Andy's Postbag and random ebay links

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ag123
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Re: Andy's Postbag and random ebay links

Post by ag123 » Fri May 26, 2017 12:31 pm

if you use SSID, may be you could even get their *location* to within decimeter accuracy :lol:
http://web.stanford.edu/~skatti/pubs/si ... spotfi.pdf

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zoomx
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Re: Andy's Postbag and random ebay links

Post by zoomx » Fri May 26, 2017 1:45 pm

ag123 wrote: ^^ perhaps you could even 'see through walls' with those :lol:
Holography of Wi-fi Radiation
https://journals.aps.org/prl/abstract/1 ... 118.183901

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RogerClark
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Re: Andy's Postbag and random ebay links

Post by RogerClark » Fri May 26, 2017 9:44 pm

I am not sure if doppler of WiFi would work, as wifi is very wide spectrum, and the doppler shift is only a few MHz

You would also need to quickly detect which channel the approaching car was using.

It sounds very difficult to do.

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ahull
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Re: Andy's Postbag and random ebay links

Post by ahull » Fri May 26, 2017 11:20 pm

Directing the output of one of those little modules sounds like a much easier task. I wonder if I could re-purpose a microwave oven waveguide. Given that the wavelength of the microwave is roughly twice that of the nominal wavelength of the module (12cm or so compared with 5.6cm or thereabouts), it might be possible... alternatively, some brass sheet might do the trick. Maybe the waveguide of the microwave could be split in four with a brass cross shaped insert to produce up to four beams.. I wonder if anybody would notice if I nicked the wave guide out of the microwave... ;) Alternatively a hand cobbled together yagi antenna made from copper wire of copper foil might give you a nice narrow beam. I would need to be careful not to make it too efficient, I wouldn't want to cook the passing motorists. :lol:
- Andy Hull -

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RogerClark
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Re: Andy's Postbag and random ebay links

Post by RogerClark » Fri May 26, 2017 11:41 pm

I recall those articles about wifi antennas made from "pringles" tubes as they are foil lined cardboard.

But as I don't eat Pringles that would greatly increase the price of the project.

I could make a curved reflector from some aluminium flashing (plate) that I bought on a large roll, which I used for home renovation.

Or just stick some aluminium foil to something curved, e.g. perhaps get old of an old football etc

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RogerClark
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Re: Andy's Postbag and random ebay links

Post by RogerClark » Sat May 27, 2017 9:39 pm

Just a thought... But but after reading some other blogs etc about these devices, I suspect they are not operating in a way that could be made directional.

There is just one transistor that appears to operate as TX, and RX+mixer. So it seems far more likely that its only operating as an oscillator, but that anything inductive or capacitive in a 4 meter range in any direction is part of its tuned circuit.

Somewhat like a Theremin

https://en.wikipedia.org/wiki/Theremin

Except that has 2 oscs and a mixer.

i am not quite sure how the change in oscillator frequency is detected, but I suspect the easiest way would be to look at the current being consumed

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ahull
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Re: Andy's Postbag and random ebay links

Post by ahull » Sat May 27, 2017 10:13 pm

I suspect you are correct about the mode of operation, however since it appears to rely on detecting a relative change in the oscillator, rather than an absolute frequency, then so long as the reflector is as effective at gathering the reflected signal as it is at boosting it in the wanted direction, then it should theoretically work...

Only one way to find out though..

Edit: The detector on these boards looks suspiciously like the ubiquitous BIS0001 - > Datasheet.. and may rely on a low pass filter and integrator, picking up the heterodyne signal from mixing a small portion of the transmitted signal with the received one. This heterodyne signal is effectively the doppler shift caused by a radio reflective object passing through the radiated energy from the transmitter. In other words, there is no signal from the "mixer" unless something reflective moves, and when it does, the reflective signal is proportional to the speed of the moving object. In a differential PIR, pairs of sensor elements are wired as opposite inputs to a differential amplifier. In this "radar" module I suspect our differential amplifier is used to spot the phase shift caused by movement in the radio beam. It will be interesting to see what appears on pin 12, the 2nd stage op-amp output.. if it is proportional to the rate of movement, then we may be able to estimate speed, though to what degree of accuracy remains to be seen.
- Andy Hull -

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RogerClark
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Re: Andy's Postbag and random ebay links

Post by RogerClark » Sat May 27, 2017 11:49 pm

I'm not sure how it can be mixing the TX with the RX signal.

There is only 1 transistor, hence only one SHF oscillator. If it has 2 transistors I could see how it could be transmitting on one and receiving + mixing on the other, but with only 1 transistor I can't see how I can do that.

I'm not sure precisely what the chip is doing, I think perhaps they are just using it because it contains several opAmps and an input for the LDR to disable the whole unit.
And somehow one of the opAmps behaves as a differentiator , but C9 / R3 appear to be a low pass filter, where I had expected a high pass filter / differentiator.

But this branch of electronics is IMHO rather esoteric, and more like quantum than Newtonian physics

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ahull
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Re: Andy's Postbag and random ebay links

Post by ahull » Fri Jun 02, 2017 11:36 am

Looks like the latest version of the "Transistor tester" that ebay is awash with, can do some new tricks.

http://www.ebay.com/itm/2017-DIY-KITS-G ... SwFc5XwSfU

It seems it can talk to DS18B20's and IR Decoders..
I resisted the urge to purchase, but it was a close run thing :D

What I couldn't resist was a faulty 2.4GHz mini drone and two faulty IR helicopters.. Winning bid £1.46 - I am of course old enough, but clearly not wise enough to know better.

"Remote control helicopters + drone for spares or repair" appears to be a Syma X4 2.4GHz mini drone including remote and two helicopters one that looks like its probably a Syma S107, or similar including one IR remote.. and one unidentifiable one.

Will they all be junk?
Are they heading straight for the parts bins?
Will I ever learn?
Watch this space.
- Andy Hull -

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Re: Andy's Postbag and random ebay links

Post by zmemw16 » Fri Jun 02, 2017 12:59 pm

@1.46 probably not, nor me either if i looked at such things :lol:
srp

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