Product Magnetic Stirrer DocID D0001 Version 0.1 Author Malcolm Watts Status Pre-release Editor Tekla Labs Date 7/11/2011

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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.

1 Product Overview 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. Figure 1. Tekla Labs magnetic stirrer with 500 ml sample Figure 2. Internal view of design Acknowledgements and Creative Commons Licensing Designed by M.J. Watts (formerly of Massey University). Design open- source courtesy of Massey University New Zealand. This blueprint is made available under a Creative Commons Attribution- NonCommercial- ShareAlike 3.0 Unported licence. 1

2 Navigating the Blueprint Document Product Overview... 1 Acknowledgements and Creative Commons Licensing... 1 What Skills and Tools Do I Need?... 2 Specifications: What can this magnetic stirrer do?... 3 What Materials Are Needed?... 3 Details of Materials... 5 Module 1: Motor Assembly... 6 Module 2: Motor Controller (Speed Adjustment) Final Assembly How do I know it is working correctly? Now how do I use this, troubleshoot and perform maintenance? Feedback What Skills and Tools Do I Need? Safety considerations during construction The soldering iron becomes extremely hot. Hot solder and flux can give off harmful fumes. Conduct soldering in a well ventilated environment and take standard precautions to secure the hot iron. The motor can become hot immediately after operation. These safety considerations are, of course, in addition to normal precautions necessary when using the various tools needed for this project. Required skills Skill Module(s) Soldering to printed circuit board 2 Press- fitting shaft 1 Drilling holes in sheet metal 1 Required tools Tool Module(s) Screw driver 1,2,3 Drill (hand drill or drill press) 1 Soldering iron 2 Method of cutting and bending sheet metal 1 2

3 Specifications: What can this magnetic stirrer do? List of specifications 1. Magnetic stir bar rotates at an adjustable speed. Maximum rotation rate depends on the components used, including motor, magnets, and stir bar. Rotation will be optimal in low friction containers such as glass containers. 2. Creates stable vortex in beakers up to 1000 ml. Optimized for use with 250 ml beakers. 3. Stirrer operates on battery power, not plug- in wall power. 4. Interior of device is as isolated from outside environment as possible so that device is resistant to spills. What Materials Are Needed? Figure 3. Overview of materials used 3

4 List of Materials. Note that in many cases, alternate materials can be used, as long as changes in properties such as dimensions are accounted for in the building process. Please add you comments to the online document if you find alternative materials or solutions. Part Part Description Module PartID Quantity Cost (USD) Rotor body Used to hold magnets, cut from light wood 1 M mm diameter $0.25 or hardboard (fiber board) 5mm thick. Rotor magnets Magnets that spin on rotor body. Circular 1 M002 2 $2.60 or bar magnets may be used. See text, DC motor Brush or brushless DC motor, body M003 1 $2.99 mm long, with mounting screws Sheet metal 16 gauge or similar, for motor mount. Also 1 M x 70 mm $0.25 for metal strip connecting magnets Printed circuit board (PCB) Connects electronics; can also wire by hand 2 M005 1 Spare, or handwire Plastic casing Encloses unit, see Material Hints 2 M006 1 $14.90 Battery clip Holds and connects the battery 3 M007 1 $0.89 Knob To set various speed settings 2 M008 1 $5.30 Potentiometer Speed regulation, can be bought with knob 2 M009 1 M2.5 screws M2.5 x 10 mm machine screw 1 M010 2 $0.20 M2.5 screws M2.5 x 20 mm machine screw 3 M011 4 $0.40 M2.5 nut M2.5 hex nut 1 M012 2 $0.20 Stir bar Magnetic stir bar Final M013 1 $2.00 LED light Optional. Lights up when motor is getting 3 M014 1 $0.40 power Battery D size Final M015 1 $2.00 Transistors, capacitors Tekla Labs is working on a detailed list of these parts 2 M017+ $4.00 4

5 Details of Materials This is a more detailed explanation of the key materials used in the design. In general, equivalent options can be bought from many other suppliers and dimensions and other parameters can be readjusted as needed. M001 Rotor body Cut from bulk material, can use scissors or hobby knife. M002 Rotor magnets Small rare- earth magnets are necessary to couple the rotor and stir bar in this compact design. The author suggests removing the magnets from magnetic battery connectors. Magnets can be circular or bar magnets.these are available from Science Lab Supplies, sciencelabsupplies.com, # ($1.30 each). Individual magnets are also available directly from other suppliers, e.g. magnets.amazonwebstore.com/ M003 DC motor Author s initial design used a generic toy motor available from JayCar (jaycar.com.au), YM2706 or YM2707 ($3.25 or $4.25), VDC operating voltage Prototype contains HTI Model 32 DC motor marked HTBRR F Radio Shack to 3VDC Hobby Motor ($2.99) may be a suitable replacement Motor case should have threaded mounting holes on the shaft side. This will allow it to be attached to the sheet metal mount. M005 Circuit board If a printed circuit board cannot be obtained, hand- wiring electronic components is straigthforward. Tekla Labs will provide wiring instructions in the future. M006 Plastic box Bud Industries PN DG (DigiKey ND) $14.90 Tekla Labs unit is a 12 cm wide x 17 cm long by 5.5 cm tall case made of ABS plastic. These dimensions are not critical but the surface area has to be large enough to hold the size glass beaker you want to use. M007 Battery holder Memory Protection Devices BHDL (Digikey BHDL- ND) $0.89 M009 Potentiometer Tekla Labs will confirm resistance range and dimensions of potentiometer. M010 M2.5 Screws DC motor screws need to be flat heads, otherwise you will have excessive rubbing between the mounting screws and the rotating hardboard. M013 Stir bar Science Kit and Boreal Laboratories (sciencekit.com) WW46316M14 SpinBar Magnetic Stir Bar $2.00 M014 LED light Panasonic LN21RPH (DigiKey P374- ND) $0.40 M016 Knob Optional. Standard knobs are available from DigiKey for a few dollars. A cylindrical rubber stopper or cork is a cheap substitute, and can be pressed directly onto the potentiometer. 5

Module 1: Motor Assembly Approximate time = 1 hour Step-by-step instructions Figure 4. Close- up of motor setup. 1. Cut sheet metal into two pieces: a small metal strip for use on top of the rotor body and a larger piece for mounting the motor.

6 Module 1: Motor Assembly Approximate time = 1 hour Step-by-step instructions Figure 4. Close- up of motor setup. 1. Cut sheet metal into two pieces: a small metal strip for use on top of the rotor body and a larger piece for mounting the motor. 2. Cut holes in the small metal strip for the motor shaft and for the screws to hold the motor. The small metal strip is important as it creates a low- profile horseshoe magnet, which greatly enhances the pull of the magnets as compared to the case where no metal strip is used. If the magnets are put directly onto the hardboard without a metal strip, one has to be careful not to change the power setting too quickly on the finished unit or else the magnetic stir bar will jump off target.. Figure 5. Top and sideview of small metal strip, magnets and hardboard. 6

7 3. Drill holes for motor screws and shaft in the motor mount sheet 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. Figure 6. Topview schematic of flat metal mount sheet, indicating locations of drill holes and metal bending. 4. 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. Figure 7. Sideview schematic of bend- lines of metal mount sheet. 5. 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. 7

8 Figure 8. Final sideview schematic of motor and sheet mount. 6. Cut the hardboard to form the rotor body The rotor body s diameter is dependent on the size of the stir bar you will be using. The magnets on the rotor body should be 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. Our rotor is 38 mm in diameter and 5 mm thick to fit a magnet center- to- center distance of 24 mm. 7. Drill two screws (or holes, if you re using machine screws) through the bottom base of the hardboard. 8. Install the magnetic strip rotor atop the drilled hardboard using the corresponding nuts 9. 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. 10. 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). 11. Attach the motor mount sheet (with attached motor) to the printed circuit board with screws. 8

9 12. Glue magnets onto the metal strip on the hardboard rotor. Magnet distance should be about 80% of the length of the stir bar. Figure 9. Exploded view of Module 1 components. 1) metal sheet for mounting motor 2) motor 3) hardboard 4) small metal strip 5) magnets 6) screws, 6 total 9

Module 2: Motor Controller (Speed Adjustment) Approximate time = 1 hour Step-by-step instructions 1. Construct a PCB according to the circuit shown below.

(Tekla Labs is working on converting this circuit diagram into a more accessible form and photographing the hand- wiring step- by- step.) 2.

10 Module 2: Motor Controller (Speed Adjustment) Approximate time = 1 hour Step-by-step instructions 1. 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.) 2. Drill a hole through the plastic casing to connect the speed control knob to the potentiometer. 3. Attach knob to potentiometer shaft outside of case Figure 10. Schematic circuit diatram of components.. Figure 11. Wiring diagram for hardwiring in the absence of access to a printed circuit board KEY capacitor resistor transistors potentiometer motor pnp transistor battery LED light npn transistors Figure 11 ground 10

Module 3: Battery Power and Power Indicator Light Approximate time = 1 hour Step-by-step instructions Battery power 1. Solder the two battery casing wires to contacts on the printed circuit board.

Hotmelt, a hot glue gun can be used for this step. LED light (optional) 3. Drill a hole on the side of the plastic casing to fit the LED light to provide a tight fit. 4.

11 Module 3: Battery Power and Power Indicator Light Approximate time = 1 hour Step-by-step instructions Battery power 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. 2. Glue the battery casing to the PCB. Hotmelt, a hot glue gun can be used for this step. LED light (optional) 3. Drill a hole on the side of the plastic casing to fit the LED light to provide a tight fit. 4. 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. 5. Carefully press the LED light into the hole using pliers. Figure 11. Close- up of finished battery clip. Figure 12. Close- up of finished LED light connection 11

Final Assembly Approximate time = 30 minutes Step-by-step instructions 1. Screw circuit board into bottom of plastic case 2. Place D- battery into battery clips 3. Screw top onto plastic case body a.

12 Final Assembly Approximate time = 30 minutes Step-by-step instructions 1. Screw circuit board into bottom of plastic case 2. Place D- battery into battery clips 3. Screw top onto plastic case body a. 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. Figure 13. Finished, closed magnetic stirrer with magnetic stir bar on top. 12

How do I know it is working correctly? Inspection 1. The unit should be completely sealed, but without over- tightening of the screws used to attach the top surface.

13 How do I know it is working correctly? Inspection 1. 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. 2. When turning the unit on, with a working battery in place, the LED indicator light should turn on. Testing 1. Insert stir bar into one liter beaker with one liter water. 2. Stir rod should not rotate when knob is turned to zero. 3. Gradually turn knob to higher setting. Stir bar should rotate faster as motor is operated at a higher speed. 4. At maximum speed setting, the magnetic stirrer should be able to form a large vortex within a one liter volume beaker (see picture). Figure 14. Testing the finished magnetic stirrer: at full speed, a large vortex should form in 1 L of water 13

14 Now how do I use this, troubleshoot and perform maintenance? Operator safety 1. When working with hazardous chemicals, glassware should be covered. Stirrer should be kept in a secondary container to catch any spills 2. Retrieve stir bar with a retriever, not by hand. This can be made by taping a magnet to the end of a plastic rod or pen. Maintenance 1. Change battery when necessary (indicated by LED light). 2. When changing battery, also confirm that magnet rotor still makes tight fit with shaft. Troubleshooting 1. Problem Stir bar does not move at all when knob is turned up. Solution Remove cover and check that motor and rotor body are spinning. If not, check battery and motor connections. If they are, check that magnets used are capable of moving stir bar and are close enough to top of plastic casing. If current is delivered to the motor but it does not spin, the motor may need to be replaced. If rotor spins but does not catch stir bar, try moving rotor further up along motor shaft. 2. Problem Stir bar moves when knob is turned up, but does not rotate fully. Solution Double- check that the beaker and stir bar are both centered over the motor shaft. Gradually turn up rotation speed until rotor catches stir bar. Feedback The Tekla Labs community wants to hear from you! Please give back to the Tekla Labs community by letting us know if you: find alternative materials or building tools to use use another approach for building a module or part of a module come up with a whole new design for this type of equipment (or another equipment piece!) had any other problems or solutions to share need help with specific parts of this Tekla Blueprint. What is unclear? How can we make this better? To get involved in the Tekla Labs community, visit our website 14

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

Magnetic Stirrer. Designed by M.J. Watts (formerly of Massey University). Design open-source courtesy of Massey University New Zealand. Magnetic Stirrer Designed by M.J. Watts (formerly of Massey University). Design open-source courtesy of Massey University New Zealand. Written By: Tekla Labs 2017 guides.teklalabs.org Page 1 of 10 INTRODUCTION