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[wxsig] N4XI X1W-1 kit and T238

sv2dof at raag.org sv2dof at raag.org
Wed May 31 15:59:02 UTC 2006

Quoting John Vause <jdv at iglou.com>:

> Anyone know if TAPR's N4XI X1W-1 barometer kit is compatible
> with the original T238?
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> *   John Vause  KF4ERV  jdv at igloudot.com                  *
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T238/T238+, for barometric pressure, reads a DS2438 10bit A/D and
interprets its value as follows:
P (mBar) = VADIN(Volts) * 14.42 + 931

Thus a barometer to work with T238/T238+ must meet the following:

   1. Based on a DS2438 10bit A/D and optionally but preferred, an
MPX4115A Pressure sensor
   2. A contitioning circuit has to be between sensor and DS2438 VadIN,
that scales sensor output according to next two rules:
         a. When Vad = 0V, it is interpreted as 931 mBar
         b. For every 1V increment in VadIN, the pressure is increased
by 14.42 mBar

Assuming that a conditioning circuit, as N4XI's is used and
MPX4115A/DS2438 are the core components, two adjustments have to be

   1. Vref is set at Sensor's Vout @ 931mBar (3.788V)
   2. Total gain is set at 15.10852 as explained below
      Sensor has an output of 4.59 mV/mBar and we need 14.42 mBar/V =
0.01442 mBar/mV or 1/0.01442 mV/mBar = 69.34812 mV/mBar. Total gain =
69.34812/4.59 = 15.10852
see http://www.tapr.org/~n4xi/x1w-1/sheet2.html

In N4XI's barometer the amplifier has a total gain less or equal to 10
(x2x5). In order to meet the above requirements R9, R10 and R11 have to
be changed. Second stage amplifier must have a gain at least up to x8
and a combined gain of up to x2x8  to set it at 15.10852.
A starting point is R9=4,7k, leave R10=1k and R11=5,6k

see also http://lists.tapr.org/pipermail/wxsig/2006-March/000188.html

73 de George, SV2DOF

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