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Version 1.73 Copyright © 1997 Link Communications Inc. 1/18/97
One more example will illustrate how to develop a conversion that is not listed in the chart. Let
us assume that we want to read the wind speed at our site and that we have an anemometer
(wind speed detector) that reads 10 volts at 100 MPH. Since the maximum voltage is above 5
volts we will need to use a voltage divider. To avoid having to disconnect the pullup resistors
on inputs 1..3 (which is necessary to use a voltage divider with them), we will use input #5 and
make our own external voltage divider. The divider will have the input voltage coming through
a 40.2K resistor in series with a 10K resistor. The other end of the 10K should be hooked to
ground. Input #5 should be connected to the point where the two resistors are joined, where the
voltage should be about 1/5 what it is at the input of the voltage divider. We will assume that 0
volts are produced when the wind is not blowing and that it increases linearly from there. That
sets our zero point to be zero. We must calculate our full scale reading as if it will occur at 25
volts, even though the sensor will never put out that much voltage, because that is the 5 volt full
scale reading at the processor times the 5:1 voltage divider ratio. What we need to know is how
fast the wind would have to blow to give us a 25 volt signal. A simple ratio will give us the
answer:
100 MPH = X MPH
10 volts 25 volts
Cross Multiplying: 100 * 25 = 10 * X, 2500 = 10 * X, X = 250 MPH
We now know that the low point is 0, the high point is 250, and that we need to use the voltage
divider.
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