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From: Steve Turner
Subject: Re: Humidity sensor - peculiar behavior
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
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Date: Thu, 09 Jan 2003 02:47:47 GMT
NNTP-Posting-Date: Thu, 09 Jan 2003 02:47:47 GMT
Organization: AT&T Worldnet
Roger Johansson wrote:
>The most obvious solution seems to be to separate the sensor from the
>circuit, and only insert the sensor into the jar.
>But I guess you have some good reason for not choosing that solution.
Well, it's a good point. The most important reason for not using a
naked sensor was to incorporate a voltage regulator. This will
eliminate voltage drop to the sensor due to the ~30 feet of cable that
will connect it with the data collection system (although the 200 uA
power consumption of the sensor would make this insignificant anyway).
More importantly, the vreg would lend a certain degree of overvoltage
protection to the sensor. I've got sensors running every which place,
and some of them work on 12v and some work on 5v. Accidentally
feeding the humidity sensor with 12v would be a $40 accident. So
sticking in a vreg and designing the sensor board to be fed with 12v
eliminates this possibility.
As far as the follower, this was also envisioned at least partly as
[short circuit] protection for the sensor. Also, since I have no idea
what the output impedance of the sensor is, the op amp gives a low
output impedance, which I felt was desirable from the standpoint of
feeding that 30' length of cable. Feel free to correct me if I'm
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