Building a magnetic stirrer – instructions for the layman (or laybee)You can get an idea on how to build a magnetic stirrer by checking the drawing. Therefore I will rather concentrate the instructions on where to get the material and how to make the RPM regulator.
The magnetFinding a strong enough magnet should not bee a problem. They are an essential part of many machines. Notably, the strongest magnets are found in loudspeakers and DC electromotors (in cars etc.). These can be broken in order to obtain a small ~2.5cm piece.
It is however a little more difficult to get a piece of more symmetrical dimensions appropriate for a magnetic stirrer. The magnet in the bicycle dynamo is round and its magnetic field expands in four directions. It could be used but it is quite heavy and you would need a stronger electromotor to drive it. I found the best magnet in an old analog electricity consumption counter. Some contain a quite strong and square magnet (like shown on the drawing). In the case you can’t get a magnet of appropriate form you can use a piece broken from a loudspeaker that is put in a plastic round stopper and fixed with wax. It has to bee fixed so that its magnetic field expands orthogonal to the axis of rotation. This field can bee felt and checked by using another small magnet to find the attraction and repulsion sites. Try your best to fix the magnet in the center otherwise the whole stirrer will vibrate extensively when rotating. In such misfortune use some wax to balance the eccentric weight.
Avoid weak magnets like the ones used for billboards – check if your magnet is capable of moving the stirring magnet at a distance of at least 7cm before deciding that it is strong enough.
The electromotorSmall electromotors are everywhere around you. Find an old cassette player, an old electromechanical toy, a batteries powered mini-fan or whatever else and take out the e-motor. Make sure it is not a brushless or step motor as these need different speed regulation (a useful motor will only have two connecting wires). Don’t take off its transmition wheel, as you will need its upper surface to glue the magnet on it. Check also what is its working voltage U
m (if nothing is written on it assume the voltage needed is the same as the one the whole machine/cassette player/toy worked on). It shouldn’t bee wider than ~3cm so it will consume less than ~0.3A of current (I
m) – this will bee enough and will make the RPM regulator easy to make.
The electricity sourceNowadays it is so easy to buy an adjustable 3 to 12V electricity adaptor. If not adjustable, chose one that will give at least the voltage of the electromotor but not more than ~2V more. Assuming you got yourself a 3Velectromotor from a toy, you can also use the adapter of your old mobile phone battery charger or some other 3 to 5V source, as the power needed is truly small (~1W). Avoid using higher voltage than the one used in calculating the RPM regulator parts or the transistor might overheat. You can use a connector like shown on the drawing or, if you will use the adaptor only for the stirrer, you can simply skip the connector and solder its wires directly on the RPM regulator (cut the wire and find the polarity). The stirrer will only work if the polarity (+ and - side) is correctly orientated so pay attention to this.
The RPM regulatorThese e-motors can have their rotation speed regulated from min to max by a simple voltage regulation circuit. The circuit described here is the simplest possible of the useful types and it only requires a variable resistor (R1), a transistor (T), a capacitor (C) and a resistor (R2). The circuit shown on the drawing is built on an etched circuit board, but this is of course not worth the work for such a small thing. Better use a 2×3cm piece of an already etched and drilled universal board that can bee bought in any electronics shop. Actually you can even simply solder the few elements on the variable resistor if you don’t care for its “state of the art” look. The electronic parts can bee obtained from old cassette players, TV or other electronic accessory. Or you can simply buy them where you buy the board (it costs a few $ - the board and R
1 being the most expensive pieces). Choosing the types is wide open.
The transistor (T) used should support the electromotor current (~0.1 to 0.3A) and dissipate ~1W power – BC219 or similar should do (ask the vendor for analogue ones). You can either ask the vendor to mark you the position of collector, emitter and base of the three contact wires or find the specifications on the net using the type code in a search engine. Read about transistors on the net anyway in order to understand more. It is very important to connect it properly or it will not work.
The capacitor (C) should bee anything from 22microF to 220microF for 16V (or more, but they get bigger in dimensions). Mind the +/- signs on the chassis and orientate it so that the “–“ wire is connected on the minus lead on the board. Again, use the search engine if you know nothing about electrolytic capacitors.
The R
1 and R
2 properties are more specific.
Choose R
2 by assuming your e-motor works at U
m, need up to I
m and solve the equation: R
2=120*U/I
m. Lets say you have a 3V e-motor (U=3V) that need max 0.3A of current (I
m=0.3A) as these are the extremes for the most common small e-motors (like in toys etc.). This gives R
2=1200ohm (1/4W size will do).
Lets say we have a 4.5V adapter (U
a=4.5V; an old mobile phone charger for example). The voltage drop on R
1 will have to bee about U
m or just a little more. We can therefore calculate the needed resistance: R
1=R
2*U
m/(U
a-U
m). For our case this would bee 2400ohm but it should bee a little more in order to use R
1 also to switch of the stirrer at the min position. I think the nearest standard is 2700ohm, but in practice 3900, 4700 or 5000ohm would work as well (the only difference will bee the widening of the “switch off’ threshold). Now we have all the specifications needed.
That’s allWhen you have all the material it will take you a few hours to solder the RPM regulator together and glue the parts inside a plastic box (depends on your skills and imagination). Enjoy.
Edit:
PS: I just realized that the BC219 transistor is quite old and might bee hard to come by. Therefore just ask the vendor to give you any switch or low frequency transistor that can handle 0.3A and can dissipate 1W of heat. I think something like BC140 or BC 141 are much more than enough (
http://noel.feld.cvut.cz/semi/philips/acrobat/7365.pdf
). There are hundreds of other types useful like 2N1613 and others. If using a really small e-motor that consume less than 0.12A (e-motors in walkmans) you can even use BC107, but keep in mind that you should use a reduction wheel with an elastic bend (also from a walkman) as these e-motors don’t have much torque.