Hi! Please send your questions to the mailing list, too, so others might benefit. See below for (hopefully) an answer to your question.
> -----Original Message----- > From: [email protected] [mailto:[email protected]] > Sent: maandag 16 april 2012 11:33 > To: Michiel Konstapel > Subject: Re: [Tinyos-help] Error marging using REFVOLT_LEVEL_2_5 to > read a portADC > > Dear Michiel. > > Thanks a lot for your answer. I'm trying to measure a Soil probe > attached with a telosb mote. I'm sending 3V from the Exclusive Digital > I/O 2 (GIO2) telosb pin to the Soil probe. It is entering to the soil > probe as the excitation input, then the soil probe returns its sensor > value to the mote, this value is entering to the Analog Input 0 (ADC0) > in the telosb mote. > > I was reading that according to the soil probe manual it must receive > an excitation in the range of 2-5V and for 2V 2mA for 10ms > approximately. The value that the soil probe returns to the telosb is > in the ranges between > 10 and 50% of the excitation voltage, then is less of 3V. > > When I measure the soil probe when it is in the air, I must get values > in the ranges between 375-475. Against what reference? 2.5V, or against whatever you're using as the excitation voltage? > But when I put a pair of new batteries AA (3.0 > V) the value I get is approximately of 570. But when I put a pair of > batteries used with 2.5 V the measure is better I mean now is 470. For > this motive is my doubt. The reference voltage is generated from your supply voltage, so once your batteries are down to 2.5V, it won't be able to generate a 2.5V reference voltage anymore. I'm not sure what the minimum Vcc for the voltage generator is, you'd have to check the data sheet (http://www.ti.com/lit/ds/symlink/msp430f1611.pdf). To check your measurements, just measure the output of the soil sensor with a volt meter, and convert your ADC readings back to a voltage and see if they match: ADC = Vin / Vref * 4096 Vin = ADC * Vref / 4096 For example, your measurement of 470, with the 2.5V reference, would mean the input voltage was Vin = 470 * 2.5 / 4096 = 0.29 V If your volt meter tells you a very different value, something's wrong, somewhere. Since the soil probe has an output voltage that's relative to its excitation voltage, it's probably easier to use your battery voltage as the reference. To select that, use sref: REFERENCE_AVcc_AVss in your ADC config. Then, it'll work all the way down to 2V, and you can calculate the percentage output, X, directly, independent of what your actual battery voltage is: Vin = ADC * Vref / 4096 X * Vbat = ADC * Vbat / 4096 X = ADC / 4096 For example, if you're reading 470 on the ADC, that's 470/4096 * 100% = 11.4%. Best, Michiel _______________________________________________ Tinyos-help mailing list [email protected] https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
