I bought a severin a couple of weeks ago (recommended by a quite a few people on the uk coffeeforums site) for 25 quid to get back into roasting coffee. Unmodded it roasts ok, though very quick. E.g. First crack was somewhere between 3 and 4 mins. At this sort of rate it is pretty uneven and I had to roast to almost 2nd crack to get something that looked consistent.

I found this instructables piece which gives a lot of detail on controlling it with an arduino and TC4+ shield. This is quite a lot of work, but I realised you can do this stuff in stages and get something usable / drinkable at each stage:

  1. Unmodified popcorn maker, stopwatch, watch for yellow, listen for cracks
  2. thermocouple in the chamber
  3. dimmer on the AC lead, simultaneously reduce fan+heat in tandem to slow roasting speed
  4. separate the DC motor and AC heating element, control manually
  5. hook up to TC4, artisan, record roasts
  6. PID control etc

To get to (3) I bought the following which worked out well:

The cheap voltage dimmer is pretty scary. I made sure the casing was grounded, but I’m still using rubber gloves to move the dial! It was worth doing though, I was able to get a roast to stretch out to longer using the dimmer to vary the temperature climb.

Redber Colombia Excelso Huila, 100g green, 84g out Yellowing was about 160c 2 mins in, first crack at 200c 9 mins 30, dumped at 210c 17.5 mins

‘bean’ temp curve:


The beans looked quite a bit more even out of first crack than with the unmodded popper. Still a bit uneven in the early stages though.

Next step is to get control of the dc motor and heating element separately. I’m hoping that having the fan up high and the heat low in the early stages will allow for a more even roast.