Bread Machine Coffee Roaster Version 2

Update: This post has been converted into an updated, static page.

I’ve made my coffee roaster more self contained and easier to use by:

  • Moving the PID controller and solid state relay (SSR) into the unit
  • Mounting the thermocouple through the bread pan wall
  • Shortening the mixer arm
  • Separating the control of the heat gun’s fan and heating element

What’s in the Box?

Here’s a schematic diagram of the roaster,

Electrical Schematic
Electrical Schematic

and here’s a photo of the exterior.

Machine showing control panel and thermocouple plugs
Machine showing control panel and thermocouple plugs

I cut out the middle of the old control panel and bolted on a piece of aluminum to serve as the new faceplate.  To this I added the PID controller, main on/off switch, mixer switch, and the heat gun switch.  On the left side of the unit the thermocouple banana jacks are visible.

Back of Control Panel
Back of Control Panel

To make room for the SSR and heatsink I removed bread machine’s ventilation squirrel cage fan.

Cover removed shoing SSR, controls, and mixing motor
Cover removed showing SSR, controls, and mixing motor

When using a SSR it’s a good idea to either mount it to a thick piece of metal or to use a heatsink – they get pretty warm.  If you look closely you can see how its label has peeled up and warped; this is from when I ran the SSR bare on a different project.

SSR and heatsink.
SSR and heatsink.

The wall outlet is where I plug in the heat gun.  The wiring on this is really bad/unsafe.   I used the ground connector for the AC common and the main contacts for separately powering the heatgun fan & heating element.  If anyone ever plugs the heat gun into a normal outlet I think the element would turn on without the fan thus burning it out.  Also this would be putting about 10A through the ground conduit which is probably bad.  At the big box hardware stores I looked for custom 3-contact plugs but I couldn’t find any – my next McMaster order will include something like this.

Side of machine showing heat gun plug
Side of machine showing heat gun plug

I used a bus bar to try and keep the wiring tidy.  A note about wire gauges: I used 14 gauge stranded for everything leading into the heating element because it should draw about 10 amps.  otherwise I used 18 gauge stranded.  The DC stuff used 22 gauge solid core.

Wiring bus board used for splits
Wiring bus board used for splits

Fun fact: the heat gun switch (far right) is the switch off the heat gun.

Closeup of the control panel: PID, on/off, mixer, and heat gun.
Closeup of the control panel: PID, on/off, mixer, and heat gun.

Thermocouple

I got a bayonet-style thermocouple because it’s easily removable from the pan.

Bayonet style thermocouple
Bayonet style thermocouple
Exterior view of bayonet mount
Exterior view of bayonet mount

Here’s the inside of the pan with 8oz of beans.  This is the lowest I could mount the thermocouple without the probe hitting the mixing arm.  While it’s not immersed in the beans at rest, once they’re agitated they go over the top if it just fine.  In the end I feel that I’m getting a good, consistent read on the external temperature of my beans.

8oz of beans in the hopper
8oz of beans in the hopper

Heat Gun

I’m using a $10 Harbor Freight heat gun with separate power control for the fan and the heating element.  I separated them because I felt rapidly cycling the fan motor on/off would shorten its life.  So the motor is directly powered by heat gun switch while the heater has the SSR acting as a secondary switch on top of the heat gun switch.  I always run the heat gun in “high” mode because the low mode wouldn’t heat fast enough.

As mentioned above I moved the heat gun’s switch to the control panel .  Within the heat gun I simply wired the two power leads to the fan motor and element.

Mixer Arm

I cut off about 1/8″ of the mixer arm because its normal length was causing excessive bean binding between the probe and the arm.  Now when it runs I don’t hear any the loud pops caused by beans getting jammed.

The Mixer arm with 1/8" taken off
The Mixer arm with 1/8

Usage

To roast I simply stick the barrel of the heat gun into the pan as far it will go and let ‘er rip.  The thermocouple is in the top left corner of the pan while the gun is pointed in the bottom left.  I do keep the position of the gun the same for every roast so that the the pan heats the same way.  In this photo I’m experimenting with an advanced heat retention system.

How I hold the heat gun in the pan
How I hold the heat gun in the pan

Parts List

  • Hitachi HB-201 breadmaker from Goodwill
  • HarborFreight heat gun.  It’s not on their website but this is the $10 model with only a high and low setting
  • SET-620 PID controller, purchased off of eBay for about $35.  This is a made-in-China el cheapo unit but it works OK
  • 2 meter K-type bayonet thermocouple purchased from “Procon Products” on eBay
  • M12 x 1.5 hex nut used to secure the thermocouple mount.  I was able to get this at my local old-school Ace Hardware
  • The on/off switch is a beefy 20A-rated SPST toggle switch
  • The mixer switch is a DPDT switch leftover from another project.  I think it’s rated around 8A.
  • The SSR is another made-in-China eBay aquisition.  Do a search for “SSR heatsink” and you’ll see a bunch of listings for SSRs with matching heatsinks.

Conclusion

I’m pretty happy with this setup.  If anything it seems to be a bit underpowered but this could be alleviated by making a lid to retain heat (Updated).  I like it because I can keep an ear and an eye on the beans all for a pretty minimal investment.

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