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Gas Alarm with no Buzzer?

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    Deon JacobsParticipant

    I see DactylTech’s latest offering is a gas detector add-on that displays gas alarms on a mobile phone and sends out SMS’s, but no mention is made on the website of an audible alarm buzzer.

    Am I missing something?

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    Fyko
    Fyko on

    Electronic gas detectors, at least those that are commercially available, can only detect propane/butane in concentrations above about 1000ppm, at which level you or I will be gagging and passing out. If your ears were to be within audio range of a gas detector’s buzzer your nose would be aware of a gas leak at a concentration a lot lower than the detection threshold of the detector.

    The device that we offer as an add-on to the caravan mover remote control is intended to alert you to a dangerous gas situation when you are some distance away such as at the beach or the supermarket – far out of reach of the smell or the sound of a buzzer. You should then know to return to your caravan with plenty of caution and as a first step close the gas cylinder valve, and maybe open the door for ventilation, before retreating to a safe distance while the gas dissipates.

    The alarm indication on the mobile’s screen is also not intended to be much more than a convenient way to test the operability of the detector, since the same conditions apply: if you’re within Bluetooth range of the caravan you’ll be able to smell the gas that’s leaking.

    To test the detector you just emit a whiff of gas from an unlit butane lighter – you get an immediate reaction on the screen and, if the SIM card has sufficient prepaid credit, an SMS within a few seconds.

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    Simon Tasman
    Simon Tasman on

    It does in fact have a buzzer.

    What it doesn’t have is a klaxon or a siren. The buzzer is essential for testing the system – something you should do quite often if you’re going to depend on it to stop your caravan blowing up or your family being immolated.

    What it also comes with is a 4 zone intruder alarm that uses the same annunciation – buzzer, mobile screen alarm and SMS to any phone of your choice. Enough zones to cover two doors and 2 PIRs, one for inside the van and another for the side tent. Quite a must-have today in my opinion.

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    Fyko
    Fyko on

    Yes it has a little audible buzzer, not loud enough to disturb the neighbours. You can’t go letting sirens off in a caravan park when there’s a possibility of false alarms.

    The little buzzer allows you to check the gas detector – just the tiniest whiff from a braai or cigarette lighter, which takes a very few seconds, tells you the detector is working and an sms will follow within a short time.

    The intruder alarms can also be tested very easily by momentarily flipping off the zone disarming switch. You don’t need anything audible for that since there are condition LEDs to tell you what’s happening.

    The most likely source of system malfunction is from the prepaid SMS credit with the network running out. There is a SIM card with a phone number and you have to maintain a prepaid credit of about 50 SMSs to be on the safe side.

    Regular testing also keeps the SIM card from being cancelled by the network through lack of activity. They do this after about 6 months of idleness.

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    Simon Tasman
    Simon Tasman on

    A normally closed alarm circuit conveniently lends itself to a very simple method of perimeter protection.

    I learned to do this twenty five years ago when visiting the Matusadonna reserve on the southern shore of Lake Kariba. A herd of bachelor elephant would come through the camp 2 or 3 nights a week to pick up fallen maroela berries and this one single- tusker old bull would get it in his head that I had oranges hidden away.

    I’d wake up with this single tusk waving around under my awning while the trunk sniffed around in every nook and cranny searching for his favourite fruit.

    I started stringing a single thin copper wire, the kind they use for the primary windings of small transformers, around the trees surrounding my camp at knee height and connecting a normally closed circuit to a small buzzer. Just loud enough to wake me up a let me hide in the Landie while he did his searching.

    He’dbreak the wire and this would  drop out a relay which would power the little buzzer. Unless elephants are breaking branches they make no noise at all, and you can get a nasty skrik when you wake up with an elephant’s trunk sniffing half a metre from your face.

    These days I sometimes string a trip wire even in a formal caravan park if the security doesn’t look too thorough.

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    Fyko
    Fyko on

    Thanks Simon, that’s a very nice story. The reason I did not put a loud-noise device in the system is that sooner or later, whether through negligence or abuse, there’s going to be false alarms disturbing other people, and I don’t want to be the agent of that misery.

    The common scenario that we face is not with elephants in the camp seeking oranges at night, but with everyday two-legged thieves stealing whatever is not nailed down during the daytime while you’re at the beach or the supermarket.

    Since most caravan resorts are within the cell net  SMS is the most effective communication tool, and it’s available to everyone. Once you get the SMS it takes only one phone call to the resort owner (on speed dial) to get Security over to your camp to check on the situation, which is more effective than any siren could ever be.

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    Deon Jacobs
    Deon Jacobs on

    What can I do about securing the side tent?

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    Fyko
    Fyko on

    The side-tent does pose a bit of a challenge because firstly, there’s so many ways of getting in to it, whether by unzipping something, crawling under the wall, or just cutting a hole and stepping through, and secondly, a PIR will generate endless false alarms when the canvas flaps in a wind. So no magnetic switches or PIRs will work.

    Industrial grade security systems detect 4 different states of any alarm node: Short Circuit, Safe, Alarm State, and Open Circuit. Because this is a DIY environment we have to trim this back to 2 states to avoid excessive complexity. So Short Circuit becomes Safe, and Open Circuit becomes Alarm State – that way a cut wire or bad connection shows as an Alarm State and demands your attention. Magnetic switches and PIR units all provide a Normally Closed (N/C) pair of contacts to denote Safe, and open those contacts when an intrusion is detected. Well and good.

    The only  way to effectively protect a side tent is to lay a pressure mat under the groundsheet at strategic spots – places where an intruder would be expected to walk or stand while doing his thieving. The problem is that pressure mats are inherently Normally Open (N/O) devices – there are 2 grids of conductive material held by apart some mesh, and which touch each other when a weight forces them together, thereby closing a contact.

    To cater for this we provide the option of connecting a N/O device such as a pressure mat to an ‘Alt Zone 4’ pair of inputs instead of the regular Zone 4 inputs. This allows you to use a pressure mat without any other adaption or intervening device to translate N/O into N/C. Also since the mat have an N/O output several of them can be connected in parallel, so you can lay as many as you want under your groundsheet and connect them all to the same pair of terminals on the control unit.

    Each of the 4 zones is covered by a disarming switch and this switch also disarms the pressure mats. The normal procedure is to set the arming switches before you go out and then arm the entire system from your mobile phone when the last person has left the caravan/ side-tent and the last zip is done up. On your return you fire up the mobile phone app and disarm the whole system before entering the secure area, (which can even include a trip wire since that is inherently N/C).

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    Deon Jacobs
    Deon Jacobs on

    Thanks, sounds good.

    So where can I get a decent pressure mat? Those made locally are just rubbish.

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    Fyko
    Fyko on

    Sorry Deon,

    After a week nobody has come forward to offer you a local pressure mat. The best suggestion I have is that you look at SK630 SOLO, which is sold by Amazon  an will cost you about R700 landed here. It comes with a buzzer, which you will use elsewhere or just turf out.

    Here’s a pic of my unit as they ship now:

    BTW the switch at bottom left is to silence the buzzer. The yellow LED shows a disarmed zone, the red an ALARM and green for SAFE.

    Instead of a mat you may consider using a beam, not a PIR which does not work on a beam (unlike what the sales blurb and certain ‘experts’ will tell you) but detects a varying infrared source. The pebbled lens ensures that a moving heat source will vary its intensity as seen by a single infrared detector inside the PIR. The changing value of the heat source as it moves sets off the PIR detection. A static heat source is ignored.

    A beam transmitter on the other directs a stream of IR radiation into a receiver, and when that stream is interrupted, such as by a person moving his body through it, the receiver notifies of this interruption and an alarm condition is created. It’s a bit more of a hassle to align the components so that they see each other but you might be able to set the transmitter and the receiver on the same pole and use the outside wall of the caravan as a reflector so the two parts can see each other.

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    Simon Tasman
    Simon Tasman on

    Where would be the best place in a caravan to mount a PIR – looking for the most protection with the least false alarms?

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    Fyko
    Fyko on

    Let me give you some background info which will enable you to figure this out for your own unique caravan situation.

    Infrared radiation is electromagnetic radiation in the 0,78 micron to 1000 micron wavelength band that is emitted by an matter (black body) that has a temperature above 0 Kelvin, or -273 C. That includes pretty much everything. The wavelength of the infrared is determined by the temperature of the emitter – hot stuff emits short waves, and cold stuff emits long waves. The human body at 37 C emits in the 10 micron range.

    Passive infrared detectors have a single photocell that is sensitive to the 8 – 14 micron wavelength range, and a field of view about 120 degrees wide. Since everything in the caravan is warmer than absolute zero (-273 C), but normally slightly cooler than the the human body, there will be a steady influx of infrared radiation to the photosensor that will vary from day to night as well as from hour to hour. Any human bodies in the field of view of the sensor will contribute to that influx. You can see that the radiation level can vary greatly but will always be present to some extent.

    What the PIR sensor must do is detect a change to that background radiation level such as would be caused by an intruder moving around in the field of view, but not the change caused by the sunshine moving slowly across the furniture  as the day passes. This can be a bit of a challenge since the total radiation emitted by the intruder will not add substantially to the total radiation flying around inside the caravan and bouncing off all it’s interior surfaces.

    The way the PIR does this is with a Fresnell lens, which act on light the same way as a pebbled glass window pane on a bathroom – it refracts the light waves differentially depending on their position. Putting this lens between the sensor and the radiation source will vary the intensity of the inflowing radiation with the position of its source. So a moving body will create a wave of incoming on top of the steady state from whatever is not moving.

    So to get the best sensitivity to moving bodies you should position the sensor where those bodies are likely to move across the field of view rather than towards or away from it. A radiation source moving directly towards or away from the sensor will not be detected. The height of the sensor should be at the geographic center of the heat source i.e. belly height.

    To avoid false alarms there should be a minimum of other moving radiation emitters in the field of view of the sensor – like flapping curtains or tent sides. Remember – everything is a radiation source, and every radiation source that moves or changes rapidly is identified as an intruder.

     

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    Simon Tasman
    Simon Tasman on

    That’s very informative. Thank you.

    But how difficult can it be to detect 95Kgs of moving meat at 37 deg C? Surely there should be sufficient heat radiated to be easily detected even coming straight ahead?

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    Fyko
    Fyko on

    Your PIR has a few challenges to face, here’s some of them:

    You know that everything everywhere that is not at absolute zero (0 Kelvins) emits infrared radiation at some wavelength or other. The interior of your caravan on an average day will be at 296 Kelvins (23 C). That means that from the entire 120 degree field of view of the PIR it’s being bombarded with IR within its sensitivity range. Remember the PIR has only one receiver, so all inputs are aggregated into one photovoltaic output.

    Now along comes you with your healthy 90Kg bulk at 310 Kelvins (37 C), which is only5% higher than the background temperature, a very small increase. Not only that but at 4 metres from the PIR you occupy only 7 degrees of arc, which is about 6% of the PIR’s field of view. So when you appear on the scene the increase of energy seen by the PIR increases by  only 0.3%.

    But wait, there’s more – you’re not naked, much of the heat-emitting real estate is covered by clothes, leaving only your face, arms, and maybe your legs as uninhibited emitters of IR, so that the 0,3% increase probably drops to about 0,1%. Bear in mind that light colours of clothing radiate less readily than dark colours, and shiny surfaces emit less than rough ones.

    And that’s on a not-too-hot day. If it was 37 C inside the caravan there would be no temperature differential between you and the background so you would be totally invisible to the PIR no matter how much you moved.

    So, to get the best from your PIR you should:

    • Limit the breadth of its field of view to only the area you expect to see the intruder. This will maximise the radiation differential when an intruder does appear because he will be occupying a greater portion of its viewing arc.
    • Have the PIR look at the least emissive background you can find, like the white fridge door or a white curtain (one that doesn’t move in the wind).
    • Know that on hot days the intruder will be less detectable than on colder days.
    • Know that intruders wearing dark apparel will be more easily detected than those wearing cricket whites.
    • Know that there is a better chance of detecting an intruder if you can force him to move across the face of the PIR, where the Fresnell lens will cause an apparent flashing to the photovoltaic receiver, which will see a repeated rapid change in input as a result.

    It’s not a good idea to place too much reliance on PIRs alone. They are useful, but only in conjunction with magnetic switches on doors and windows.

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    Simon Tasman
    Simon Tasman on

    What about vibration switches on the windows? Will they work?

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    Fyko
    Fyko on

    To reduce to frequency false alarms we have a switch de-bounce delay of one second, which means that all inputs of less than 1 second duration will be ignored. Consequently vibration switches cannot be used on windows. They would need some additional device to detect a series of vibrations and translate that into a continuous input.

    We do have a device that attaches to the clamping strut that hold the window open, which detects when the window is being forced, so you can go out leaving the window a little bit open for ventilation and still have protection. Though not from monkeys, when there are monkeys about you need to close everything up really tight.

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