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[Jörgen Karlsson]

Hi,

I have been thinking...

I think that the obvious choice is to use a Atmel microcontroller, these are
as inexpensive as the cheaper PICs and have the performance of the most
expensive ones. They are made to be programmed with a parallell cable, the
only electronic parts is three resistors and they are optional. Finnished
circuit boards are available from dontronics.com, check the simmm100. When
you are at Dons site check out the demo version of Bascom AVR, pretty large
code can be built with the demo version, up to 2KB code can be used with the
demo. That should be enough...

Since this is a diy project I think that one of the most importand aspects
is to keep the initial cost down, in production that is not a problem. By
using an atmel processor we _can_ get started with an AVR, an old printer
cable and a broken power supply for a computer... Lets say $10. Or we can do
it simple, we get a complete simm100 simstick from Dontronics, if you are in
scandinavia get it from the manufacturer directly, that is www.lawicel.se. I
think that the fully assembled board are around $35, the circuit board it
self are around $10. I think that it is beneficial to the project if no
programmer has to be bought or built, being able to get an assembled
computer for $35 is good too.

We cannot pwm anything across the pump cell, it has to be a constant flow
there, I think that the way to go is to filter the pwm output to get a
stable current. If the sensor ground is put at 2.5v above the circuits
ground the processor can drive the pump cell and filter circuit directly.

We know that Bruce and the guys used up towards 1.1A heater current, lets
assume that is max. Let someone else do the damage ;) First of all we filter
the pwm output from the mcu and then we use the filtered voltage to drive
the power transistor.

We also need to measure the current across the sensor, we only put a series
resistor and measure the voltage drop. By using a 'current mirror' we can
put the voltage potential of the resistor almost where we want it, if we
connect one of the pins to ground we can use a simple op amp circuit to get
the voltage drop over the resistor. We can probably omit the amplifier
completely,  but I am not sure. I think that we need to use the drive
transistor as one of the transistors in the current mirror, we might have
some problems with the headroom available if we don't.

Or we can use a more complex op amp circuit on the unmirrored heater drive
to get a ground referenced voltage across the resistor. This kind of
circuits are available in the LM124 data sheet from national.

We also need the voltage across the heater element it self, if we have the
current and the voltage across the heater we know the resistance, that we
can translate to temperature. I think that aiming for a set resistance here
is the way to control the heater correctly, If the resistance goes up we
must lower the heater current and the other way around. I think that we can
be pretty lay back while doing this, I think that we will use one voltage
across the sensor for initial fast heating, then at some resistance we drop
the voltage to prevent cracking, the correct values can probably be found in
the base shematic. After that we let the raising resistance of the sensor
take care of the adjustment, we just put two more regulation points, one
that lowers the voltage a bit when the sensor is close to maximum temp and
one that almost completely shuts of the current when it gets even hotter.

What do you think?

Jörgen Karlsson
Gothenburg, Sweden

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