Continuous alarm with MQ6 gas sensor

[embeddedlover]

We are using MQ-6 Gas sensor in one of our project. We tried to re-use the same circuit found in one of the sites.

Normally, the coil resistance is 30 ohms and when i provide supply, it goes above 500ohms. so, the output on "B' pin is high and counter starts operating.. Because of this, led is glowing and buzzer in ON.
This all with the gas burner being off and no gas in the air :roll:. Can someone help me why the gas sensor is showing that high resistance and thus giving an indication :?:

Please, note that i bought a new gas sensor (MQ6) and directly used it in the circuit.

 

[adi0803199]

I referred to this datasheet and https://www.sparkfun.com/datasheets/Sensors/Biometric/MQ-6.pdf from the datasheet provided and from my understanding of working of this sensor ,the sensitivity of this sensor depends on the temperature that is required for it to operate.From the data sheet ,the impedence of the heating coil in the sensor is around 33 Ω.Now if you increase the resistance that you connect to this heater branch, the current through the Ni-Cr alloy will reduce and the heating will reduce (Joules Law).So I would suggest you increase the resistance connected to the heater part. I don't know whether this will work.It is only a suggestion.(maybe around 10 mA).
Hope this helps.

 

[Jestin_cubetech]

Try this connection.

 

[embeddedlover]

At present, with external 100ohm and coil resistence of 33ohm 1.2V is getting supplied to the coil. But if we increase this series resistance the coil may not get sufficient current to heat up. With added resistance, we are limiting the current into the coil. what do you say?

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Try this connection.

I think current limiting resistors are required on input for other reasons.

 

[Jestin_cubetech]

is it a battery operated device ?

heater consumption is near 1W [<=900mW] . resistor dropping is not a good idea.
you can use lm317 based regulator instead of heater filament series resistor.

 

[embeddedlover]

is it a battery operated device ?

Yes, it is a battery operated application finally.. I guess you might be talking about the series element on the heater element.. And we removed 100 ohms.. have a series resistor only on the A branch..

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@Jestin_cubetech : any specific reasons for using 1K in the circuit connection suggested by you?

 

[FvM]

I don't use MQ-6, but the datasheet doesn't say anything about heater resistance rise to 500 ohm. It's rather unlikely that he resistance changes by this large factor.

The datasheet also clarifies, that 5 V should be applied to the heater, so the 100 ohm resistor shouldn't be there.

The sensor resistance will be much higher than 500 ohm, and further increase without heating. So the report in the initial post can't be brought into line with the datasheet. Either the real circuit wiring is different from the schematic or the sensor defective.

 

[Jestin_cubetech]

datasheet
1K it works fine.

 

[embeddedlover]

The sensor resistance will be much higher than 500 ohm, and further increase without heating. So the report in the initial post can't be brought into line with the datasheet. Either the real circuit wiring is different from the schematic or the sensor defective.

I could see a max. of 650ohm. I verfied the circuit and all the connections seems fine. The main doubt for me is why there is high on 'B' pins of the output, i could measure +1V. If at all sensor is damaged, i shouldn't see any voltage on output in.

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datasheet
1K it works fine.

This gave me little hint and i started tuning my load resistance. At some point of load resistance value (measured around 745ohm), the output current was not enough to switch on the NPN . Then i turned burner on and it caused the B voltage to go high.

The whole problem was with the output voltage was too high and even though there is no gas in the air, this voltage is causing transistor ON condition.

What is this pre-heating for 48 hours about? I feel MQ6 is working even without any heating. This is quite mis-leading, we may not know if circuit is not working because of this pre-heating or any issue with connectivity..

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thanks a lot Jestin_cubetech, FvM..