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hacking a smoke alarm

Malcolm_McGrath

Nov 1, 2023
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I have created a home alarm system that consists of an arduino, blue tooth board a LCD screen and 4x3 touch panel, it currently monitors two circuits and triggers an alarm if the circuits become open. The arduino is wired up to two custom circuit boards.

If the alarm is triggered the arduino sends a message to the base mobile phone by BT which in turn passes that message on to the rover mobile phone and an alarm is triggered

The mobile phone app is MIT App Inventor App.

I would like to add detect and pass on messages from smoke detectors and to that end I tried using a rf sniffer but had no great success so my latest though is to simply stick a rf smoke detector next to the uno and trigger an alarm by tacking a circuit off of the piezo buzzer. But am a bit unsure about how to do it.

Thought 1 is to add an opto coupler to the piezo line and then just connect an alarm circuit to the other side , but I figure that the piezo side is not just power on but fluctuates in some manner so maybe I need some sort of "latch"



 

crutschow

May 7, 2021
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You said smoke detectors, so one way would be to add a microphone and amplifier to detect the alarm sound from the detectors.
A simple analog band-pass filter would likely need to be added to suppress other than the alarm frequency (typically between 2kHz to 3.5kHz).
Also have a delay in the Arduino responding so it ignores any momentary noise in that frequency range.
 

kellys_eye

Jun 25, 2010
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What country?

In the UK (well, in Scotland for sure), fire alarms have to be 'linked' i.e. one goes off, they all go off. Accordingly they are all wireless (or use a signal through the mains power cabling). Mine are after-market units and fully wireless.

You may be able to 'sniff' the protocol and adapat through that process. It could also be as simple as detecting an increased current flow when they alarm.
 

Malcolm_McGrath

Nov 1, 2023
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A microphone and band-pass filter seems to me a little over complicated, the smoke alarm has two wires coming from it to its speaker, when they speaker is on there is some sort of signal/power going down the line, so my thought is that i could use that as the trigger. I suspect that an optocoupler is not enough though, the alarm side is going to be oscillating? so it would perhaps be sensible to turn that oscillation into a steady voltage, maybe the band-pass filter thingy, but I do not know how to do that.

My tech expertise is maxed out at, connect a led to a resistor, lets try this resistor, oops, wrong resistor.
 

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Malcolm_McGrath

Nov 1, 2023
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What country?

In the UK (well, in Scotland for sure), fire alarms have to be 'linked' i.e. one goes off, they all go off. Accordingly they are all wireless (or use a signal through the mains power cabling). Mine are after-market units and fully wireless.

You may be able to 'sniff' the protocol and adapat through that process. It could also be as simple as detecting an increased current flow when they alarm.
Australia.
My smoke detectors are wireless 433 Mhz , I tried using a rf niffer and had mixed results. I can send and receive rf and I can sniff rf but I could not work out how to capture the signal from my detector and am concerned about how much overhead is involved in adding a potential rf capture program to my monitor program, which is why I was hoping an opto coupler might do the trick, there will be a current of some sort and all it has to do is close a circuit, not additional programming.
 

Bluejets

Oct 5, 2014
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Thought 1 is to add an opto coupler to the piezo line
Mains connected smokies have a green and a red LED, one for power indication and the other for alarm/zone indication.
All should be interconnected and audibly alarm on smoke condition as suggested above but only the dodgy zone alarm shows red flashing LED.

One could opto this red LED ..........BUT........ I'd hate to think what insurance companies would make of any smoke alarm hacking, especially in Aus. if the aforementioned smokies are your main legal smoke alarm source.
I imagine you could play to your hearts content on a separate system not relied upon for main detection and alarm.

Just be aware the red flashing LED also activates (allbeit on a different , slow flash rate) when the internal backup battery is below safe level.
Piezo also beeps at these intervals.
Some smokies will also give a brief red flash every 1 to 2 minutes to show alarm is monitoring correctly.
This latter condition could be ignored in the code.
 

Malcolm_McGrath

Nov 1, 2023
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This is NOT a mains connected smoke alarm, rather a wireless interconnected smoke alarm. My hack will simply monitor the peizo line on a SPARE detector, it will in NO way affect the efficacy of the alarm system. This alarm has a led on its board but I don't wish to touch the board, I am the worlds second worst solderer, I only want to monitor the wires to the piezo buzzer.

This particular smoke alarm, Diamond, is a 433 Mhz wireless photoelectric device, they interconnect with a wireless remote, supposed to be good for 10 years.

The intent is, if the smoke detector goes buzz, a circuit gets closed, optocoupler or something, that closed circuit is detected by the arduino the arduino talks to the blue tooth module which talks to the base cell, the base cell sends a message to the rover cell, the rover cell then alerts the user who probably then swears mightily.
 

Bluejets

Oct 5, 2014
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This is NOT a mains connected smoke alarm, rather a wireless interconnected smoke alarm.
Either way, if it part of the required smoke alarm system for the dwelling, and you "fiddle" with it, and insurance find out under any inspection, either before or after any incident, you can be certain they will wipe any insurance claim.
That would be the least of your problems as building requirement departments whichever operate in your area, would definitely impose their own checks and balances which could see legal action against you.

Arduino as well, don't encourage in any way, any item used in what may be considered a life threating situation.
Not difficult to imagine how they would consider one "fiddle" with a regulation smoke alarm system.
 

Malcolm_McGrath

Nov 1, 2023
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A few points

1 The smoke alarm is not a wired system it is wireless
2 I have said that I will be using a SPARE smoke alarm
3 The SPARE smoke alarm basically becomes a 433Mhz rf sniffer that can tell that the alarm has been set off and will send a notification to my arduino.
4 The alarm system is in no way compromised by the SPARE system that just happens to be LISTENING to the same circuit. There are another half a dozen smoke detectors on the same network.
5 If in the unlikely event what I am doing was to trigger an alarm on the SPARE it cannot NOT warn me it can only set the others off.
6 The hack is an introduction of an optocoupler on the piezo circuit, the input side of an optocoupler is passive, it cannot provide a current so it cannot effect the circuit, except by a slight additional resistance in that circuit.

As it happens whilst waiting for some advice on how to do what I want to do I got myself some 4N25 optocouplers and did a quick test with an UNO to see if the signal that triggers a peizo buzzer can be used on the input side of the optocoupler. And it can.

The diagram on the left shows the connection for the optocoupler via a switched 3.3v input to the optocoupler to switch on an off a LED, the UNO is simply a power supply,

The diagram on the right shows the connection for the optocoupler using a piezo circuit as input to the optocoupler to switch on an off a LED.
 

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AnalogKid

Jun 10, 2015
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A microphone and band-pass filter seems to me a little over complicated,
Not as complicated as you might think:


ak
 

Malcolm_McGrath

Nov 1, 2023
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Not as complicated as you might think:


ak
Thank you!
 

Malcolm_McGrath

Nov 1, 2023
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Don't see that it's significantly more complicated than using an RF sniffer or hacking into the alarm.
I had thought that what you had suggested might be more complicated than adding the circuit board that AnalogKid above has referenced.

The board at 1.70 is actually cheaper than the optocoupler that I bought yesterday at 4.90 (cheaper from China but didn't want to wait).

I had a look at a few utube vids and implementing seems it would not be too difficult, but it's another library and more code to be included into my program, maybe another day,

The optocoupler hack works. it's what I have handy so it's what I will use in the short term, but I will get a LM567 from ebay and have a play. Maybe a future improvement might be, "House alarm, on", but that might need a different gadget.
 

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DanielCross

Nov 3, 2023
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To add smoke alarm detection to your Arduino-based home alarm system, you can use an optocoupler to isolate the Arduino from the smoke alarm's circuitry and protect it from damage. Connect the optocoupler's output to a digital input pin on the Arduino and write a sketch to monitor the pin. If the pin goes high, the Arduino knows that the smoke alarm has been triggered and can send a message to your mobile phone by BT to trigger the alarm. You can also implement a latch to prevent the Arduino from sending multiple messages in a short period of time.
 

Delta Prime

Jul 29, 2020
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To add smoke alarm detection to your Arduino-based home alarm system, you can use an optocoupler to isolate the Arduino from the smoke alarm's circuitry and protect it from damage. Connect the optocoupler's output to a digital input pin on the Arduino and write a sketch to monitor the pin. If the pin goes high, the Arduino knows that the smoke alarm has been triggered and can send a message to your mobile phone by BT to trigger the alarm. You can also implement a latch to prevent the Arduino from sending multiple messages in a short period of time.
Regenerative AI.
Variational autoencoders?
Me thinking...
 
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