[Weather] calibration for 1.2a version Bray barometer

[Steve Hendrix]

At 2007-04-21 23:23, you wrote:
>I am seeing Vad fluctuate all over the place as the unit just sits 
>untouched on the bench with just power and the 1wire.

Without a good look at the unit it's hard to say, but I can offer two 
suggested possibilities:

1) An open or almost-open connection, like a cold solder joint or a 
cracked trace. My favorite tool to track down such problems is a 1K 
(or 10K, or 100K, or etc. depending on the drive circuit) resistor. 
Analyze the circuit to see what effect 1K to Vcc should have, and 
what effect 1K to ground should have. Then try it. If the result 
differs markedly from expected, go halfway back from the output to 
the source, re-analyze, and test again. At some point you'll get the 
expected result. Now you've narrowed the problem to somewhere between 
the test point that gave the expected result and the nearest one that 
didn't. I've found cracked traces that way, where the crack was so 
small that it took a microscope on very high magnification, and 
knowing exactly where to look, in order to see the crack.

2) A nearby radio station. I happen to be about 2 or 3 miles from one 
of the most powerful AM radio stations on the planet. I've been able 
to receive audio on a pressure sensor before, with enough oomph to 
drive a piezo earphone. If you can get a scope on the Vad input you 
can get a much better idea of what's going on. Since the pressure 
changes you're looking for occur very slowly, you can eliminate 
virtually all interference with aggressive lowpass filtering. Keep 
all wires very short, and hang some capacitance across the pressure 
sensor and various intermediate points and see what effect it has. 
I've got a 40V 0.1 Farad capacitor I pulled from some old equipment, 
that will quiet down just about anything, at least up to a few hundred KHz.

Hope these ideas help!

Steve Hendrix