How does plugging in a clock radio, dvd player, tv know what time it is when you plug it in?

Question

The time is always the correct time when you plug it in. How does this work?

Answer 1

They don't know. Someone has to set it (either at the factory or at home). Some models have a tiny battery (or a disc battery like the CR2032) to keep the circuit alive....

Answer 2

I've never noticed any of those devices having the correct time when you FIRST plug it in. It is only after you program the correct time that some of those devices remembers it.
In the case of the clock radio, a battery is used to power the memory used to keep track of the time when the clock is unplugged.
When talking about the television time, are you considering the time shown on the channel, or the actual time displayed by the television? Because, the channel time comes from a tv network that displays it, and the tv being off or on has no effect on it.
Cell phones always display the correct time because, most of them get a time signal from the relay tower in their network. You ever notice if you travel into a different time zone, say new york to cali, your cell phones time automatically changes.
Some cell phone do not get their time from the tower, instead it must be reprogrammed everytime the phones power is turned off, or the phone goes into a different time zone.
There are also some devices out there that get the exact time from a radio signal that constantly sends out the correct US time. You can actually go online to find out the radio frequency.

Answer 3

In fact, it doesn't always work; most devices show 12:00 when plugged in and you have to set the time. However, some more complex appliances (especially video appliances) look for a broadcast from an educational TV channel, which often contains time information buried in the signal. Also, more appliances are listening to time signal broadcasts from WWV or other Bureau of Standards transmitters --but these may take a while to lock onto the signal and set themselves.

Answer 4

The time of day is broadcast by radio and on PBS television channels. These devices tune into these broadcasts to know what time it is. Some of my toys do this correctly, for some others it doesn't seem to work. You can also access the exact time through the web and have you computer's clock be set automatically as well.

Answer 5

Either you have a battery in it that remembers the time or you have one of those clocks that picks up a local time signal and sets itself that way.

Answer 6

Tiny little battery - doesn't take much power to ron that little circuit

Answer 7

either rom chip, tv may get a signal from cable, thats how mine keeps time.

Answer 8

12:00....u gotta set it!