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From: "Jack Denver" <nunuv...@netscape.net>
Newsgroups: alt.coffee
References: <lYha6.145498$P82.17780626@news1.rdc1.ct.home.com> <bnka6.334541$_5.75702984@news4.rdc1.on.home.com>
Subject: Re: "Aftermarket" idea
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Date: Sat, 20 Jan 2001 19:03:59 GMT
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Looking up at my wall, I just thought of what may be an easy way to kludge a
prototype...use an electronic thermostat meant to control electric baseboard
heaters.

Like this one (the first one that showed up in a search):
http://www.energyautomationinc.com/products/thermostats.html

"These thermostats are equipped with an electronic controller to send only
the right amount of power required by the heating system to maintain the
desired temperature. This state-of-the-art technology offers the smallest
temperature variations ....Regulation: 0.3 °C (0.5 °F) .....1250 watts @ 120
volts
10.4 A [maximum] resistive load ."
"

Unlike furnace thermostats, which only control a low voltage relay, these
devices are meant to directly control line voltage electric resistance
heating elements .

Of course, they are built to work in the range of 50 to 80 degrees F  and
not at 200F, but I'm guessing that this could be overcome quite easily.
These thermostats generally take their temperature readings from a
thermistor.  What they really are are ohmmeters, with resistance as a proxy
for temperature.  Therefore, all that's needed is to "fool" the device into
seeing its expected range of resistances, but have those resistances instead
correspond to the desired control range for the espresso machine, by means
of a different thermistor and fixed resistors.  I think that a couple of
hours playing with an ohmmeter would get you there. The digital readout
would of course be wrong...it would still say "70 degrees" but you would
know that 70 corresponds to 200, 71 to 202, etc.


"Jeff Cooper" <jeff.coo...@home.com> wrote in message
news:bnka6.334541$_5.75702984@news4.rdc1.on.home.com...
> Cool idea.  I like the digital temp display.  While you're at it, you
could
> have both an adjustable setpoint for getting the temp exactly right and
the
> actual temp display.  Another possibility is a two stage heating circuit
> instead of the typical on/off, instead full/half/off.  This plus a
> microcontroller with a relatively simple fuzzy logic algorithm could give
> very precise temperature control as I suspect overshooting is the main
> limiter to accurate temp control with a cheap thermocouple circuit.  The
> adjustable setpoint and external temp display makes this idea appealling
> even to HE home users.  Price is obviously an issue, so I agree that a low
> cost version with no external displays is needed.  Perhaps it could even
be
> offered in kit form, i.e. sell the plans to build it and the option of
> purchasing an assembled one.  Of course the ability to interface through
> RS-232 to my PC and program temperature ramps when the brew switch is
> activated would be the ultimate... but that's extreme.
>
>