Re: [ATM] Electronics help needed - temperature control

[Mark Holm <mdholm@telerama.com>]

Don,

I have been looking at your dew heater circuit as shown at atmsite.org.
 It seems that to make it work for fan driving, one needs two changes

1. The response of the thing needs to be reversed, so that it comes on
when the temperature difference between air and mirror rises.  That is,
when (air - mirror) > threshold output curent turns on.

2. very likely, most people will want the circuit to run a two-speed
system rather than an on-off system.  Couldn't this be done by putting a
shunt resistor around the power transistor to carry the low speed
current?  The shunt resistor would probably have to be sized for a
particular fan, and would have to have enough power rating for the job.

I don't have enough experience with op amp circuits to know how to do
the logic inversion while maintaining the Schmidt trigger feature.
Could it be as easy as just moving the temperature gain adjust pot over
to the other sensor?

Mark Holm
mdholm@telerama.com

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