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I'm working on a mobile device. Essentially it has nothing to do with WiFi communication. It doesn't transmit any data over the air.

It has to know however if it's in- or outside of the range of a given WiFi base station. (So basically it must be able to tell whether a WiFi network with a specified SSID is in the air or not, nothing more, nothing less.)

Do I need to put a WiFi module into the device, or are there any PCB components made specifically for this purpose?

(Power efficiency is a very important aspect as the device is pretty small and uses kinetic energy to charge its battery.)

Update: The device has an 8cc harvester that operates around 2 hours a day and is capable of outputting approx. 200 µW during a normal cycle. About once in every second month, the device connects a cellular network and does GPS positioning too. The whole process takes up 2-3 times 30 seconds on average. This results in a total power cons. of ~27mW/h. (If circumstances turn out to be fortunate it might be that this "energy-hungry" operation doesn't need to be done for a whole year or even for a longer period.) WiFi-monitoring described above must be continuous. (Manual charging can be a possible option if necessary, but it would be great to eliminate this circumstantiality.)

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    \$\begingroup\$ This on Kinetic energy only ? Perhaps if you were an experienced application engineer, but a beginner like you ? No I don't think so. Also the amount of useful answers you get is proportional to the amount of effort YOU have shown. And since the latter is zero... \$\endgroup\$
    – Bimpelrekkie
    Nov 30, 2015 at 15:11
  • \$\begingroup\$ You might check ESP8266. But expect to supply a few 100's of mA while it is operational. You might be able to reduce that by disabling the transmitter with some fancy programming, but that is largely unknow territory. \$\endgroup\$
    – Wouter van Ooijen
    Nov 30, 2015 at 16:12
  • \$\begingroup\$ Sorry @FakeMoustache, I'm just an economist and software engineer, not too experienced in electrical engineering so far, that's true. Yet, I'm working on a mobile device, assessed the requirements already, know what components to install, but wasn't sure if i'd need to implement a Wi-Fi module or some simpler circuit (maybe even a power-friendlier one) could also do this work. I was only expecting a list of options, different modules and ICs that could be used I didn't know they'd exist so I can read more about them. I don't think it's necessary, but updated my question with some more info... \$\endgroup\$
    – tom
    Nov 30, 2015 at 16:59
  • \$\begingroup\$ @Wouter van Ooijen, 100s of mAmps are way too much unfortunately. As I see 802.11 modules already that consume only a couple of µAmps, (or even less than a µA) in sleep, I thought it might be a viable option to stick with kinetic charging only if I find a solution that does the work for me only for 5-10 µA or less. Considering that it doesn't need to radiate, "only scan the signals on its antenna"... Do you think that it's impossible to do with such a low power consumption? ...It would be good to know how much energy wifi modules need to radiate, and how much they use for the other tasks... \$\endgroup\$
    – tom
    Nov 30, 2015 at 17:18
  • \$\begingroup\$ I feel it is important to remember that energy harvesting specs(when you are designing it) are never reliable unless you have measured them in your system. It can be very tricky to do kinetic harvesting properly as you will need to know your resonant frequency, which one could assume is someone's gait. \$\endgroup\$
    – mcmiln
    Nov 30, 2015 at 18:01

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Regular 802.11 Wifi networks use something called a beacon frame to transmit all of their "housekeeping" information. This includes info like the SSID, the current time, and the supported data rates. It gets transmitted every 102 ms.

WiFi receivers can only detect nearby networks by watching for these beacon frames. There is no difference between receiving ordinary data and receiving beacon frame data - it's transmitted over the air in the same way, so there can't be an SSID "sniffer" without a full-on receiver. That means that you really need a proper WiFi module with a regular receiver.

With all that said, the lowest RX current that Digikey sells in a WiFi module is roughly 2 mA. As Wouter said in the comments, you may be able to make your module hibernate to save power, but while receiving, you're pretty well guaranteed to be spending quite a bit of power searching for WiFi. I think your engineers need to go back to the drawing board.

PS: you said that the GPS module consumes 27 mW/h. This number doesn't make sense! Watts are already a measurement of energy per time. I think you mean 27 mW.

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    \$\begingroup\$ Thanks! 27 mWh (= 97J) I wanted to write. That's the amount of energy that is consumed during the process when the cellular module and the GPS module work together for about 1-1.5 minutes. Your writing about the operation principles of SSID broadcasting was very useful. \$\endgroup\$
    – tom
    Nov 30, 2015 at 19:04
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I will say abandon hope of an energy harvesting version of this. Start by accomplishing your goal and then work on power consumption. You are interested in a sniffer. Look up wifi sniffing and have a go. In general, yes this will mean having some sort of wifi module. You will not want to layout your own antenna and deal with that as a first project, so make sure it is a module. Then, with your module attached to maybe a raspberry pi or something you can give this a start.

From here, iterate and see what you truly need for this device. You start moving into low power microcontrollers, energy harvesting(maybe a rectenna and still going to need a battery). You'll get there eventually.

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