Interested to know how a card only needs to be in close proximity to send a signal to the terminal yet doesn’t appear to have any internal power of its own to send the signal? Could understand if contact was necessary to complete a circuit and data could transfer but don’t quite get how it can wirelessly send/receive data with no power source?
In: Technology
It is the radio waves from the terminal that provide the power that that card use.
When an antenna picks up a radio signal it creates an electrical current in the wire. In most situations that is too little power to do anything useful so let it be the input to an amplifier.
But for a short distance where the power can be relatively high and low power requirement lithe the card has it is enough. You send out a radio signal that is used to power the card, the card can store the power in a capacitor and use it to do the operation and transfer data back.
The technology a type of RFID. IT is a quite common technology. There are standards with passive (in internal power) RFID tags that can be read from 10 meters.
There is another example of this being used and that is in a [Crystal_radio](https://en.wikipedia.org/wiki/Crystal_radio) where you can demodulate a AM radio signal and power headphones with the energy in the radio signals.
They have been used since 1902 and was popular in the early day of radio as the was very simple to build at home and not radio is required.
You can compare it to a wireless phone charger. That user changing magnetic field instead of radio waves but is another application of wireless power at a short distance.
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