Hello @ all
i have a question and i hope somebody can help me!
which sensors can i use with k8055 and where i can bay it
temperature sensor
and
moisture sensor
maybe digital or analog
Thanks Alex
Any sensor that has a voltage output.
Hi,
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Measuring temperature is easy using a suitable thermistor for the desired temperature range.
Unfortunately measuring relative air humidity may get rather tricky and inaccurate.
The usual humidity sensors are based on a variable capacitor like the Humirel HS1101 (costs only $5, so not unreasonable to get one for playing around with it). The datasheet says that it has a capacitance of 180 pF at 55% RH and then about 0.34 pF per %RH above or below. That is a tiny capacitor with an even smaller humidity dependent range.
There are two (relatively) simple ways to measure its actual capacitance and derive the RH from that. At least in theory. Both are based on the idea to charge the capacitor, then let it discharge over a resistor and measure how long that takes. That can be done by using an output port to charge it, turn the output off and use an input port to measure how long the capacitor can hold it in high state. At the sampling interval of the K8055, this won’t work since the discharge resistor would have to be in the tens of mega-Ohms if not higher, at which point the input resistance of the K8055 ports would completely mess up everything.
The other (and more often seen) method is to use the capacitor with suitable resistors in an Astable 555 configuration, which then outputs a frequency that depends on the capacitance. The higher the capacity, the lower the frequency. So now we need to implement a frequency counter.
The average 555 based solution uses frequencies in the 4 kHz to 10 kHz range (any configuration that generates lower frequencies would have the 555 input impedance interfere with the circuit). Those frequencies are way beyond what the K8055 Experiment board can count. I have reliably counted up to 280 Hz with the K8055. So in order to use such a frequency setup, the 555 output signal would have to be routed through a frequency divider to get it down around 250 Hz.
Assuming the output of the frequency divider is driving a K8055 hardware counter, the program would reset the counter, wait a few seconds, read the counter and divide it by the number of seconds waited to calculate the frequency. The capacitance range of the HS1101 however will limit the accuracy of the whole thing, since +/-1 Hz of that input frequency now equals about -/+2% humidity.
The third circuit you probably want to have is a piezoresistive pressure sensor to build a barometer. That will take up the second analog input of the K8055.
Building such a “weather station” for the K8055 would still be a cool project. Even with the expected inaccuracy of the humidity measurement.