention to the exhaust fan current when selecting the relay, and similarly when selecting the transistor that will turn on the relay.


I hope all of you have understood how to design an Automatic exhaust Fan circuit for Kitchen. We MATHA ELECTRONICS will be back soon with more informative blogs soon.

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