# MiCS-4514 Gas Sensor with a FET explanation

Hi,

I am planning to use the MiCS sensor in a project, however I am confused my the circuit diagram that has been provided. I have attached a snap of the datasheet.

To my knowledge this device will increase or reduce the voltage on the output depending how much gas is in the air. What I don’t understand is why is there a fet after the voltage divide? Couldn’t you just have that going straight into a ADC on a MUC?

Also would a mosfet be more suitable?

Any help would be apprenticed.

Cheers,

As the subtitle of that schema with the FET says that’s the way to power the NOx sensor part and with the FET you can increase the current during pre-heating and once your sensor has reached target temperature you switch off the FET and lower the current for operating temperature.

This is not he sensing setup. That’d be the right schema.

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Hmmmmmm it seems my knowledge on this is wrong then. If you know could you explain the operation of this device in detail please?

@Link068, the “whole” circuit is obtained when you combine both circuits. This sensor has elements which vary their resistance based on the concentration of some gases. In order to operate the sensors need to be heated above a specified temperature. Thus the circuit on the left. The FET in that circuit, as @ScruffR pointed out, allows you to “accelerate” the pre-heating of the NOx element by allowing more current to flow through the heating resistors (82 ohms) after which it can be disabled for normal operation. The CO sensor doesn’t seem to need this.

The right side of the (whole) circuit creates a voltage divider between each sensing element and a resistor of no less than 820 ohms. The voltage is measured at the junction of the element and the associated resistor. As you may have deduced, the voltage at each junction is proportional the concentration of the (reactive) gases on each sensor.

So basically, you heat up the sensors to their operating range and read the voltage on the output of each sensor. FYI, many resistive type sensors operate this way.

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@Link068, it never hurts to look at the actual datasheet!