In today's How-To we'll cover converting a popcorn popper... into a home coffee roasting machine. If you love coffee like we do (and we have a feeling ya do), try upgrading your beans by roasting your own. Roasted coffee can go stale within a week or two, which is exactly why freshness is of the utmost importance when it comes to your brew. Plus, high quality green coffee beans are generally half the price of roasted, and can be easily stored throughout the year. Click on to read how! For today's How-To you'll need:
  • Hot air popcorn popper (read on to make sure you get the right kind)
  • Two gang plastic electrical box
  • Two hole switch cover
  • Basic wall dimmer
  • Light switch
  • Two computer power cables, or one fifteen foot extension cord.
  • Radio Shack  Model: 273-1512 Transformer or similar (25.2V Center tap 2.0A.) [Note: we originally grabbed the wrong part number]
  • Electrical tape or heat shrink tubing
  • Soldering iron or twist on wiring nuts.
  • Green coffee beans

Find the right popper

Not every hot air popper is suitable for roasting coffee. In order to safely roast coffee, the hot air chamber should look like this:


The hot air enters from the sides, allowing the beans to heat evenly and rotate. If the air enters directly from the bottom, the concentrated heat can become a fire hazard. We used a West Bend Poppery II for our roaster. Finding a suitable popper can be a challenge, but thrift stores often have suitable poppers for a few dollars. eBay is a great source for poppers, and Walgreens has reportedly been selling a suitable machine.

Normal air poppers won't get quite hot enough to fully roast coffee; some modifications are necessary . We'll be removing the "safety" features of the popper, so be aware that overheating the unit is possible (you know we're not going to be held responsible, yadda yadda). So, most poppers are built with two heating coils. One is used to reduce the voltage powering the fan motor. Sometimes one of the coils has failed, so test the popper to make sure it gets hot enough to pop popcorn. If the test resulted in a nice snack, it's probably in full working order.


When the project is done, the popper will have two separate circuits. The dimmer will be used to control the fan, while the switch will turn the heating coils on and off. For more detail peep our super-detailed schematic above, that should give you a very precise technical explanation.

Radio shack sells a couple of 25.2 volt center tap transformers. Make sure to get the 2.0 Amp version. If you pick up the 450ma version, the fan on the popper won't go fast enough (and the transformer will get pretty warm). Get an outdoor style plastic two gang electrical box (some people get a 3 gang and mount the transformer inside the box).

Mount the transformer on the control box

We drilled two holes and used a pair of 6-32 nuts and machine screws. There are a couple of wire to wire connections. You can twist these together and use wiring nuts, or solder them and insulate them with electrical tape or quality heat shrink tubing.

Prepare your power wires

You need four total or two pair of wires from the popper to the control box, and a pair of wires leading to a power plug. Keep in mind that green is used for a safety ground in AC wiring. If you're using a cord with a three prong plug on it, the other two will be the 'hot' wires.


One of the hot leads will connect to one pole of the switch and the dimmer. The other will connect to one lead from the heating coil in the popper and a primary wire to the transformer. The other pole of the switch connects to the second lead to the popper's heating element. The other wire of the dimmer will connect to the other primary lead of the transformer. Finally, the two outer leads (yellow in the photo) are connected to the other pair of wires to the popper. Those two will be connected to the fan motor. Label each pair of wires "fan" and "heat" so you don't have to trace which is which later on.

Double check all of your wiring for safety!

The popper needs to be opened up so that the fan can be separated from the heating element and the new wires from the control box can be connected. Poppers vary, but the most use a few screws to hold the body together.


Our Poppery II used three phillips screws. Remove them and the top should easily separate from the base. The wiring will usually keep the heating chamber from being removed from the body until you remove the power cable strain relief.


On ours, the stress relief clip was easily pried up using a regular screw driver. If you can't get a screw driver under it, try a good pair of pliers. Don't worry about damaging the cable, we'll be replacing it. Once it's out, just pull the two halves apart and the heating chamber should now be easily removed.


The fan is attached to the bottom of the heating chamber with a few screws. Before we start, note the three wires that connect to the two heating elements, and the two leads to the motor. Cut off the power cord and cut all the wires near the crimp connectors.


Remove the three screws that hold the fan to the heating chamber. Now the heating chamber can be pulled apart to reveal the heating coils. When the chamber comes apart, you'll find a metal spacer and a fiber gasket, made of the same heat resistant material that the heating coils are mounted to. On reassembly, they should line up easily. Just in case, the order is: heating coils, metal spacer, fiber gasket, fan assembly.


The silver component with the red labeling is a thermal fuse. The black component with the brass track on it is a thermal switch. The thermal fuse is the SAFETY feature of the popper. If the popper overheats, the fuse kicks in and power to the heating coil is stopped.

The thermostat works by opening the contacts once a particular temperature is reached. Use a piece of stiff wire (uninsulated if it's not rated for the heat) and connect the terminal of the black wire to the terminal at the end of the thermal switch, next to where the white wire is terminated. Make sure it won't short across the other terminals! You have now DISABLED the SAFETY feature of the popper in order to gain full manual control. Now the popper can get hot enough to roast the coffee beans. It can also easily get hot enough to start a fire if you don't pay attention to it. In a nutshell: never run the heating element without the fan blowing. For safety, use a power strip with a circuit breaker in it.

Pull the two new pairs of power leads through the hole in the body previously used for the power cord. Connect the fan leads directly to the original fan wires. For the heater, connect the wires to their respective devices. Now reassemble the popper.


To add some stress relief, we use zip ties on the control wires inside and outside the popper body. There are two posts on the bottom of the fan base that fit into two molded towers inside the body. Once you line these up, you can put the top back on the body and screw the unit back together.


If you're satisfied with your wiring job, install the switch plate cover on your control box, along with the knob for the dimmer. Once you feel confident, test the unit out in safe conditions. We suggest placing it on concrete, away from anything flammable just in case! As long as you have good home wiring, a short will probably just trip a breaker. If you have problems, unplug it, make sure everything is cooled off and trace your circuits.


Now that the roaster is built and tested, you'll probably want to roast some coffee. One of our favorite coffee suppliers is Sweet Maria's.

Turn the fan dimmer all the way up and turn the heater on. Add green coffee beans until they just stop spinning around. Controlling the heat level will seem a bit un-intuitive. The faster the fan is going, the cooler the beans will be. In order to heat the beans up more, you need to slow down the fan slightly. It will take a batch or two of beans to get used to the process. When the beans have reached the roast level you want, crank the fan on high and turn off the heater to cool the beans.

Sweet Maria's has a short how-to on air popper roasting and a great pictorial showing the stages of roasting beans. Make sure you check them out. If you're not sure what kind of beans to get, try one of their sample packs. For testing out your new roaster, they offer a cheap coffee called Ugh! If you just want to try out the whole roasting experience, try sweet talking a local roaster into a few greens for educational purposes. Happy caffeination!