I’ll be interested to see how you handle the feedback loop in software because of the extreme lag in response from the skillet: it’s not like a position sensor on an actuator where you start moving it and stop when the feedback says you’ve got to where you want to be. Overshoot could be a real problem.
Actually, thinking about it a bit more I’d have taken a totally different approach. I *love* the engineering in your servo-controlled temperature knob, but what I’d have done is put a mains-rated relay in the supply line (could even be just an X10 module if you don’t want to touch mains) and turn the skillet control to max. Then use the temp sensor as input and just turn the skillet on and off to stabilize it around the target temperature, since that’s all the in-built temperature control does anyway. Attempting to manipulate the built-in control puts you an additional step removed from the output you’re measuring so you have to deal not just with the hysteresis of the heater coil and temperature sensor but also the inbuilt controller itself.
most people do a relay/triac with thermocouple type thing. i wanted to try this out.
reflow isn’t that delicate, theres a bit of leeway. i’ve been doing ‘hand reflow’ by watching my multimeter and turning the knob slowly. now just cuz i’m doing something don’t mean its the right thing to do. im going to try this out & report back.
Nice video. Always interesting to see the thought processes people are going through…
Also jealous of your Epilog laser and the nice space that adafruit occupies. I really like the skylight.
Comment by Robert Boerner — March 27, 2009 @ 12:00 pm
You might check out this instructable. He puts together a uC hot-plate controller that’s not quite as nasty as some AC solutions that you may not want to touch (literally). And it’s even ATtiny2313-based.
I’m not personally a huge fan of steampunk/servopunk solutions when a pure electronic one is the more natural fit, but if you added some drawdio-style audio to match the servo motion, it’ll be huge with kids…
im heading towards using an SSR, i bought one for $10. i found an SSR chip from mouser but its like $6 and after the heatsink and protection circuitry its about the same price.
anyways itll show up soon. i hope is zero crossing, if not i can pretty easily make the 0-cross circuitry.
Understanding it’s relative, I never though household circuits as HV, but an interesting solution to deal with something that you said you where uncomfortable with. I never knew an electric skillet could be used to reflow, so I may get around to trying SMD projects. Garage sale season is around the corner, if the $ store doesn’t carry inexpensive electric skillets. Thanks for taking the time to detail your projects, and posting them to the web.
It’s a photoelectric lamp control. I buy them at my local hardware store. You plug it into a standard outlet and then plug your load (up to 300 watts) into it. I glue a rubber grommet over the photocell, then glue a green LED into the grommet. LED ON = AC OFF.
Don’t laugh – I hatched a number of button quail eggs in an incubator improvised from a crock pot using this and a simple diode temperature sensor circuit.