Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, "demolitron" <ahardinger@...> wrote:
>
> --- In Homebrew_PCBs@yahoogroups.com, DJ Delorie <dj@> wrote:
> >
> > 
> > Anyone hack digital temperature control into a laminator?  My GBC 9"
> > laminator seems too cool for toner transfer and too hot for photofilm.
> > 
> > I'm pondering adding something to mine to let me control and monitor
> > the temperature more accurately, using a K thermocouple where the old
> > temperature switch is and a triac to power the heater.  Add an MCU,
> > LCD, potentiometer, and external enclosure...
> > 
> > Anyway, anyone done this before?  Any gotchas I should be aware of?
> >
> 
> 
> Personally, I love this project as I seem to want to MCU everything
I see!
> 
> One advantage to a MCU control would be precision.  With a static
> temperature sensor the heater would turn fully on till it reaches the
> set point + hysteresis and then turn fully off till the temperature
> drops below the set point - hysteresis.  This means the temperature
> actually swings, sometimes this is very little sometimes this is a
> lot, above and below the set point.
> 
> With an MCU you could implement a Proportional Integral Derivative
> (PID) algorithm and apply a PWM signal to the Triac giving very fine
> and nearly linear (depending upon the PWM frequency) power control to
> the heater element.
> 
> Using the SPI thermocouple interface would eliminate the need for an
> MCU with an ADC as well as the commensurate fussy analog signal
> conditioning.  If you wanted to save even more cost you could replace
> the LCD with three 7-segment LED displays and replace the POT with two
> switches, one up one down.
> 
> 
> I don't know what your MCU background is so please forgive me.  The
> math for a good PID control is a bit complex for a little 8-bit MCU
> but well within reach.  I've done several 0.18 degree per step Brushed
> DC Servo Motors with all digital PID positioning on 8-bit mid-grade
> PICMIcro MCU's with 2kHz+ update rates.
> 
> I would recommend, as a starting point, the PIC16F677 MCU from
> microchip.  It is a 20 pin device, has ample program space, RAM, and
> EEPROM storage as well as an internal 8MHz clock including a hardware
> SPI interface. The cost is under $1.50 so that makes it nice.  I like
> microchip because of the huge community of hobbyists and the large
> amount of development software available for little to no cost.
> 
> Here is a link regarding 7-Segment LED Displays and how to drive them
> with an MCU...
> 
> http://www.melabs.com/resources/articles/ledart.htm
> 
> Hope you give it more thought, MCU projects really are fun.
> 
> Aaron
>

Here is a link regarding the interfacing of Thermocouples directly to
the MCU, this could also be used to reduce the cost... 

http://ww1.microchip.com/downloads/en/AppNotes/00844a.pdf