From: "Dr. David Kirkby"
X-Mailer: Mozilla 4.78 [en] (X11; U; SunOS 5.9 sun4u)
Subject: Re: indoor outdoor thermomoter
Date: Mon, 06 Jan 2003 16:40:09 +0000
NNTP-Posting-Date: Mon, 06 Jan 2003 16:40:16 GMT
Organization: ntlworld News Service
> I also, have been looking for that type of "gadget"
> It's easy to design, just a pain to put it togethers.
> A couple of opamps to use as amplifiers for the sensors
> ,one more opamp as a analog comparator, a relay, two matched sensors
> and a power supply.
> If I design it cam you put it together ?
I don't think it needs to be that complex ? Two identical thermistors
connected in series across a supply would give half the supply when
the two are at the same temperature and more/less when they are not.
Two identical resitors across the supply will always give half the
supply voltage at the centre point (basically making a Wheatstone
Just use one power op-amp as a schmitt trigger, comparing the voltage
between the two resistors, with the voltage between the two resistors,
with some hyterisis (otherwise it will be foreever switching on/off).
Use the output of the power op-amp to drive the relay. So the
component count is
power supply (unregulated).
two termistors as sensors (identical)
two resistors (identical)
one power op-amp
A few resistors around the op-amp to give hysteresis.
The only issue I see is the hyteresis would be a constant voltage, not
a constant temerperature. However, given a room temperature is not
likely to vary wildly, this should not matter.
Dr. David Kirkby,
Senior Research Fellow,
Department of Medical Physics,
University College London,
11-20 Capper St, London, WC1E 6JA.
Tel: 020 7679 6408 Fax: 020 7679 6269
Internal telephone: ext 46408