DIY Stir Plate for Yeast Starter

Below is a link to a video showing the inside of the DIY Stir Plate I built for stirring my yeast starter.

Parts:

1. One fan from pc case (case cooling fan with two wire: hot and ground)
2. One 12v power supply from an old wall charger
3. Two earth magnets from an old hard drive
4. One 1.5-inch washer (to super glue to fan)
5. Some 18 gauge wire (similar to power wires inside a PC)
6. An old DVD case (I recommend something without a metal top)
7. An on/off switch
8. A 12 ohm potentiometer (optional) to control fan speed
9. Bolts and nuts to mount the fan.
10. A way to connect wires (solder, or wire nuts or electrical tape)


Special Tools Required
1. Torx #9 screwdriver to remove hard drive screws (Torx size could vary by manufacture)
2. Two large pliers to bend hard drive brackets to remove magnets
3. Super glue

Matching Power Cord with Fan

Before you start, make sure the power cord will work with your fan. The voltage must match exactly. In other words, probably a 12v DC power cord. However, it must be able to supply at least as much amperage as the fan requires. This is usually printed on the fan. For example, my fan reads: DC 12V 0.14A. This means my fan requires 140 milliamps. My power cord has printed on it 12v DC 300 mA, which means it can deliver up to 300 milliamps.

Wiring Assembly Tips:

1. Connect the black wire (or ground or neutral wire for your fan) to the ground wire of your power cord.

You should have two wires on the power cord, and two wires on your fan. If you only have two wires on your fan, you probably will not hurt the fan by test touching bare wires to the fan's two wires (one wire to one wire) to see which wire connects to which. If your fan wires are not red and black, Google the manufacture and model number of the fan to see which wire is hot.

Note: If you have more than two wires on your fan, the other wires are probably for sensing fan speed and temperature. You can probably ignore the speed and temp wires, as long as you can figure out which wires are hot and ground.

2. If you are installing a potentiometer, connect the fan's red wire to the middle leg (usually has three legs) of the potentiometer, and connect the right leg (when viewed from the bottom, opposite of the turn knob) of the potentiometer to one side of the on/off switch. The other side of the on/off switch will connect to the hot wire of the power cord.

Tip: Before you connect the potentiometer, make sure the switch works as you expect by turning on the switch, touching the hot wires to each side of the switch to make sure it works. Do not connect the black wire to the on/off switch.

3. If not using a potentiometer, connect the red wire to one side of the on/off switch, and connect the power cord's hot wire to the other side of the on/off switch.

Removing the rare-earth magnets from a hard disk

Note: You can buy rare-earth magnets at hobby stores like Michaels, or here in Minneapolis, at Axe Man Surplus. They are not expensive. If you must buy a torx screwdriver to remove the rare-earth magnets, it may be easier and less expensive to just buy the magnets.

1. Use a Torx #9 screwdriver to remove all screws from the top of the drive, and from beneath any labels that hide screws (there are usually one or two under labels).
2. Remove any screws that secure the read arm inside the case.
The magnets are part of the read arm, usually located in a the corner that serves as the base of the read arm.

Caution: Rare-earth magnets can really pinch your fingers if you get trapped between them. Be careful.

3. Remove the magnets from their brackets by bending the bracket edges using two sets of pliers.
Each magnet is mounted on a crescent-shaped bracket. Grip each side of the bracket, bend one side down until you can see a gap beneath the magnet. Don't bend so much that you crack the magnet.
4. Slide a razor knife under the edge of the magnet and pry up to loosen the glue.
They usually come loose easily once you can get a blade under and edge. You shouldn't have to use much force with the blade. If you do, you risk cutting yourself. Bend the edge until you can slip the blade under easily.

Fan assembly

1. Super glue the washer to the top of the fan, taking care to center it.
2. Center the rare-earth magnet on top of the washer.

Note: In the video I mention that I used Velcro. This did not work. Do not use it. It causes the magnets to warble and the stir magnet will go flying when the fan gets up to speed. The magnet on the fan must be rock solid in place and it should be centered.