Roaster #1

Hot Air Popcorn Popper

After buying green coffee beans I decided to modify a hot air popcorn popper for home roasting (instead of stove top or oven roasting). The first major modification is to remove the thermal switch so that the device will not automatically shut off after long periods of use and during roasting. This is inherently dangerous, and for that reason, the device will only be used outside, connected to a power strip with a surge protector, and with rapid access to water in case the device begins to melt or combust.

After removing four screws from the base plate (2), the outer plastic shell can be separated from the metal column and additional components (3).

Then, the base plate can be separated from the internal hardware by removing four additional screws from the base, and also removing two screws from a plastic clip that keeps the power cable in place (4).

The heating coil, fan motor, and metal cone (6) can all be separated after removing the previous screws.

Thermal Switch:
On the base of the heating coil component is a ceramic resistor in parallel with a thermal switch (1). The metal strip inside the thermal switch can be easily removed with a small flat-head screwdriver (2).

Metallic strip expanding and contracting due to applied heat from a soldering iron

After removing the metal strip from the thermal switch, the device was reassembled and tested by running continuously for 10 minutes outside without the clear plastic cover in use. Further testing with coffee beans is required to determine if the hot air popper will be able to roast green coffee beans sufficiently.

Remote Arduino Control

After roasting a few batches in the popcorn popper it was determined that the heating coil needed to be controlled as the roasts completed in 3-5 minutes, far faster than the goal of 8-12 minutes. There are many tutorials online and many different ways that heat control can be achieved. I settled on toggling the entire device on and off at repeated intervals controlled by an Arduino as I already had a remote switch and the parts necessary to set it up.

First, the remote control was disassembled and its circuit was analyzed to determine how to interface with the Arduino:

The top button turns on the switch, the bottom button turns off the switch
The switch wiring was determined by bypassing the switch with jumper cables to activate the remote device

After determining the correct wiring, jumper cables were soldered to the battery terminals and the switches on the remote such that the Arduino can trigger the device and power it from its 5V output. The remote was then reassembled with electrical tape.

The device is fully functional. Needs to be tested when roasting beans to determine the optimal switching rate in order to roast the beans in a more controlled fashion.

AC Dimmer and Yocto Thermocouple

The previous remote Arduino switch was replaced with an AC dimming control unit, as roasting was very inconsistent and difficult to fine tune with the previous method.

At the same time, I purchased a Yocto Thermocouple to record the temperature of the beans while roasting. This thermocouple can monitor two probes at the same time, however I will initially only monitor BT (bean temp).

Here is the positioning of the two temperature probes:

And here are the results of measuring the long term temperature of the air in the two positions. The temperature readings for the exhaust probe were wildly inconsistent (2), due to the hot air moving in an indirect path that could have not been detected by the probe:

After roasting multiple batches, it was determined that with this specific air popper and AC dimmer combination, setting the dial to ~25 or lower resulted in the most controlled roasting environment and was used for the majority of the batches.

Coffee roasting data will be posted here.

Sadly, while attempting to roast a large batch of beans, the thermal failure point of the device was reached. The fan broke and the heating element is no longer functioning correctly. I constructed a new device, posted here.