Magnetic Stirrer

Designed by M.J. Watts (formerly of Massey University). Design open-source courtesy of Massey University New Zealand.

  • Author: Tekla Labs
  • Time required: 1 to 4 arbitrary periods of time
  • Difficulty: Moderate

Uses: Magnetic stirrers (also known as magnetic stir plates) are very common in experimental chemistry and biology. They are used to mix components (solids and liquids) to get homogeneous liquid mixtures. Common examples include bacterial growth media and buffer solutions.

How They Work: Magnetic stirrers mix solutions using an external magnetic field that rotates a small magnetic bar that has been placed in the mixture of interest.

Advantages: Magnetic stirrers minimize the risks of contamination since only an inert magnet bar, which can easily be cleaned, is put inside the sample/fluid. In addition, using a magnetic stirrer rather than manual stirring is critical for consistent, reproducible mixing or mixing over long time scales. Protein dialysis, for example, requires multi-hour or overnight sample mixing and is sensitive to bacterial contamination.

This Magnetic Stirrer: Can rotate an internal magnet at variable speeds, mixing volumes up to one liter. Glassware with a stir bar is placed directly on top of the stirrer unit. The device is battery-operated and largely isolated from spills by its plastic case and gasket.

Image #1

Edit Step 1 Module 1, Motor Assembly (photo 1)  ¶ 

  • Component list: (1) motor mount - sheet metal, (2) DC motor, (3) rotor body, (4) rotor mount - sheet metal, (5) magnets, (6) screw

  • Task 1, Rotor Mount Fabrication

    • Cut rotor mount metal strip from bulk strip metal. (photo 2)

      • Rotor mount length is set by the by desired magnet location. It is spaced approximately 80% of the stir bar length. Using longer bar magnets on the rotor body allows for a wider range of magnetic stir bar sizes.

    • Drill Holes

      • The motor shaft through hole (red) should be larger then the shaft (R).

      • The screw holes (r) to attach to the Rotor Body are spaced symmetrically (L) around the center. Be careful not to place them too far, where the magnets will be attached.

Image #1

Edit Step 2 Module 1, Motor Assembly  ¶ 

  • Task 2, Rotor Body Fabrication: part numbers 3, 4 & 5 in the image in step 1

  • Cut hardboard to form the rotor body

    • The rotor body’s diameter is dependent on the size of the stir bar you will be using (as discussed in Rotor Mount Fabrication). Our rotor is 38 mm in diameter and 5 mm thick

  • Drill two screws (or holes, if you’re using machine screws) through the bottom base of the hardboard.

  • Install the magnetic strip rotor atop the drilled hardboard using the corresponding nuts

  • Glue magnets onto the metal strip on the hardboard rotor. Magnet distance should be about 80% of the length of the stir bar.

Image #1

Edit Step 3 Module 1, Motor Assembly  ¶ 

  • Task 3, Motor Mount Fabrication

  • Cut out a metal sheet for the Metal Mount. Length should be enough to realize the structures depicted in photo 1.

  • Drill holes for motor screws and shaft in the motor mount sheet (photo 1)

    • CAUTION: clamp sheet metal to work surface while drilling. Keep hands free of sheet metal in case it comes free and rotates with the drill.

    • Positions of holes for motor screws and shaft are critical. These depend on the exact model of motor that is used. For most precise placement, first mark hole position in pencil. Then indent the sheet metal surface with an awl or nail. Finally, position tip of drill bit in the indentation.

  • Bend the motor mount sheet to form the metal motor mount shape described below. The mount height (30 mm in this case) should be high enough to fit the motor without compressing its solder leads. The angle you bend the sheet is not critical. (photo 2)

  • Attach motor to the motor mount sheet with screws. Screws with short heads should be used so that there is enough clearance to the rotor body. (photo 3)

Image #1

Edit Step 4 Module 1, Motor Assembly  ¶ 

  • Task 4, Final Motor Assembly

  • Press-fit the rotor body onto the motor shaft.

    • This is a friction fit. The motor shaft can be covered by a piece of heatshrink plastic to provide a firm fit to the hole in the center of the hardboard rotor body.

  • Solder the two motor wires to contacts on the printed circuit board (one to ground and one so that it can be electrically connected to the potentiometer).

  • Attach the motor mount sheet (with attached motor) to the printed circuit board with screws.

Image #1

Edit Step 5 Module 2, Motor Controller for speed control  ¶ 

  • Construct a PCB according to the circuit shown below. Alternately, a hand-wired circuit may be simpler to construct with available parts. (Tekla Labs is working on converting this circuit diagram into a more accessible form and photographing the hand-wiring step-by-step.)

  • Drill a hole through the plastic casing to connect the speed control knob to the potentiometer.

  • Attach knob to potentiometer shaft outside of case

  • http://www.pcb.com

Image #1

Edit Step 6 Module 3, Install Battery and LED  ¶ 

  • Task 1, Battery Installation (photo 1)

    • Solder the two battery casing wires to contacts on the printed circuit board. One wire must be connected to ground and one must be connected to a contact so that the wire can be electrically connected to the potentiometer.

    • Glue the battery casing to the PCB. Hotmelt, a hot glue gun can be used for this step.

  • Task 2, LED installation (photo 2)

    • Drill a hole on the side of the plastic casing to fit the LED light to provide a tight fit.

    • Solder the two LED light wires to contacts on the printed circuit board (one to ground and one so that it can be electrically connected between the potentiometer and the motor.

    • Carefully press the LED light into the hole using pliers.

Image #1

Edit Step 7 Module 4, Final Assembly  ¶ 

  • Screw circuit board into bottom of plastic case

  • Place D-battery into battery clips

  • Screw top onto plastic case body

    • Do not over-tighten as this could cause the uneven pressure load along the perimeter and thus bowing (outward bending) of the top that affects the distance between the glass beaker and the magnetic rotor.

  • Device Inspection

    • The unit should be completely sealed, but without over-tightening of the screws used to attach the top surface. There should be no entry points into the unit other than the holes cut for the indicator light (if used) and for the speed control knob.

    • When turning the unit on, with a working battery in place, the LED indicator light should turn on.

Image #1

Edit Step 8 Module 5, Testing  ¶ 

  • Insert stir bar into one liter beaker with one liter water.

  • Stir rod should not rotate when knob is turned to zero.

  • Gradually turn knob to higher setting. Stir bar should rotate faster as motor is operated at a higher speed.

  • At maximum speed setting, the magnetic stirrer should be able to form a large vortex within a one liter volume beaker (see picture).

For more information, check out the Science Lab Equipment category page.

Comments Comments are onturn off

hi,

you forgot to give the voltage of the Zener diode.

j.l.

jean louis, · Reply

You specified the wrong battery in 'relevant parts', it's a 1.5V 'D' cell, not a '9V battery'.

RealGene, · Reply

Thanks for the catch!

Kate Lovero,

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