Heating Controller

---------- Forwarded message ----------
From: "/PN=Steven.S.AMOR/OU=ENG4/O=SHELL AUSTRALIA/"@shell.otc.au
Date: 29 Jan 97 09:30:32 +1100
Subject: Heating Controller
To: mike at microspan_com (Non Receipt Notification Requested),
        Aquatic-Plants at actwin_com (Non Receipt Notification Requested)

G'Day Mike,

I designed an undergravel heater a few years ago as a university project.  The
basic design could easily be modified to become a temperature controller.

The principle is that you use a thermister (probably no more than $5 if that)
sealed somehow in silicone gel which passes through a analogue to digital
converter (There
are many many different ICs which do this).  This  digital signal is then
by an LCD display for temperature readings.  There are many combinations of
this - 
some LCDs are able to take analogue signals and produce an output, others 
(more common) have a driver built in and others again need a separate driver
driver purely determines which 'cells' are active depending on the digital
input) -
e.g. a "1" needs two segments to be lit.

The thermister is simply a resistor that changes resistance with temperature
(all resistors do - it's just that a thermistor does it at a known rate). 
you add a bit of circuitry (ie op-amp) to make this progression more linear.

Then you just use a comparitor to compare the voltage that corresponds to the 
water temperature to the voltage that corresponds to the temperature that you
the water to be.  Add a bit of hystersis (ie +- say 1 degree to stop the heater

going on and off all the time)

I managed to get an accuracy of +- 1 degree over a range of 0 to 100 degrees

Regard Steve