Ask HN:Lowest cost hware tool for: validate physical presence and online web app

- the lowest cost (in dollars and human processes for validator and validated)

-for several distinguishables certifications of the fact that a web app is present physically in a determinable timeframe.

So far I've came up with :

- I mount a screen placed in a determinated physical place connected to internet with 3s interval refreshing QR code that web app have to scan

- validator scan with specific app a barcode on web app (but non cheap on human cost)

- ibeacon with web bluetooth api (but only newest-ish mobile browsers)

- human validator gives a specific code input in web app

basically as simple and cheap as possible digital signature of time and place, how would you do it ?

Ideally you tell me that there is a mini 2" x 2" screen with online capabilities on batteries with juice for several months that can display qr code. To be validated web app has the incentive to scan QR code to prove the web app is present physically present "there" in a determinable datetime.

Lets brainstorm :)