# Resistive sensor reading

I have to read the resistence of three different electrochemical gas sensors. The ranges are:

1. 3k - 60k Ohms MiCS 2611 for O3 reading
2. 100k - 1.5M Ohms MiCS 4514 for NO2 reading
3. 0.8k - 20k Ohms MiCS 4514 for CO reading

Some suggestion for an easy to use circuit? I think to interface this sensor board with an atmega8.

• a part number for the sensor would eliminate any false starts – Scott Seidman Jul 9 '13 at 17:57
• Just something to note; the lower the resistance the higher the concentration of gas, the signal produced is very noisy (perhaps as much as +/- 20%) and its not a linear device. – JIm Dearden Jul 9 '13 at 18:48
• Where did you get the sensors? – starblue Jul 10 '13 at 20:12

## 1 Answer

I'll go for the low hanging fruit here.

I suggest voltage dividers for each of the three sensors. Be sure to use stevenvh's formula for determining your other resistors for each divider: Resistor formula for voltage dividers

In short:

$$R = \sqrt{R_{min} * R_{max}}$$

The formula will help you obtain the greatest range in your Vout.

Then read your voltages with the ADCs in the AVR.

• I read the voltage divider is so much noisy. What about the Wheatstone bridge? – Maicol Jul 9 '13 at 21:48
• You have to be careful here. Electrochemical sensors can require bias currents which will add another contraint to your system. – DrFriedParts Jul 9 '13 at 21:51
• @Maicol The gas sensors mentioned are inherently noisy, both due to their physics and due to the vagaries of target gas molecule distribution. The noise levels involved are so significant that resistive divider noise is irrelevant in comparison. – Anindo Ghosh Jul 10 '13 at 10:22