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37

A "denial-of-service" wireless attack is very easy. It will disrupt radio communication between sensor and panel. Hopefully, the panel is smart enough to detect that one (or more) of its sensors has failed to report-in. A non-reporting sensor should be assumed under attack. Ask your supplier what protocol is followed if your panel reports that a sensor has ...


35

Yes, WiFi and Bluetooth can disturb each other. But both are equipped to handle that. A standard that is not capable to handle disturbance and/or interference will simply be unusable under many circumstances. The 2.5 GHz ISM band is also used by Microwave ovens and other wireless standards like Zigbee. Wifi transceivers are able to detect when certain ...


27

Oh, but they do interfere! There are several mechanisms in play permitting the sharing of the airwaves by the various radio sources mentioned - the keyword being multiplexing, in its various flavors. Frequency bands: Different RF devices use different "bands" of frequency, which are typically allocated and governed by the relevant local authorities, e.g. ...


21

Your schematic is excessivly large and layed out in a confusing way, which discourages people from responding. Don't draw grounds going upwards, for example, unless the parts really are coming from a negative voltage. If you want others to look at a schematic, give them some respect. Don't make us tilt our heads to read things and make sure text doesn't ...


20

First of all, to clear some things up: All digital signals are built up by analog signals. As already mentioned in the comments, all wireless communications can be jammed, encrypted or no. And last but not least, jamming is not the same as hacking into. Jamming is just "stopping" the signal. Now, if the alarm central is good, I would expect it to expect a ...


19

Summary In a balanced signal both wires carry the signal, with one wire the negative of the other. In the receiver both are subtracted, giving the double of the positive wire's signal. If both wires pick up a disturbance this will be cancelled by the subtraction. Twisted wires reduce the inductance because the field inverts with each half twist. An ...


17

[ added 2_D resistor_grid methodology for exploring shielding topologies ] You want that IR receiver to respond to photons, not to external electric fields. Yet the photodiode is a fine target for trash from fluorescent lights (200 volts in 10 microseconds) as the 4' tube has that restrike-the-arc action 120 times a second. [or 80,000 Hertz for some tubes] ...


16

Wifi uses a listen before transmit system. If the channel is busy, it holds off transmitting. Eventually it gets through. Each channel is fixed. If it tries to use a channel that is busy, from bluetooth, then it will wait. This may reduce the data speed for wifi if it has to wait too much. Bluetooth for over a decade now uses adaptive frequency hopping (...


15

What the schematic does not show is the huge common mode 60Hz hum and how the grounds are interconnected with proximity to AC lines & power line transients. Your example is a good example of how not to interface long wires. May I suggest in future; prefer twisted pair prefer balanced lines avoid high impedance inputs prefer matched impedances avoid ...


15

A number of techniques are used, often in combination. The available frequency spectrum is divided in a large number of bands that can each be transmitted and received independently. This is how radio stations and your WiFi can operate undisturbed by other (nearby) radio stations and WiFi sets. (Frequency division multiplexing) Cell phones aren't called ...


14

Developing a workable RF communications protocol is apt to be a tricky but educational exercise. A few additional points to consider beyond what's been said: On some radio hardware, it takes a lot of power to listen for a signal. With many if not most small radios, listening for a second is going to take more energy than transmitting for a millisecond; on ...


13

FCC Title 47, Part 15 section 5.b should clarify this for you: (b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions ... that interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiator, by industrial, scientific and ...


13

The follow-up question... but what I don't understand is why the flow of electrons that are physical entities result in emission of these EM waves Why does "radiation" occur? Let's look at this specifically, because it is a common (and excellent) concern. Here is a simple wire, instantly connected to a voltage source: simulate this circuit – ...


12

You should not expect any problems, other than the same problem any piece of metal poses. The magnetic field does not mask or influence any GHz transmissions. TV, Radio, 60Hz power lines should not pose a problem either. Think of the magnet as 0Hz; a DC electromagnetic field that is stuck in the magnetic form, with zero electrical component.


11

Instead of a rigorous mathematical treatment, here's a somewhat hand-waving explanation: Any wire has a magnetic field around it (perpendicular to the length of the wire) when there's a current flowing through it. However, efficiently launching an electromagnetic wave also requires a voltage drop (E field) at right angles to the M field (along the length of ...


10

SPI bus at 50MHz can easily run a couple of inches thru a few vias without hitch. Wavelength of 50MHz is 6 metres but realistically because fast edges are used you need to think ten times faster. Even so that's a wavelength of 60 cm. Rule of thumb is keep tracks smaller than a half of a quarter wavelength (other folk will use other rules of course) and this ...


9

The reason RS-422 is so much better at rejecting interference is because it is what is known as a Balanced Line transmission system. In a balanced line system the signal is transmitted twice. Once as a normal "positive" signal, and once as a "negative" signal. Imagine if you will a signal that is made up of the binary digits 01110010 (we will ignore any ...


9

1) As much as possible. If possible, nothing should be to the side of the device for as many mm as possible 2) This depends on the antenna used and its radiation pattern (where the lobes are). You want to interfere as little as possible with these lobes 3) Any metal should be kept out. Copper traces could make things worse since it would cause noise and ...


8

If you're not using a standard protocol for this then you're going to have to design and implement one, e.g. a simple example: before transmitting, a node should listen to check that the channel is free if after transmitting a message no acknowledgement is received, the node should wait a random period of time and then try again, up a some maximum number of ...


8

There are two issues with crosstalk between conductors of a cable: capacitive coupling and inductive coupling. Inductive coupling happens because current flowing thru a wire creates a circular magnetic field around that wire. This also works in reverse. If a wire is subjected to a changing circular magnetic field around it, a voltage will be induced. ...


8

A simple solution may be to leave the doorbell system how it was before you started this project and just use the micro controller to kill power to the door bell speaker with a relay when you don't want to hear the bell. Update I'm not very familiar with doorbell systems (other than a quick google) so the following assumes you have at least two conductors ...


8

This is a simplistic answer to suit people who describe themselves as newbies in radio Imagine the radio spectrum to be your hi-fi playing music. If you had a graphic equalizer on it you could do obscene things to the tone of the audio like just enhance stuff at 1kHz - slide the 1kHz control to max and reduce all the others to minimum - this is how a radio ...


8

Putting this in an answer just because it's easier to type here. I did a similar automation project at home like you describe, so I'll share how I felt when the project was finished. I did custom boards with a TI MSP430 @ 900Mhz, just point to point with a MSP430 with ethernet as the access point. I wish I had picked 433Mhz, I expect it's range would have ...


8

The wire has inductance and, the longer the wire is the more inductance it has. An inductance likes to maintain the current flowing through itself so, when your switch opens, the small stored energy in the cables magnetic field tries to maintain current flow and produces a sizable voltage (aka back-emf) in doing so. This can easily exceed the maximum voltage ...


7

It sounds like the price was more a function of the brand name than good design. There are two possibilities: The noise is coming in via the audio cable or into the circuitry of the amplifier directly over the air. The first is relatively easy to deal with. Try putting a 100pF or so ceramic cap accross the audio leads going into the speaker system as ...


7

If the bulk of interference is coming from circuit connections (a schematic would help), you can either add inductance to the connections to filter out high-frequency feedback or attempt to isolate the discharge circuit and control & monitoring circuit. Adding inductance can be as simple as wrapping wire around a ferrite bead. Care must be taken to ...


7

From the comment "Could you please explain how your loop was created? Through an answer with some pictures? It would be helpful to know." Erm. It's embarrassingly boneheaded actually. I don't have the PCB - I think I jumped on it and I never checked the original design file into source control. I just have a partial backup: That file is partially routed - ...


7

Single-ended signalling is the source of your troubles. Differential signalling will drastically improve crosstalk and noise. RS-422 would be an appropriate choice. I2C is not ideal, as its bidirectionality (and, consequently, weakly-driven signals) does not lend itself well to differential signalling. However, it seems you only need one direction. And if ...


7

The usual problem with interference from GSM cell phones is caused by rectification of the strong RF signal at the semiconductor junctions at the input of the amplifier. GSM uses Time Domain Multiplexing to share the RF frequency with other uses - it only turns the RF signal on for a short time at a 217Hz rate with other users and the base station using the ...


6

There is actually a very simple solution to this problem. The key to understanding this solution is to think about why a plain electro-mechanical doorbell does not ring due to similar interference. The answer of course is that it requires the actual ring power to flow through the closed circuit - interference will not couple enough power into the open ...


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