Carbon monoxide sensor for garage.

[cupoftea]

Hi
We wish to design & install a CO sensor in a working car garage.
(eg using CO sensor and opamp(s).)
There are no circuits online showing CO detection.
Though do you know what is the nature of the main types of CO sensor?
Ie, do they have impedance which varies with CO?
Or do they have capacitance which varies with CO exposure.
Or do they put out a voltage dependent on CO exposure levels?

I am speaking of the two terminal ones only.
I cant find a datasheet which tells how any of them work....do they work like a photodiode and have increased current when exposed to CO?

OK finally found one....but why is the circuitry around the CO sensor so simple....in the schem our consultancy gave us it is a very intricate opemp with feedback cct

https://www.ti.com/lit/ug/tidubx8c/tidubx8c.pdf

The actual feedback loop has a large number of components and switchable transistors in it etc. It also has positive and negative feedback paths.

 

[ahsrabrifat]

It will be easiest to make a carbon monoxide detector using an MQ-7 sensor and an Arduino.

MQ-7 Carbon Monoxide Sensor: Datasheet, Pinout & Working - The Engineering Projects

Today, we will have a look at the MQ-7 Carbon Monoxide Sensor, will discuss the MQ-7 Datasheet, Pinout, Working, Features, Applications etc. in detail.

www.theengineeringprojects.com

Carbon monoxide (CO) is not usually present in fresh air therefore, the sensor does not provide any signal until it senses any leakage or danger. Just like other MQ sensors, MQ-7 also has a sensitive element made of ceramic that has a layer outside it. This layer consists of tin dioxide (SnO2) and has the feature of absorbing the target gas. In the case of MQ7, the tin dioxide absorbs the carbon monoxide and indicates its presence as soon as it comes in contact.

 

[FvM]

MQ-7 sensor has cross sensitivity against combustible games, electrochemical sensor is more sensitive, accurate and has low power consumption.

 

[cupoftea]

Thanks, i know you mean "gases".

..if one observes the schem on page 17 of the TIDA-00756 in the top post, it is pretty simple....but the circuit that our consulatncy has done for us is way way bigger than this.....it has more than 6 times that component count. Would you say that page 17 schem of TIDA00756 is a very over-simplified schem?

In the circuit that they have done for us, i am trying to trace the actual feedback circuit of the opamp....(they use one opamp like in TIDA00756)...but it seems un-trace-able....it seems to go off in every direction with all kinds of subcircuits thrown into it. I just spent an hour trying to re-draw it but still cant get the hang of it. So do you say that the TIDA00756 schem is over-simpliifed?

The output of the cct is read by micro ADC, and when the micro wants to read the output, it appears that it pulses a FET on/off at the output and this fet shunts the signal to ground and the fet is in series with a BJT which also has its base capacitor coupled to the output. The Micro output that pulses the cct when it wants an adc reading also pulses an RC cct to ground which is partially across the CO sensor.......the sensor is not to ground, but is on top of a divider voltage...like its biased up above ground (maybe they originally used a opamp which wasnt R2R input?
Anyway, what say you about this consultancys cct?...does it sound laudible?
Also, their cct only uses a 1n feedback cap on the opamp. Also, the RF appears to be some 800kR.
But their look to be some other Rs in pllel to this and are switchable.
There are also other weirdness's about the circuit but i harp on too much already.

So , whats wrong with the TIDA00756 CO sensor cct.....is it over-simplified. ?

 

[FvM]

So , whats wrong with the TIDA00756 CO sensor cct.....is it over-simplified. ?

Nothing wrong, just appropriate.