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Want CONSISTENTLY GREAT home-brew? build this magnetic stirrer! Design by Michael Burton You probably know that magnetic stirrers are widely used in chemistry and biology laboratories – but a more down-to-earth reason for having one is if you brew your own beer. Why? It is great for making activated yeast, as Michael Burton explains. He came up with the basic design and we refined it (a bit). S o what is a magnetic stirrer? In essence it is a small machine which produces a rotating magnetic field. On it you place a flask or beaker containing the liquid(s) you want to stir. The stirring action is produced by a short bar magnet encapsulated in an impervious plastic. It is spun by the rotating magnetic field and you can leave it to do its job for as long as you like. Why stir a mixture yourself when a machine can do it for you? Magnetic stirrers used in chemistry and biology labs often incorporate a temperature-controlled hotplate. Typically, they use four electromagnets which are alternately switched 40  Silicon Chip to provide a pseudo rotating magnetic field. In other words, the magnetic field does not actually rotate but by suitably fiddling with the speed knob you can get the mixture to start spinning and then you increase the speed knob to its desired setting. We include a photo of the workings of one of these machines. So now that you have a broad picture of how magnetic stirrers work, you are probably wondering how they are used in home brewing. In general, beer is made of four things: sugars, hops, water and yeast. When you make a batch of home brew, you mix sugars, hops and water in a big barrel, add your yeast, then soon enough the yeast will “wake up” and A commercial unit which was the inspiration behind this project. It uses four electromagnets which are sequentially switched to simulate rotation. siliconchip.com.au OK, we admit it: we’re not stirring yeast wort – in fact, it’s orange cordial! This photo was taken so you can see the stirring action (the vortex) created by the spinning magnet in the solution, driven by the specially modified fan in the box underneath. The only controls are an on/off switch and a speed controller knob. start fermenting the sugars to make alcohol. The trouble is that until the yeast is active you have a barrel of sweet vulnerable wort, just asking for any bacteria to gorge itself and start multiplying. If unwanted bacteria gets a foothold before the yeast takes over you will get a beer that tastes anywhere from mediocre (if you’re lucky) to yukkk! Interestingly, every batch of beer is infected to some extent with unwanted bacteria. The job of a good brewer is to keep unwanted bacteria to a minimum. This is why sanitation and cleanliness is paramount in home brewing. Second to this, we can minimise the chance bacteria will have to get a foothold by pitching yeast that is already activated (awake and active). This way the yeast will start fermenting the beer right from the start and drastically reduce the opportunity for unwanted bacteria to multiply and spoil your beer. Why isn’t yeast already active? Without the presence of sugars to consume, yeast cells become dormant and the longer they are dormant, the longer they take to become active again. siliconchip.com.au So even if you buy a fresh liquid yeast culture from a home-brewing store, it has usually spent weeks or months in a dormant state and it will take numerous hours to become active after being pitched into the wort. Many brewers also use dry (dehydrated) yeasts. If you ever buy a home brew can from your local supermarket you will find a sachet of dehydrated yeast under the lid. These can take a very long time to activate which can spell bad results for your pride and joy if it is pitched in directly (and often that’s just what the instructions will tell you to do). The solution to this trouble is to activate your yeast well in advance of when you have to pitch it. So you need to plan ahead to ensure that your yeast culture is awake and ready to start fermenting immediately. And they will be in good shape to compete with any undesirable bacteria strains that have crept in to your wort before they take hold. Activating yeast cultures involves providing them with a sterile, sugarrich, well-oxygenated solution in which they can wake up at their own leisure. For this you can use a 1 to 2-litre flask; fill it half with water, add some sugar and pitch the yeast. Dormant yeast tends to sink to the bottom of the vessel and it needs to be stirred up – which will drastically speed up the awakening process. Here is where your magnetic stirrer comes to the rescue. It offers a way to keep the starter mixture moving and holding the yeast in suspension, while also being completely sterile. You simply sterilise the stir bar (“flea”), drop it in the mix and it will get the yeast mixture fully active so it can be pitched into the wort, ready to work its magic to ferment the brew. Making your own stirrer You probably already have some of the components you need for a magnetic stirrer, such as a surplus 12V computer axial box fan, a 12V DC plugpack and perhaps a suitably sized The stir bar, actually called a “flea” is simply a small bar magnet encapsulated in some material that is impervious to the solution being stirred – PVC, for example. It is absolutely essential that the flea is sterilised before use to ensure that harmful bacteria are not introduced into the mix. An encased rectangular bar magnet may work even better than the rounded one shown here, because it will create more turbulence. December 2011  41 Just about any old (or even new!) 12V DC fan will do the job – this one was salvaged from a junked computer power supply. The two rare-earth magnets are glued to the rotating motor hub (not the blades!) using a super-strength adhesive such as JB Weld. The last thing you want is those magnets flying off! plastic jiffy box. What else do you need? A pair of super-strong magnets to be glued to the fan and a speed control circuit. The magnets (which are often referred to as “rare earth” magnets, super magnets or even scary magnets!) are easily obtained: try Jaycar or even eBay. The speed control is easy too – we have used the nifty LM317 adjustable regulator PCB featured elsewhere in this issue (originally from May 2007). In fact, to make the job easy we simply purchased the Jaycar kit for that project; Cat KC-5446. However, we have made a few simple changes. The main one is to substitute a 2k linear potentiometer for the 2k trimpot which would normally be installed on the PCB. Apart from that, the prototype made by Michael Burton incorporated a DC socket for the 12V plugpack, a 12V LED bezel, an SPST toggle switch for power on/off and a 2-pin header to suit the plug for the 12V computer fan he used. Assembling the PCB takes only a few minutes. Since a typical 12V fan is not likely to draw any more than about 250 to 300mA, no heatsink will be required for the LM317 regulator. The prototype magnetic stirrer was assembled into a plastic case obtained from Futurlec. Alternatively, you could just use an appropriately sized plastic zippy box to accommodate the fan you decide to use. Whichever zippy box you use, the lid should be used as the base of the finished unit. The reason for this is that there will inevitably be spillages and by having the lid as the base you reduce the possibility of any fluids getting into the box. We have not included any PCB component or wiring details with this article as we will let the photos tell the story. Assuming you have assembled the LM317 regulator PCB, the next step is to work on the plastic case and the fan. You need two small high power magnets and they are glued to the rotating hub of the fan, as shown in one of the photos. Don’t use just any adhesive because the magnets will be subject to high centrifugal forces. We used JB Weld epoxy adhesive which gives a really strong bond. Gluing the magnets does tend to be a little tricky. You obviously cannot use Here’s how it all goes together inside the box, which also becomes the base for the container being stirred. The fan is held in place using doublesided adhesive foam pads on the four corners. These not only hold it tight but also give a couple of millimetres of clearance for the magnets glued to the fan hub. Test for clearance before finally securing the fan – if you need to, add a second layer of pads. 42  Silicon Chip siliconchip.com.au Est.1978 5th Generation MR16 & GU10 5 Watt LED Replacements Ultra bright 400 lumens =45W Wide beam 60° Long life 35,000 hours Another view of the case, this time from above showing the mounting of the regulator PCB, power supply socket and the three controls. a steel tool to apply the adhesive to the magnet because it will inevitably jump on to the tool and then it is a messy job trying to extricate the two. A better approach is to put two small dollops of adhesive on the rim of the hub where you want the magnets positioned and then place the magnets. Even then, the magnets will tend to move slightly, to be “where they want to be” as they interact with the magnetic rotor of the fan. If there is room, a clamp may help hold them in place while the glue is setting. Leave the adhesive to cure for at least 10 hours (24 is better), just to make sure that the magnets are rigidly in place. The next task is to drill the case with the various holes needed to mount the hardware: DC socket, 12V LED bezel, potentiometer, miniature toggle switch and the holes for the screws to mount the regulator board. We elected to mount the regulator board vertically at one end of the case, adjacent to the DC socket. Again, the photo tells the story. Wiring the regulator board to the hardware does present a problem with the original design, simply because there are not enough points on the board to terminate the DC supply input, the 12V LED bezel and the switch. Fortunately, there are extra ground points on the board which cater for different types of trimpot, sizes of elecsiliconchip.com.au trolytic capacitor etc and this made it possible to wire everything up. However, we have now revised the design of the regulator board to provide 2-pin headers for these connections. The modified design is featured elsewhere in this issue. The fan needs to be centrally mounted on the base of the case (which becomes the top of the finished magnetic stirrer). The fan can be attached with double-sided foam tape which also provides the necessary clearance so the magnets on the fan do not foul the case. SC Parts list – Magnetic Stirrer 1 Voltage regulator kit (Jaycar KC5446 or similar) or the new MiniReg elsewhere in this issue 1 ABS or similar case, size to suit 1 12V DC computer-type axial box fan 1 12V DC plugpack (500mA or so) 2 mini “rare earth” magnets (eg Jaycar LM1618, 1622) 1 bar magnet, encapsulated in PVC etc. 1 SPST mini toggle switch 1 2k linear potentiometer with knob 1 LED with bezel 1 DC input socket (size to suit your 12V supply) JB Weld 2-part adhesive hookup wire, nuts & bolts, etc Cool operation Cool, natural & warm white 2 year conditional warranty MR16 Dimmmable 7 & 9W (also dimmable) available MR16 (1+) $24.00 (10+) $22.00* GU10 (1+) $25.00 (10+) $23.00* Incandescent & CFL Led Replacements Long life 30,000 hours Cool operation Cool & warm white 6 Watt 620 lumens (cw) = 60 Watt 7 Watt 740 lumens (cw) = 70 Watt 9 Watt 915 lumens (cw) = 90 Watt 5 year conditional warranty 6W E27/B22 $19.00* 7W E27/B22 $24.00* 9W E27/B22 $28.00* *Prices valid until 31/01/12 Queensland Bowen Hills Ph: (07) 3252 7466 Southport Ph: (07) 5531 2599 New South Wales Homebush Ph: (02) 9704 9000 www.prime-electronics.com.au December 2011  43