--- 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.htmHope you give it more thought, MCU projects really are fun.
Aaron