Building a Stir Plate

2021-05-09 758 words 4 minutes

Contents

Introduction

One fun aspect of homebrewing is working through problems and engineering solutions. Like most hobbies, there is a DIY aspect to a lot of issues that, in a more professional environment, would be solved with money. One of the first things I built was a stir plate in order to help in yeast propagation and starter creation. Constantly stirring a slurry of yeast and nutrient will help introduce plenty of oxygen that the yeast needs to reproduce. There are many benefits to making your own starters.

Ensure high cell counts to guarantee healthy fermentation
Save money by re-using yeast from a prior brew
Propagate a specialty strain

Of course you can just buy one that will work great, however what is the fun in that? A stir plate is nothing more than a spinning magnet which magnetically couples to a pill shaped sir rod in a beaker. As the magnet in the stir plate spins, it rotates the stir rod.

Making plans

There are many tutorials and plans online that cover making a stir plate. The basic concept for the majority of them is to take a computer fan, attach magnets to it, wire in a motor controller, and stick the whole contraption in a box. I planned to follow a similar build using whatever parts I had laying around.

Part of the fun in building something like this is making it your own. Working in tech provides me with ample pieces of obsolete junk hardware. For the stir plate case I found an ancient Nortel Network router. Using this was an easy choice. All metal construction, built in switch and power jack as well as the ability to have fun blinky lights on the front while running.

The plan is to gut the router PCB of components minus the power jack, power switch, and front facing ports and lights. Wire in the fan and motor controller to the switch and power jack. Finally, add in a small circuit to randomly blink the link and activity LEDs on the front facing RJ45 jacks.

Parts list

Stir Plate Parts
- Nortel Networks Router (stir plate case)
- computer case fan
- 12v -> 5v step down converter
- Motor speed controller (PWM)
- Magnetic stir bar
- 2L Erlenmeyer flask
- 2 Small neodymium magnets
- Acrylic (to use as a magnet mount)
Blinky lights parts
- TI CD4026BE CMOS Decade Counter/Divider with Decoded 7-Segment Display Outputs
- 555 timer
- Adafruit mini proto board
- Various capacitors
- Various resistors

Case prep

The first step is to open up the router, pull the PCB, and remove all components except the front ports, lights, power switch, and power jack. This is accomplished easily enough with a hot air gun and some pliers.

Next up, I needed to cut two holes in the case. One for the motor controller knob and the other for the stir mechanism. The only trouble with using a metal case is that you can not easily pass a magnetic field across a metal plate. In order for this to work, a hole will need to be cut in the case above the spinning magnets.

Electronics

Now that the case is ready to go, it is time to wire up the insides. The primary mechanism is very simple. Power is fed in through the barrel jack in the back of the device, is routed to the motor controller, which then sends a PWM signal to the fan allowing you to adjust speed.

The real fun is driving all the blinky lights on the front of the case. I mean, what is the point of using some obsolete tech if you can not flash all the blinky lights. To make it appear that the lights are doing things, we will need to make a circuit that randomly flashes them. This is accomplished using a 555 timer to provide the randomness and a CD4026BE to drive the LEDs.

Magnets

The heart of this build is the magnets on the fan which spin the stir rod in the beaker. It is important to attach the magnets centered on the fans axis of rotation. If you place them off center, they will cause the stir rod to spin unevenly and fly off. To accomplish this, I made a mount for the magnets in CAD and laser cut it in acrylic. This allowed me to be accurate in my placement. The acrylic puck was glued to the fan with some epoxy.