The magnetic stirrer is a useful device for any home scientist; it is effective, cost-efficient, and is easy to make and use. The DIY magnetic stirrer attracts a lot of attention, but is often disregarded as being too difficult to make, usually because of the potentiometer used to control stirring speed, and the challenge one faces in calculating the right potentiometer to use. In this Instructable, we will not use a potentiometer, and will still be able to control stirring speed.
The stirring action is achieved through magnetism. Inside a project box, there are two magnets glued to a fan which spins, powered by the 12V DC power supply, and controlled by resistors on a rotary switch. Above the lid of the project box, a beaker sits, and inside the beaker, a magnetic stir bar is placed. The magnetic stir bar corresponds with the magnets inside the project box, and will move as the magnets on the fan move, as it is attracted to the poles of the magnets. The movement of the stir bar will cause the movement of the surrounding liquid, therefore stirring the liquid, as it moves in a circular motion above the hidden magnets.
The DIY magnetic stirrer is perfect for dissolving stubborn solutes, stirring for long amounts of time, and allowing stirring of covered mixtures to prevent evaporation.
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Step 1: Materials
Most electrical parts were purchased from Jaycar Electronics.
Helpful photographs will be included in the method.
- 12V LED
- FEMALE DC power port
- 12V DC power supply (PSU) - the PSU must fit the female port tightly.
- Project box - I used an 11cm (wide) x 25cm (long) x 6cm (deep) box, which can stir quantities of up to 1L easily.
- 1-input rotary switch
- Head for rotary switch.
- Regular 2-way switch
- 12V, 0.19 amp 80mm x 80mm computer fan, purchased on ebay. Amperage does not matter, as resistance needed can be calculated for different fans: http://www.ebay.com.au/itm/80mm-x-80mm-x-15mm-New-...
- Magnetic stir bar, purchased on eBay: http://www.ebay.com.au/itm/Magnetic-Stir-Bar-28mm-...
- Resistors (values will be calculated further in Instructable)
- Insulated copper wire
- Heat shrink
- 2 x neodymium (rare earth) magnets, purchased on eBay: http://www.ebay.com.au/itm/20-x-Disc-Rare-Earth-Ne...
- Soldering Iron
- 5 minute epoxy
- Rubber or foam spacers (I cut sections from an old Nerf dart).
- Standard drill bit
Step 2: Calculating Resistance
The aim of using the rotary switch is to avoid tricky calculations and problems associated with the potentiometer. Instead of having many different speeds, the rotary switch will allow for 3 different speed settings by running current through different resistors in series that can be selected by rotating the rotary switch. To reduce or increase fan speed, the fan will be supplied with an increased or decreased amount of voltage; the amount it is decreased by is determined by the value of the active resistor. Remember: The higher the resistance, the lower the voltage.
So, different resistors will amount to an amount of voltage supplied to the fan. I chose to run my fan at 8V, 10V and 12V, as well as leaving one channel disconnected to open the circuit, so I could leave the stirrer on, without it stirring anything.
For those who would like to simply make the stirrer, using the fan purchased from the link previously provided (12V, 0.19 amp), purchase these resistors to allow these voltages:
0V (off) - N/A
8V (low) - 32 ohm resistor
10V (medium) - 13 ohm resistor
12V (high) - no resistor
For those who would like to understand the science and calculation behind these values, or for those who have a a different fan they would like to use, use this simple calculator to calculate the value of the resistors you need: http://www.blackfiveservices.co.uk/fanspeed.shtml
Step 3: Part Placement and the Fan
It is easiest to solder the connections for the stirrer when all parts have been placed and secured in the the project box. Most belong in the lid of the box, while some belong in the sides. Placement of each part is up to the desire of the maker, but how I placed each part is shown in figures 1.2, 1.3 and 1.4
Epoxy is added to each corner of the fan frame, as well as to two opposite points on the fan head. The foam slices (fig. 1.5) are cut and placed on the epoxy in the corners of the frame; they must be slightly higher than the magnets, so as to allow a gap between the magnets and the roof of the box (fig. 1.6).
One magnet is placed on one dot of epoxy on the fan head, and another on the other dot. Ensure the stir bar is attracted at both ends to the magnets before sticking them down, or the stirrer will not work properly. After the foam has stuck to the fan frame, and the magnets have been stuck to the fan head, the foam is glued to the roof of the box with more epoxy, and is left to fully adhere.
Ensure there is a gap of at least 2mm between the magnets and the roof of the box, so as to avoid scratching, inefficiency and a terrible noise. Location or adjacency of the fan is not important, as the placement will not disrupt the circular movement of the fan head. General layout is shown in fig. 1.7.
Step 4: Connections
All connections in the circuit are soldered together, and protected with a jacket of heat shrink to prevent exposed wires from shorting. The schematic for the circuit is shown in figure 1.8.
Firstly, the positive terminal of the female power port (figure 1.9) is connected to one terminal of the switch. Another length of wire continues from the other terminal of the switch to the positive terminal of the LED. From the same positive terminal of the LED, another wire is soldered, and connects to input of the rotary switch, shown as the red wire in figure 2.0. The input will be in the middle of the base of the switch, while the numbered output channels will surround it. You can restrict the number of channels you wish to use with a washer that will come with the rotary switch.
Channel 1 is left open, channel 2 has the 32Ω resistor soldered to it which, channel 3 has the 13Ω resistor soldered to it, and channel 4 has a wire leading straight to the positive terminal of the fan. All resistors are also wired to the positive terminal of the fan and soldered, protected with a length of heat shrink, shown in figure 2.1.
The negative terminal of the female power port is connected to the negative terminal of the LED, and continues from the negative terminal of the LED to the negative terminal of the fan.
Overall view offered in figure 2.2.
Step 5: Finishing Touches
The lid is screwed down to the project box and the holes are plugged (screws and plugs are provided with the box).
All fittings and hex rings are tightened.
The control arm of the rotary switch is filed down, and the control head is added. You may add one of any style - figure 2.3.
The stir bar is placed on the lid, and a marking may be made of where it naturally sits, for ease of use later on.
To test, place a beaker with some water and the stir bar inside on top of the box. Ensuring the stir bar is centered, plug in the power supply, turn on the stirrer, and click the rotary switch to channels 2, 3, and 4.
Changing the position of the rotary switch should alter stirring speed.