5.11. Analog Input
The GSM Commander provides a single 0-10.8V analog input, which can be used to measure incoming
voltages from sensor devices or power sources (example: battery banks). Using an appropriate
resistor, one can also connect 4-20mA sources to the GSM Commander.
(See application note under
section 18)
What makes this input so special, is that the GSM
Commander can interpret the analog value on your
behalf. It is perhaps the best to explain this at the
hand of a good example.
Suppose you have a water tank, with a level sensor
that is connected to the analog input of the GSM
Commander. The sensor is such, that when the tank is
empty, the voltage is 1.2V, and when the tank is full,
the voltage is 7.8V. The configuration software allows
you to request the current reading in a scaled format.
You would thus tell the software that 7.8V should correspond to 5000 litres, and 1.2V should
correspond to 0 litres, and leave the rest of the calculations to the GSM Commander.
You can now receive a message from the GSM Commander, detailing the actual contents of the tank
in litres, instead of just a raw value.
(Which you would otherwise have had to convert back to
“real” numbers yourself)
.
Please note:
This feature assumes a linear relationship between the input voltage and the desired
output units. Thus, this will not work very well for a cylindrical tank that is mounted on its side,
as the voltage from the level sensor is not linearly proportional to the actual contents of the tank.
The user is also able to define the message to precede, and the message to follow the analog value.
The output of the circuit above feeds to an Analog-to-Digital converter with a reference voltage of
0.98V, and a resolution of 8 bits. The full-scale input voltage to the GSM Commander is therefore
10.8V. The range of the analog input can easily be increased by the addition of a resistor in series
with the analog input.
Below are some examples of resistor values, and the resultant input ranges.
RESISTOR
MAX VOLTAGE
RESISTOR
MAX VOLTAGE
1K Ohm
0 – 11.8V
18K Ohm
0 – 28.4V
3.3K Ohm
0 - 14.0V
22K Ohm
0 – 32.3V
6.8K Ohm
0 – 17.4V
39K Ohm
0 – 49.0V
10K Ohm
0 – 20.6V
56K Ohm
0 – 65.6V
15K Ohm
0 – 25.5V
68K Ohm
0 – 77.4V
The configuration software assumes no series input resistor, when defining behaviour statements for
the analog input.
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