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Intelligent Toaster Project

Thu, 12/09/2010 - 16:33 — Michael J. Haney

"May I Just ask one question... would anyone like any toast?"

In so far as

 

  • most electrical components today are offered as surface mount technology (SMT)
  • fine pitch surface mount components require special care in soldering, to avoid solder bridges (shorts) between leads
  • high pin count SMT devices, and especially four-sided components (PLCCs, quad flat packs, etc.) can suffer from assembly skew if the leads are soldered "sequentially"

 

it follows that a bulk reflow mechanism is a desirable resource. Specifically, a device that can

 

  1. warm an entire (small) PCB to a target preheat temperature (perhaps 150 F)
  2. rapid heat ("flash") the top surface of the board to solder reflow temperature (e.g. 400 F) for a controlled amount of time

 

which will cause all of the solder joints to melt concurrently. The SMT component(s) will then be free to self-center on the copper lands of the PCB, courtesy of surface tension. Further, as long as the amount of solder (paste) on the board is minimal, the individual puddles of solder should collect on the individual copper lands, and not bridge across them. Also courtesy of surface tension.

In commercial facilities, this can be done with hot air, infrared, vapor phase, and convection ovens. Of these, the easiest to fabricate (personally) is the convection oven.

 

Mission statement

Adapt a toaster oven for computer control. Provide temperature sensing and heater element (on/off) control; develop a "temperature profile" editor (software) to allow the user to "draw" a target temperature profile for the oven to follow; develop real-time software to control the oven according to the profile.

 

First guess at required resources

 

  • toaster over
  • PC
  • temperature sensor
  • solid state relay (120V, 1300W switching capability)
  • small PCB to serve as cable connection point (with DB9 serial port connector)
  • (maybe) microcontroller, to sit between the PC and oven, to provide USB interface as well as watch-dog shutdown (failsafe mechanisms(

 

Note: the following content belongs on separate pages. However, at this time, I am unable to create visible, persistant content. But as long as I remember the URL to this page, I can keep coming back to it...

 

9 Dec 2010

Purchased 2nd-hand toaster oven from Twice is Nice, the First Presbyterian Church resale shop on Elm Street, in Urbana.

  • Hamilton Beach model 336. 1300W. $2

Spent 30 minutes with steel wool (and sand paper) removing burnt-on food residue. This unit will characteristically be operated at high temperatures; best to remove organics now...

 

Test run. 2"x1" pcb (scrap) with a 0.5" piece of lead-free solder (wire) on top.

  1. "Baked" until 250F (assuming that the front panel thermostat still works). 5 minutes.
  2. Cranked thermostat to "Broil". Solder melted with 30 seconds!
  3. Cool down (to the point that I felt was safe for a stranger to touch) took 20 minutes Frown

It may be desirable to position the PCB closer to the top heater element. However, it may also be wise to provide tin-foil "hats" for silicon, to avoid top-browning...

 

Need for next time:

  • oven mits
  • tongs (needle nose pliers are not adequate)
  • oven thermometer (to check front panel thermostat)