The Project
You know that moment when you're stirring a cup of coffee or a beaker of solution and you think, "There has to be a better way"? Well, there is. A magnetic stirrer is one of those devices that feels like magic the first time you see it work: a tiny magnet spinning inside your liquid, driven by a rotating magnetic field from below, no moving parts touching the liquid. It's a staple in chemistry labs for mixing reactions, but it's also incredibly useful for home brewers, coffee enthusiasts, and anyone who needs hands-free, consistent stirring.
The problem is that commercial magnetic stirrers cost anywhere from $50 to $300, and most of them are overkill for home use. But here's the good news: you can build a perfectly functional magnetic stirrer for under $20 using parts you probably already have lying around. This project is perfect for a weekend afternoon, and it teaches you the fundamentals of magnetic coupling, variable speed control, and basic electronics. By the end, you'll have a device that can stir your morning coffee, mix epoxy, or even agitate a small chemical reaction.
Why is this trending now? Because the maker community has rediscovered the joy of repurposing old computer parts. With the rise of home labs, DIY chemistry, and coffee culture, people want affordable, customizable tools. Plus, it's a gateway project — once you build a stirrer, you'll start thinking about what else you can motorize. It's satisfying, it's practical, and it's a great conversation starter.
What You'll Need
Let's get specific. You'll need a few key components, and I'll tell you exactly where to source them and what alternatives work.
**Materials:**
- **A small DC fan** (40mm to 80mm, 5V to 12V) — scavenge from an old computer power supply or buy one for $3 on Amazon. The fan's motor is the heart of the stirrer. Look for a brushless fan (common in PCs) because it's quieter and lasts longer.
- **Rare earth neodymium magnets** — two small disc magnets, 10mm diameter by 3mm thick. These are critical. Get N52 grade if you can; they're stronger. A pack of 10 costs about $7. Avoid cheap ceramic magnets — they won't couple well through a cup bottom.
- **A small stir bar** — a Teflon-coated magnetic stir bar, typically 10-20mm long. You can buy a pack of 5 for $5 on eBay. Or improvise with a small steel bolt wrapped in Teflon tape (not as durable, but works in a pinch).
- **A potentiometer (10k ohm)** — for speed control. A simple rotary knob. Cost: $1.
- **A small enclosure** — a plastic project box or even a sturdy cardboard box lined with foil. I prefer a 3D-printed case, but a $2 plastic box from a hobby store works fine.
- **Wires, solder, heat shrink tubing** — standard electronics fare.
- **A 5V power supply** — an old phone charger or USB cable. Most fans run on 5V, but check your fan's rating. If it's 12V, use a 12V wall wart.
**Tools:**
- Soldering iron (25W or higher) and solder.
- Wire strippers.
- Hot glue gun.
- Drill with a 1/4" bit (for the potentiometer hole).
- Multimeter (optional but recommended for testing).
**Budget:**
- Fan: $3 (or free if salvaged)
- Magnets: $7
- Stir bar: $5
- Potentiometer: $1
- Enclosure: $2
- Power supply: $2 (if not salvaged)
- Total: around $20. You can't beat that.
Step-by-Step
**Step 1: Disassemble the fan.** Remove the fan blade from the motor hub. Most fans have a small sticker on the back covering a retaining ring or a snap ring. Pry it off carefully with a small screwdriver. The blade assembly usually pulls straight off. Keep the motor and the blade assembly — you'll reuse the blade as a magnet holder.
**Step 2: Attach the magnets.** Take the fan blade (the plastic hub that held the blades) and glue two neodymium magnets onto it, opposite each other, with opposite poles facing up. How do you know which pole is which? If you have a compass, the north pole of the magnet points to the north pole of the compass. Or just stick one magnet to the other — they'll attract if opposite poles face each other. You want the two magnets to attract each other across the center, so they should be opposite poles facing outward. Hot glue works fine, but epoxy is better for long-term use. Make sure the magnets are centered and balanced — an unbalanced rotor will vibrate like crazy.
**Step 3: Wire the potentiometer.** Solder the potentiometer in series with the fan's positive wire. The typical circuit: power supply positive -> potentiometer input -> potentiometer output -> fan positive. Fan negative to power supply negative. That's it. If you want an LED indicator, add a resistor and LED in parallel across the power. The trick most pros won't tell you: use a 10k ohm logarithmic potentiometer for smoother speed control at low RPMs. Linear pots make fine adjustments tricky.
**Step 4: Mount everything in the enclosure.** Drill a hole for the potentiometer shaft. Mount the fan motor inside the enclosure so the magnet hub is near the top surface. You want the magnets to be as close to the top of the enclosure as possible — ideally less than 5mm. Use hot glue or standoffs to secure the motor. The enclosure top should be thin plastic or wood; metal will block the magnetic field. If you must use metal, cut a hole and cover it with a thin plastic sheet.
**Step 5: Test and calibrate.** Place your stir bar in a cup of water (or coffee) and set it on top of the enclosure, centered over the magnets. Turn the potentiometer slowly. The stir bar should start spinning. If it doesn't, check magnet polarity — you might have them repelling instead of attracting. Also ensure the gap between the magnets and the cup bottom is minimal. Adjust speed until you get a nice vortex without splashing.
Safety First
This is a low-voltage project, so electrocution risk is minimal. But there are still hazards. First, neodymium magnets are incredibly strong and brittle. If you let two snap together, they can shatter and send sharp fragments flying. Always handle them carefully, and keep them away from pacemakers, credit cards, and magnetic storage media. Second, soldering irons get hot — 400°C hot. Use a stand, and don't touch the tip. Third, if you're using a 12V fan with a 12V power supply, the motor can get warm during extended use. Ensure ventilation in your enclosure. Finally, never immerse the electronics in liquid. The stirrer is for use under a cup, not inside it. Keep liquids away from the enclosure.
What NOT to do: don't use a fan with a plastic hub that can't hold magnets securely. Don't skip the potentiometer — running the fan at full speed is noisy and unnecessary. Don't use a metal enclosure that will block the magnetic field. And don't expect this to stir thick substances like peanut butter — it's for low-viscosity liquids.
Troubleshooting
**Problem: Stir bar doesn't spin.** Check magnet polarity. The magnets on the rotor should attract the stir bar, not repel it. Also check that the gap between the rotor and the cup bottom is less than 10mm. If it's too thick, the magnetic coupling weakens. Solution: use a thinner enclosure top or mount the motor closer.
**Problem: Stirrer is noisy or vibrates.** Imbalance. The magnets might not be perfectly opposite each other. Remove them and re-glue, ensuring they're 180 degrees apart and at the same radius. Also check that the fan blade is seated squarely on the motor shaft. A tiny wobble at low speed becomes a big vibration at high speed.
**Problem: Speed control is jumpy.** The potentiometer might be dirty or the wrong type. Try a linear pot instead of logarithmic, or add a small capacitor (100µF) across the power supply to smooth out noise. Also check your solder joints — cold joints cause intermittent connections.
**Problem: Motor gets hot after 10 minutes.** Your fan might be rated for 5V but you're running it at 12V, or vice versa. Check the fan's voltage rating. Also, running at very low speeds can cause some fan motors to overheat because the internal driver circuit isn't designed for PWM-like control. Solution: add a resistor in series to limit current, or switch to a different fan.
**Plan B:** If you can't get the fan to work, buy a cheap DC motor from a hobby store (like a 3V to 6V motor) and glue magnets directly to the shaft. It's less elegant but works. Or, if you want a dead-simple version, use a computer fan with a built-in magnet rotor — some fans already have magnets inside the blade hub. You can skip the magnet gluing step.
The Result
After about two hours of work, you'll have a magnetic stirrer that spins a stir bar smoothly at variable speeds. The first time you drop that stir bar into a cup of coffee and see it whirlpool without any moving parts above the liquid, you'll feel like a wizard. It's quiet, efficient, and surprisingly satisfying.
The finished device is about the size of a deck of cards. I used a black plastic enclosure with a brass potentiometer knob, and it looks like a piece of lab equipment — minimal, functional, and cool. It stirs a 500ml beaker of water with a 20mm stir bar at a steady vortex without splashing. For coffee, it's perfect: set it on your desk, drop in the stir bar, and sip while it keeps your drink mixed.
What would I do differently? I'd 3D-print a custom enclosure with a recessed top to hold the cup in place. I'd also add a USB-C port for power instead of a fixed cable. And I'd probably build a second one with a larger motor for thicker liquids. But for a first build, this is a solid success. Cost: $20. Time: 2 hours. Satisfaction: priceless.
Now go build yours. And when someone asks, "How does that work?" you can smile and say, "Magnets."
