pbo april DIY gas detector electronic circuit - for electronically clever people

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tim_ber

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Just bought the mag for first time in a long time and saw the article about making one's own gas detector using a gas sensor MQ-6 FROM TECHNOBOTS 3801-301:

http://www.technobotsonline.com/lpg-isobutane-propane-gas-sensor-mq-6.html

Is this circuit any good? I am worried that the sensor has 5V across it to heat the sensor. I know nothing about conventional gas detectors, but would have thought that it is a good idea to have no electrical sources that could allow a spark anywhere near a potential leak.

Or is it safe? Or should I get a canary bird?

More interested in messing around with the electronics than having a gas detector, so I am most interested in what the electronic experts think of this.

I can't post the circuit because it is in the mag and under copyright of course.

Also, one would either have this permanently switched on (a siren sounds if gas is detected), or one would get on the boat, flick an outside switch (this circuit not being isolated via the main isolating switch, I guess, and by flicking this outside switch, would that not create a spark? There is no relay in this circuit.

Hope that made enough sense.
cheers
 
tim_ber said:
I am worried that the sensor has 5V across it to heat the sensor. I know nothing about conventional gas detectors, but would have thought that it is a good idea to have no electrical sources that could allow a spark anywhere near a potential leak.
Click to expand...

Given that conventional gas detectors have a cable running from control box to sensor, it seems likely that they use electricity. How else would they communicate, carrier pigeon? :)

tim_ber said:
by flicking this outside switch, would that not create a spark? There is no relay in this circuit.
Click to expand...

Hopefully you should never climb on board a boat to find it full of gas, because the bottle would be turned off while it's unattended. So you'd turn the detector on with the battery master switch (they're not usually wired with their own switches on the panel) when you get on board, before ever opening the gas bottle.

If you had forgotten to turn the gas off before leaving, and it leaked into the cabin while you were away, I would hope the smell of gas on opening the hatch would alert you.

Pete
 
tim_ber said:
Just bought the mag for first time in a long time and saw the article about making one's own gas detector using a gas sensor MQ-6 FROM TECHNOBOTS 3801-301:

http://www.technobotsonline.com/lpg-isobutane-propane-gas-sensor-mq-6.html

Is this circuit any good? I am worried that the sensor has 5V across it to heat the sensor. I know nothing about conventional gas detectors, but would have thought that it is a good idea to have no electrical sources that could allow a spark anywhere near a potential leak.

Or is it safe? Or should I get a canary bird?

More interested in messing around with the electronics than having a gas detector, so I am most interested in what the electronic experts think of this.

I can't post the circuit because it is in the mag and under copyright of course.

Also, one would either have this permanently switched on (a siren sounds if gas is detected), or one would get on the boat, flick an outside switch (this circuit not being isolated via the main isolating switch, I guess, and by flicking this outside switch, would that not create a spark? There is no relay in this circuit.

Hope that made enough sense.
cheers
Click to expand...

To make it safe enclose all the electronics in a sealed metal case. The measuring head is covered with a gauze which makes that safe.

Pretty standard method of flammable gas detection ... at one time the measuring element was a hot wire ( but safe under the gauze except for hydrogen) I guess this one is solid state.


I used to use MSA explosimeters ( Google) among other types. All built in a metal case to make them intrinsically safe
 
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The manufacturer of the MQ-6 Sensor is Hanwei: http://english.hwsensor.com/English/veren/Gas-Sensor.asp I have emailed them for a better data sheet, but there is one at SparkFun https://www.sparkfun.com/products/9405

My main concern is that the resistor selection seems arbitrary, the 15kΩ in the sensor feed assumes that the sensor also switches around this value at the required gas level, but I can't see confirmation of that in the data sheets. I would also like to see some decoupling on the sensor rail. There is also no calibration when the unit is powered, a short on the sensor output would give a false negative.
 
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All good stuff as usual.

I know I am getting old - I the cage around the sensor; I used to study / do a lot of work on that and I'd completely forgotten it all.

I do worry about my memory at times.

Nigel's stuff was excellent as usual, but way way above my level ("decoupling on the sensor rail") so a bit of research for me to do there. All good fun.

Going to make one after a visit to rapid electronics etc and give it a good testing.

cheers all.
 
cpedw said:
This kit http://www.quasarelectronics.co.uk/Item/4055-gas-sensor-detector uses the same type of detector; all the bits delivered to you for £25. But no box to put it in.
Derek
Click to expand...
Interesting thanks.

No fun just buying something though; much better to solder away, drip solder on places that don't need solder, dodge the lead fumes and so forth.

Take glasses on and off (yup, going long sighted now - too much close up work I reckon). Good product for comparison of course once I have made the PBO one.

I hope I can make it for less than that. In fact I have most of the parts already, just need the MQ-6 FROM TECHNOBOTS.

Thanks for the link.
 
I've read this article with interest and intend to make a couple with two sensors on each.
I've searched for the components on ebay and the internet and can get the dc-dc convertors from china via ebay for around £1.50 including postage, the resistors, capacitor, piezo and chip can be found at www.bitsbox.co.uk for less than a pound and the mq-6 sensor again from china on ebay for £2.50.

Having not a lot of electronics knowledge, I do have a question; I noticed there is another type of sensor, the MQ-2 which senses LPG/i-butane/propane/methane/alcohol. http://www.ebay.co.uk/itm/320891249468?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649 Would there be any advantage using this one over the MQ-6?
 
tim_ber said:
Just bought the mag for first time in a long time and saw the article about making one's own gas detector using a gas sensor MQ-6 FROM TECHNOBOTS 3801-301:

http://www.technobotsonline.com/lpg-isobutane-propane-gas-sensor-mq-6.html

Is this circuit any good? I am worried that the sensor has 5V across it to heat the sensor. I know nothing about conventional gas detectors, but would have thought that it is a good idea to have no electrical sources that could allow a spark anywhere near a potential leak.
Click to expand...

With any circuit diagram published in PBO, the established procedure is to wait for the correction article in a subsequent magazine. ;)
 
Okay, from the top:

1) Is it any good / will it cause a fire? The idea is that it will squeal loudly if there is any gas present and therefore you will not switch on (or off) any circuits etc etc. Relays are bad btw as is any switch. Yes, leave it on all the time you are aboard.
2) Detectors are no substitute for care and common sense. if it's making a noise there is a problem, if it's not making a noise there MAY be a problem !!!!
3) Thanks, Viv - I owe you one :-) Keeping me honest again...
4) Good ole Humphry Davy! Seems the element is resistive.
5) Would be nice to see a better datasheet - I couldn't get them to give me any info other than the two published websites but good luck - PM me if you get any response please. These have to win prizes for being the most obstruse datasheets I've ever seen. Is the resistor choice arbitrary, yes it is, but look at it the way I did: what you don't know is the value of PPM of gas the sensor will detect when it is in series with a 15K resistor. But you do know it will be between 10000 and 200ppm. 10% LEL for both propane and butane is about 2000 ppm and if you want to hit this number accurately then buy the Figaro sensor I refer to in the article. Otherwise accept that you may have a sensitive or insensitive sensor but it will still detect a leak. Calibrating the beast is beyond me (do you have a 1000ppm reference tank or something?) Again, if you want precision buy the Figaro sensor from RS (£20), use the same circuit but leave out ALL the resistors, RS has a good datasheet / circuit diagram.
6) Everything is resistive and fed from a stable supply so I didn't see a need to decouple the sensor signal. Shorts are a problem (why should you get one?) so test it every time you get on board and at random intervals between (we use an extinguished gas lighter)
7) Didn't see the kit but don't fancy it. It doesn't seem to address any of these points and you need a stable 12V supply so what happens if your batteries get a bit low? I bet it gets a bit warm too! I'm always sceptical of other people's designs though so don't let me put you off - just don't feel you are safer because you bought a kit! Shouldn't be lead in solder these days - hope you're not hooked on the stuff :-)
8) Don't know about MQ2 - I did read the blurb but since MQ6 is designed for LPG maybe stick with it?


David Berry
 
David,

Many thanks for the post here and the contribution to PBO. Without the contribution I wouldn't have a new project to work on.

I bet PBO are grateful too, because it made me buy a mag for the first time in MANY years.

cheers
Tim.
 
I've been considering Nigel's point about decoupling (filtering) the sensor signal line and I think he has a point. The circuit I put together has the sensor close to the chip so shouldn't need a filter for the line, however, if you build the circuit and position the sensor remotely (say anything over 2ft) then there is a possibility that the cable could pick up electrical noise or spikes which would best be dealt with. Luckily it's simple: put another 0.1uF capacitor in parallel with R1. You might have to bend the legs a bit to make it fit (depending on what the Maplin thing looks like) but this will ground any spurious a.c. For a remote sensor choose a screened twin cable. Connect the cores to the sensor element and the screen to ground (the green track on the board). Leave the screen unattached at the sensor end.

Have fun,

David Berry
 
I will do that, thanks, David, as I would have the sensor quite a way from the actual circuit board.
Many thanks,
Tim
PS About this bit: "For a remote sensor choose a screened twin cable. Connect the cores to the sensor element and the screen to ground (the green track on the board). Leave the screen unattached at the sensor end."

When you say : "For a remote sensor..." does that mean when the sensor is over 2 foot away as in your description.

Also, screened twin cable gave me this as an image:
http://www.rapidonline.com/audio-visual/heavy-duty-twin-screened-cable-62366/
obviously too heavy duty, but
when you say: "Connect the cores to the sensor element and the screen to ground (the green track on the board). "
Am I right in thinking that one core goes to one pin of the sensor and the other core to the other pin? (The 2 yellow wires in the mag picture) And twist the two screens together and solder to ground?

EDIT: JUST THOUGHT, DO YOU MEAN, WHEN USING THE SENSOR OVER 2 FEET AWAY, REPLACE THE YELLOW WIRES WITH TWIN SCREENED WIRE - sorry - being dense I imagine aren't I?

Thanks, Tim


whiteoaks7 said:
I've been considering Nigel's point about decoupling (filtering) the sensor signal line and I think he has a point. The circuit I put together has the sensor close to the chip so shouldn't need a filter for the line, however, if you build the circuit and position the sensor remotely (say anything over 2ft) then there is a possibility that the cable could pick up electrical noise or spikes which would best be dealt with. Luckily it's simple: put another 0.1uF capacitor in parallel with R1. You might have to bend the legs a bit to make it fit (depending on what the Maplin thing looks like) but this will ground any spurious a.c. For a remote sensor choose a screened twin cable. Connect the cores to the sensor element and the screen to ground (the green track on the board). Leave the screen unattached at the sensor end.

Have fun,

David Berry
Click to expand...
 
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tim_ber said:
EDIT: JUST THOUGHT, DO YOU MEAN, WHEN USING THE SENSOR OVER 2 FEET AWAY, REPLACE THE YELLOW WIRES WITH TWIN SCREENED WIRE - sorry - being dense I imagine aren't I?

Thanks, Tim
Click to expand...

Try this one http://www.maplin.co.uk/2-core-overall-lapped-screen-127 two cores with one overall screen. Take the screen to ground and replace the two wires going to the sensor with the cores. Yes, over about the two foot mark. Doesn't have to be Maplin of course, many sources for this stuff.

Cheers, David
 
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