A small fan that switches itself on when your desk gets warmer than the temperature you pick, and spins faster the hotter it gets. A screen shows the room temperature, your target, and how hard the fan is working.
Building with an Elegoo kit instead? It works with two part swaps. See the Elegoo changes.
The Uno is the brain. Two things feed it information, and it controls two things. Everything else on the breadboard is there to protect the parts or keep power steady.
The Uno reads the sensor 16 times and averages them to smooth out noise. The TMP36 gives 0.5 V at 0 °C plus 0.01 V per degree, so a 75 °F room reads about 0.74 V.
Fully left is 64 °F, fully right is 90 °F, snapped to half-degree steps (in °C) so the number doesn't jitter.
The fan turns on a little above your target and off a little below it. That gap stops it from clicking on and off when the room sits right at the target.
Pin 9 switches the MOSFET on and off about 490 times a second. More "on" time means a faster fan. From a stop it gets a 0.2 s full-power kick so the motor actually starts.
Legs are drawn in the direction they'll point into the breadboard. The coloured dot under each leg matches the wire colours on the build diagram further down.
Feels hot? It's in backwards. Unplug USB right away and turn it around.
1 VSS→GND · 2 VDD→5V · 3 V0→contrast · 4 RS→12 · 5 RW→GND · 6 E→11 · 11–14 D4–D7→5,4,3,2 · 15 A→220Ω→5V · 16 K→GNDStripe must face 5V. Backwards, it shorts 5V straight to GND every time the fan turns on.
Polarity matters. Backwards, this kind of capacitor can swell or pop.
Click a step (or just scroll through them) and the diagram shows only what's in place so far, with the new parts glowing. Switch between the Breadboard view (where every leg and wire physically goes) and the Wiring map (what connects to what). Hover or tap any wire on the breadboard to see its two ends.
Holes are named by row letter and column number, like c4. Each column of five holes is joined inside the board: a4 to e4 are one connection, and f4 to j4 are another. The gap down the middle separates them.
The two long rails along the far edge run the whole length of the board. Blue line = − rail (GND), red line = + rail (5V). This layout fits a 30-column half-size board. On a bigger board, use the same hole names counting from the left end.
Hover or tap a wire or part to see exactly where it goes.
This is a wiring map, not a photo of the breadboard: every line is one jumper or leg. Black dots are shared breadboard rows.
Wire unplugged. Most mistakes are harmless if nothing is powered while you fix them.
5V pin to the breadboard's red + rail.GND pin to the blue − rail.A0, right leg → GND rail.If it ever feels hot, it's in backwards. Unplug USB and turn it around.
A1. This knob picks the target, about 64 °F to 90 °F.a, columns 1–16. The screen hangs off the near edge of the board, so slide something the same height under it (a book, the kit box lid) to keep it level. If it has no header pins, they need soldering on first.12, E (pin 6) → 11, D4 → 5, D5 → 4, D6 → 3, D7 → 2. Leave pins 7–10 empty.desk_fan.Now and Set lines. Blank or solid blocks? Turn the contrast knob slowly.temp=24.6,set=23.0,fan=0 appears each second (those are °C).Check: the LCD shows the room temperature, and turning the target knob changes the Set line.
~9 → 220Ω resistor → a free row (the gate row). Put the MOSFET's gate leg in that row.Diode stripe must face 5V. Backwards, it shorts 5V to GND whenever the fan switches on.
Check: the fan starts, speeds up as it warms, and stops when the target is above the room.
The exact sketch used in the build. LiquidCrystal comes with the Arduino IDE, so there's nothing extra to install.
SHOW_FAHRENHEITSet to false to show °C on the screen.MIN_PWM = 100Slowest speed that keeps your motor turning. If it hums but won't spin, raise it by 10–20 and upload again.FULL_SPEED_C = 4.0How many °C over target reaches full speed (4 °C = 7.2 °F). Smaller makes it ramp up faster.SET_MIN_C / SET_MAX_CThe knob's range, 18–32 °C (64–90 °F).The Elegoo UNO R3 board is a copy of the Arduino Uno with the same pins, so the sketch and most of the wiring stay the same. Elegoo's Super Starter and Most Complete kits don't include the TMP36 or the IRF520, so two parts get swapped. Build the circuit with the steps above, then make the changes below.
Check the transistor's print. Most PN2222s read emitter, base, collector with the flat face toward you, but some makers swap the order. If the fan never turns on, compare the legs with the picture in Elegoo's tutorial.
Replace the readTempC() function in the sketch with this one. Everything else stays the same. In the Arduino IDE, still pick Tools › Board › Arduino Uno.
float readTempC() {
long total = 0;
for (int i = 0; i < 16; i++) total += analogRead(TEMP_PIN); // average out noise
float raw = total / 16.0;
float r = 10000.0 * raw / (1023.0 - raw); // thermistor resistance in ohms
float kelvin = 1.0 / (1.0 / 298.15 + log(r / 10000.0) / 3950.0);
return kelvin - 273.15 + 0.0; // Kelvin to degrees C, plus your correction
}
Thermistors vary a little from part to part, so compare the screen with a thermometer after a few minutes. If it reads high, change the 0.0 on the last line to a negative number; if it reads low, make it positive. The correction is in °C, and 1 °C is 1.8 °F, so a screen that reads 2.7 °F high needs -1.5.
Why not the DHT11? Elegoo kits also include a DHT11 temperature and humidity module. It only reports whole degrees Celsius, which is too coarse for a fan that switches half a degree either side of the target, so the thermistor is the better fit.
Turn the contrast knob slowly end to end. Still blank: check LCD pins 1, 2 and the backlight (15, 16).
The middle TMP36 leg isn't reaching A0, or the sensor is reversed. Check it's the TMP36, not a similar-looking transistor.
Raise MIN_PWM. Also check the blade isn't rubbing.
Turn the target knob fully left. Then check pin 9 → 220Ω → gate, source to GND, and the motor between 5V and drain.
Drain and source are probably swapped, or the gate row touches 5V. Unplug and re-seat the MOSFET, printed side toward you.
USB power is dipping. Make sure the 100µF capacitor is in and the right way round, or power the motor from a 9V battery.
Swap the knob's two outer legs.
Swap the two motor wires.