From: Steve Turner
Subject: Re: Humidity sensor - peculiar behavior
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Date: Sun, 05 Jan 2003 05:16:21 GMT
NNTP-Posting-Date: Sun, 05 Jan 2003 05:16:21 GMT
Organization: AT&T Worldnet
James Meyer wrote:
>On Sun, 05 Jan 2003 01:37:01 GMT, Steve Turner
>>I've looked at both the sensor output and the follower output with a
>>scope. The sensor output shows some noise, perhaps 10 mV p/p, plus
>>very sharp spikes of about 400 mV p/p at about 5.9 KHz. The follower
>>output shows a 5.9 KHz fundamental signal as well, attenuated (maybe
>>50 mV p/p), full of harmonics with a decaying envelope. I don't know
>>if any of this is relevant to anything. It is definitely not square
>>wave and looks nothing like oscillating op amps that I've seen in the
> To me, that indicates that the sensor element is probably being driven
>with AC and its output is being converted to DC.
> Is there any information, datasheet?, for the whole sensor? I looked
>briefly at the Honeywell web site and didn't see one.
You're referring to an equivalent schematic for the sensor itself?
Not that I've been able to find. As I mentioned elsewhere, the
Honeywell web site and their device documentation leave something to
be desired. The actual sensing is capacitive, but the remainder of
the sensor operation is completely unknown to me.
> One other observation... Have you tested more than one of the sensors?
>A sample of one isn't very scientific especially when the sample doesn't do what
>you think it should.
I agree entirely. Problem is, the sensors are close to $40 apiece.
On a hobbyist budget that's a bit stiff. I've not had more than a
single specimen available for testing.
What seems suggestive to me is that the initial sensor reading is
right on the money at three different humidities. You just can't
leave the sensor powered up. :) But you're right, I have no way to
prove that this isn't coincidental.
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