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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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    Simon Tasman
    Simon Tasman on

    I thought of mounting the magnetic door switch on the hinge side of the door frame and the magnet on an arm attached to the door. Can you make up some nifty mounting arm on a 3D? I’m forced to use aluminium extrusions bent into shape, and it doesn’t look very good. White would be a good colour.

    Btw your gas alarm really saved my bacon. I was at a steakhouse on Friday night (spending a big piece of my salary) and I got an SMS on my phone about a gas alarm. I phoned my son and sent him to close the valve on the cylinder which I’d stupidly left open after I worked on the cooker. Seems like there’s a perished rubber somewhere. One spark and my neighbour would have had a large part of my Exclusive in his yard.

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    Fyko
    Fyko on

    From further research I’ve learned that predominant source of wear-and-tear gas leaks (as opposed to those from accidental damage) are caused by deterioration of the nitrile rubber O-rings around the spindles of gas control knobs on cookers,  fridges and especially on the securing thumb-nut of CADAC regulators. As the rubber hardens and maybe perishes the leaks grow bigger over time, and many of these O-rings are difficult find and hard to replace.

    Most caravans have their gas cylinders mounted out in the open air, but there are several models that have them located in the front storage compartment. In these cases there is an equal possibility of a gas leak developing in the main caravan as in the storage locker where the cylinder is located. And since these 2 areas are physically separated we really need to monitor for gas leaks in both of them.

    Consequently I’ve added a second gas detector channel to simultaneously monitor the storage locker as well as the main caravan interior and report accordingly. Since you have an Exclusive this affects you and might want to return the unit to me for upgrade. I’ll charge you for the cost of parts only, and if you estimate the cable length I can make it so it’ll be a very quick installation on your side.

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    Fyko
    Fyko on

    Sorry to take so long over the first part of your question. I’ve been working on the affected webpage. The pic is below but I’ll give you a link to the text so I don’t have to write it all again. http://www.dactyltech.co.za/vansecurity.html

    If you want one then just give me the A, B, and C dimensions. There’s no charge for you since you already have the rest of the system.

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    Simon Tasman
    Simon Tasman on

    Do you get any warning that the system is running out of SMS credits?

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    Fyko
    Fyko on

    Short answer: No.

    There’s only 2 ways you can find your prepaid credits:

    • By USSD conversation with the network
    • By checking your account on the network’s website.

    USSD (Unstructured Supplementary Service Data) is a real-time 2-way message channel which the SMS module in the controller does not support because there is no agreed fixed format for the messaging. Hence there is no way to program the firmware to handle it.

    Similarly there is no broadband or GPRS communication capability to access the website account, which would in any case be password protected to prevent exactly such access from a robot.

    You could take out the SIM card and put it in a phone to check the balance, but that risks upsetting something when you open the housing and start rooting around inside. The SIM card is located under the GSM module and it’s a bit fiddly to take out and put back.

    A 50 SMS bundle from MTN costs R17. Better to just be safe and give it a 50 SMS credit.

    I might just reiterate that the network will deactivate the SIM card after 6 months of no action on it – so remember to trigger a few test alarms every couple of months to stop them killing off the SIM card.

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    Simon Tasman
    Simon Tasman on

    Thanks.

    What about an ultrasonic detector for the side tent? Wouldn’t that work?

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    Fyko
    Fyko on

    Ultrasonic detectors are a bit different from PIRs, and come with their own set of potential issues.

    Whereas a PIR detects a moving heat source by virtue of the apparent pulsing induced by the Fresnell lens, an ultrasound detector sees anything standing within its range of view whether it is moving or not. It does this by emitting a very short high-frequency sound pulse, and then waiting for an echo to come back within a certain limited time. Since the ping will go out at the speed of sound (+/- 340 metres/second), and the echo from whatever this pulse hits will come back at the same speed, you can figure out from this delay how far away the detected object is (the way the sonar works in wartime submarine movies), and also determine that if noting comes back in the time the sound takes to travel 4 metres out (the practical range limit of these devices) and 4 metres back, then there is nothing there to worry about.

    On the other hand, if there is something there to cause an echo, be it an intruder, a chair, a table, or a tent wall, then there will be continuous echoes and continuous intruder alarms detected.

    So you see there must be nothing in the area covered by the detector, no chair, no table, no microwave, no tent wall. Consequently we have to limit the effective range of the device to 2 metres or less, which limits its coverage a bit, and we have to think before we attach the device at mid level to the caravan’s outer wall, to avoid placing it where it will detect anything that can cause an echo in the tent. From this spot we then have to drill a hole through the caravan wall and run a multicore cable to the control unit, without creating an eyesore inside the caravan. The best position is determined by where stuff is placed in the side tent, rather than the aesthetics of how it looks on the side of the caravan.

    Then there’s the matter of what happens when you take the side tent down. Because this device works with sound waves that cannot be impeded in any way it must have direct access to the air – it cannot be boxed in a weather-proof housing. No problem while the tent is up and protecting it, but a big problem when you go home in the rain, or the gardener hoses the caravan down when you get there. This device needs to be completely protected under a sealed cover anytime that part of the caravan is exposed to the elements, a hassle not everyone wants to accept.

    For people that understand these limitations we do offer an ultrasonic detector with a 2 metre range. Because it has a complex cable we connect it through a special plug, and it is treated as a special zone, similar to the gas detectors, but otherwise does the same thing i.e. generate an SMS, report on the mobile phone, and make a buzzer go beep.

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    Simon Tasman
    Simon Tasman on

    So what will prevent it from detecting the tent wall 3 metres away from the caravan sidewall to which it is mounted?

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    Fyko
    Fyko on

    The whole system is controlled by an MCU, a powerful little computer with loads of intelligence.

    The sonic pulse travels away from the transmitter at the speed of sound, about 340 metres per second. So after 3 milliseconds it has travelled about 1 metre, and after 6mS it has travelled 2 metres. Any solid object encountered 2 metres away will reflect some of the sound pulse, which will reach the receiver after another 6mS  – 12mS for the round trip.

    So any ultrasound reflection reaching the receiver more than 12mS after being sent must come from something more than 2 metres away. What the MCU does 12mS after sending the ultrasound pulse is it stops listening – therefore anything further away is ignored. It’s not a fade-out, it is cut off sharply by timing – the only fuzziness in the range is the result of programming variables inside the MCU.

    We established a 2 metre range because it give the best trade-off between useful coverage area and the need to keep that area free of reflective objects that could potentially be the source of false alarms. The practical range of the ultrasound detector is about 4 metres when the reflective object is both hard and large. The smaller and softer the object becomes the less energy is reflected, and with that less chance of detection, but at 2 metres a man-sized object is very reliably detected every time.

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    Fyko
    Fyko on

    UPDATE:

    We.ve now adopted a smaller, simpler housing that is weatherproofed all around except for the two sonic ports. These need to be open for the device to function but can be securely closed against the weather by just sticking 2 bits of tape over them. for the trip home and, if needs be, until the next use.

    The unit is light enough (25 grams total) that it can be attached to the caravan wall with mounting tape. Then it needs only a single 4mm hole for the cable into the caravan. So if you sell the van you only have one small hole to close, and some tape debris to remove.

    The one in the photo is mounted about midship at chest height. With a 2 metre range it covers the path to the door and protects stuff placed underneath it, like a table with a TV or a microwave on it. The cable enters the caravan in a closet and then goes down and through a partition to the dinette, so you never see it anywhere.

    This is a lot more effective than a pressure mat and, at around R300 for the extension, a lot less money as well. And it’s available locally.

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    Fyko
    Fyko on

    Simon Tasman on 9th Aug 2021 at 11:08 am
    Do you get any warning that the system is running out of SMS credits?

    There’s been a few others with the same concern so we did some work and now have an almost complete solution.

    It is still not possible for the GSM module to interrogate the network, and it never will be,  but we now sequentially number each SMS that goes out. The numbers range from 1 up to 200, and then they reset to 1 again.

    But you can reset the count back to 1 yourself from the mobile phone. So when you buy say a 50 or a 100 SMS bundle from the network you reset the counter to 1 and you know you’re OK until the counter nears 50 or 100 or whatever you prepaid at the network.

    Now you just need to remember how big was the bundle you purchased.

     

     

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    Simon Tasman
    Simon Tasman on

    How will you know whether there is network coverage at the resort, all the way into the bowels of the caravan where the box is mounted?

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    Fyko
    Fyko on

    Down on the left bottom you see the signal strength the GSM module receives from the network and sends on to the mobile phone app. It gets updates every 30 seconds but you call it up by tapping the button, because that button normally displays the battery voltage.

    I display the signal strength in dBm because that’s the way it comes from the module. If you aren’t too clear on whet dBm are then please read it on the website at http://www.dactyltech.co.za/vansecurity.html – I don’t want to retype the whole text here.

    If the signal weakens to -90dBm and beyond (towards -100dBm) it will become unable to send SMS messages. Generally you will get signal strength in the -70dBm region.

    Mounting the unit low down nearer the ground doesn’t in itself  weaken the signal appreciably, but the aluminium in the caravan walls getting relatively taller can increase their shielding effect. Plastic and fibreglass don’t attenuate RF signals.

    To solve the problem we offer antenna extension cables of various lengths that allow you to mount the antenna near the roof while keeping the control unit down under the dinette seat, where most people like to mount these units.

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    Simon Tasman
    Simon Tasman on

    Do you have any bracket or mounting that can to hold a magnet and mag switch at a window?

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    Deon Jacobs
    Deon Jacobs on

    This is a pic I lifted off their website: http://www.dactyltech.co.za/vansecurity.html

    I did a very similar thing on my windows. The windows are easier that the doors because you don’t get that thick rubber surround on the windows that you get around the door, you just screw that little box onto the window frame and stick the magnet to the window with mounting tape.

    If you want to leave windows open at night you can just disarm that circuit and leave the side tent and door circuit armed – nobody is going to get in through any window without waking you up, so you can gang all the windows onto a single circuit.

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