Thermistor Controlled Fan

Final project for E84 Circuits and Magnetic Devices at Harvey Mudd College (CA). A thermistor-controlled DC motor that serves as a fan!

Topics: Thermistor, Op-amp, buffer, transistor, dc motor

The materials utilised in this design are as follows:

1. Mini DC motor (Topoox DC 1.5-3V, 15000 RPM) x1
2. RC Drone Propellor (or similar)
3. NTC Thermistor (Strand type) x1
4. Op-Amps (MCP6004) x1
5. NPN Transistor (TIP41A) x1
6. 100 kOhm Resistor x1
7. 90 kOhm Resistor x3
8. 10 kOhm Resistor x
9. 50 Ohm Resistor x1
10. Wires as needed

Additionally, a DC power supply that can power the op-amp circuit serves as a reference voltage, and power the DC motor is needed. The DC motor requires high current so make sure to be careful when selecting your source. More information on this later.

This setup is broken down into 4 parts:

1. [Inputs] Wheatstone Bridge and Thermistor
2. [Filters] Noninverting Amplifier and Buffer
3. [Staging] NPN Transistor
4. [Output] DC Motor and Fan

To ensure that the resolution of the output is as fine as possible, a Wheatstone bridge setup will be employed. The thermistor will be in the bottom right of the bridge with the other resistor values being the same at 90 kOhm. This provides a wide range of outputs from -0.3V to 0.3V across the middle terminals. While the op amp in the next stage does not read the negative inputs, the resolution is great enough even with the positive values. A place of improvement could be offsetting this voltage.

The thermistor, NTC string type, has a room temperature resistance of around 47 kOhm, and in order to attach it to the breadboard, two leads were soldered on. Make sure to keep the thermistor in an accessible place for touching it will be your trigger for the DC fan. The left terminal of the Wheatstone bridge will be connected to the noninverting terminal of the amplifier while the negative terminal will be connected to the inverting terminal in the next step.

In this step, we will add the op-amp setup. The MCP6002 provides two op amps that can both be leveraged in this design. The first stage will be a noninverting buffer with resistor values of 100 kOhm and 10 kOhm to provide a gain of 11. The wiring diagram can be seen in step 1 in the overall circuit schematic.

The output of the amplifier will be fed into the non-inverting terminal of the second op-amp in the IC. This buffer setup will smooth out the signal and provide a more consistent output. No additional resistors are needed here, but one, 50 Ohm, resistor will be placed in between the output of the buffer and the base of the transistor.

The op-amp circuit does not provide enough current to power the DC motor so an NPN transistor will be utilized. It is important to select a transistor that is rated to your DC motor. Once selected, a 5V source can be connected to the collector while the DC motor is connected to the emitter. The output of the op-amp circuit will be connected via the 50 Ohm resistor noted before and will have an alternating current depending on the thermistor input. This altering input will control the flow of current in the transistor and either speed up the fan or slow it down. Lastly, it is important to select an NPN transistor to ensure correct operation.

This is the final step with the addition of the DC motor. The positive terminal will be connected to the emitter of the transistor and the negative terminal will be grounded. An RC airplane's rotor blades have been attached via hot glue to serve as fan blades. A pair of helping hands are holding the motor up. The motor can spin relatively fast so it is important to both secure the blades and the motor itself during operation. Always be cautious with high speed, high current parts.

When all are connected, turn on the power supply and see the results! Touching the thermistor is the easiest way to control the voltage. The oscilloscope output can be seen from the noninverting op-amp amplifier, and its response to a changing temperature is clearly seen. The video demonstrates the full functionality of the circuit.