eli5: How do cordless phones detect ringing on the phone line?

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How does the base unit’s processor detect ringing from the phone line and ring itself and the connected handsets?

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Voltage over the line at the most basic level for a land line. At 20 hz sinewave is sent over one of the wires.

Don’t even need a processor to detect that (although a cordless will have one, or possibly a microcontroller serving as one), you can do it electronechanically.

The base detects an incoming call the same way any other land-line phone does. When it detects the incoming call, it sends a signal to the handset that says, “hey, dumbass, RING!”

In old fashioned telephones with actual bells in them, the phone line would send 60-100 volts on the “ring” wire to directly power the bell. Modern electronic and cordless phones instead just have a CPU input to *detect* that voltage, and they play their electronic ringtone in response.