DuinoMite Project: Low cost Humidity measurement with SMTHS07 capacitive sensor


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SMTHS07 is low cost EUR 5.00 capacitive Humidity sensor from Smartec. The datasheet of this sensor is here.

The sensor change it’s capacitance linear in range 20-95% with 0.6 pf/RH.

Now the question is will DuinoMite be able to detect so small capacitive changes of 0.6 pf?

Let’s do some experimenting first. I made small schematic on breadboard-mini with two resistors and the sensor:

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then wrote small BASIC code:

10 PIN(1)=0 ‘sense pin will first pull the capacitor to GND and discharge
20 A=0 ‘accumulator
30 FOR I = 1 TO 1000 ‘measure 1000 times and average to remove noise
40 SETPIN 1,8 ‘discharge the capacitor
50 SETPIN 1,1 ‘start charging
60 A = A + PIN(1) ‘accumulate the readings
70 NEXT I
80 A = A / 1000 ‘now divide by 1000 to normalize the result
90 PRINT A ‘let’s see the result
100 GOTO 20

the result was steady:

> run
1.04212
1.04209
1.0416
1.04169
1.04169
1.04163

if I approach the sensor with wet fingers the reading go to
0.355777
0.347614
0.34956
0.355954

So the sensitivity is quite good! All I need is calibration of the sensor, but how to do this?

How to Preciese calibrate SMTHS07 is explained here .
Poor’s man calibration is to put the sensor it in refrigerator, the humidity in refrigerator is always 100%. As SMTHS07 is linear knowing the capacitance in just one point is enough to calibrate it.

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