Temperature-Controlled DC Fan Using Thermistor & Op-Amp (LM741)
1. Project Description
This project implements a temperature-responsive fan control circuit that adjusts the speed of a DC fan according to ambient temperature. A thermistor senses temperature changes and drives an operational amplifier (LM741) to regulate the fan’s supply voltage. The system ensures automatic cooling without manual intervention.
2. Objectives
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To design and build a circuit that automatically controls fan speed based on ambient temperature.
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To apply analog electronic principles (thermistors, amplifiers, feedback).
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To enhance understanding of sensor interfacing, voltage control, and analog signal amplification.
3. Components
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NTC Thermistor - 10k
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Operational amplifier: LM741
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NPN Transistor MJE3055
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Potentiometer – 10kΩ
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Resistors - 47Ω, 4.7kΩ
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DC Fan motor (compatible voltage)
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Power supply (e.g. 5V, 9 V or 12 V, depending on fan)
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Connecting wires and breadboard
4. Working Principle
1. Temperature Sensing via Thermistor & Voltage Divider
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The thermistor is typically of NTC type (resistance decreases when temperature increases).
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It is arranged in a voltage divider with a fixed resistor. The junction voltage between thermistor and resistor becomes a temperature-dependent signal.
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A 10 k potentiometer is included in the divider network (or connected to the non-inverting input) to act as an adjustable reference, allowing you to set the threshold temperature.
2. Comparison & Amplification using LM741 (Op-Amp)
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The LM741 is configured as a comparator / differential amplifier.
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The non-inverting (+) input (pin 3) receives the voltage from the potentiometer/thermistor network (variable signal).
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The inverting (–) input (pin 2) receives a fixed reference voltage (set by resistors).
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When the voltage at the non-inverting side exceeds the reference, the output of LM741 swings high. If lower, output is low.
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The comparator’s output then controls the transistor.
3. Switching via NPN Transistor MJE3055
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The MJE3055 NPN transistor acts as a switch and current amplifier.
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Its base is driven by the output of the LM741 (through a current-limiting resistor).
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When LM741 output is high, it provides base current to MJE3055, turning it ON.
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When ON, current flows from collector to emitter, powering the fan motor. When OFF, the fan is disconnected.
4. Fan Activation & Control
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The fan motor is connected between the supply rail and the collector of the MJE3055 (or via a driver path).
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When transistor is ON, the fan receives voltage and spins; when OFF, no current flows.
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Because the comparator threshold is adjustable (via potentiometer), the fan turns on only when ambient temperature exceeds the preset level.
Circuit Diagram
Circuit diagram of the temperature-controlled DC fan using thermistor, LM741, potentiometer, and MJE3055 transistor.
5. Hardware & Results
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The hardware is assembled on a breadboard (or PCB) with thermistor, LM741, resistors, and fan.
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As ambient temperature rises, the thermistor’s resistance resistance decreases, triggering the amplifier to increase the fan’s speed (or turn it on).
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In tests, the fan adjusted speed smoothly as temperature increased in a gradual manner, demonstrating automatic control.
Hardware of temperature-controlled DC fan assembled on a breadboard.
6. Applications
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Smart cooling systems in electronics, CPUs, or power supplies.
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Climate-sensitive control in small enclosures or devices.
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Automatic ventilation or exhaust systems where fan activation depends on temperature.
7. Conclusion
The Temperature-Controlled DC Fan project successfully demonstrates analog sensing and control by integrating a thermistor sensor and LM741 operational amplifier to regulate fan speed according to ambient temperature. This project demonstrates a practical application of temperature-dependent circuitry and dynamic voltage modulation, which is useful in various electrical and thermal management systems.