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driving 7 segment LED

I am a total novice at electronics so bear with me. All the digital clocks I've looked at online are very $$ which got me to thinking...Why couldn't I take any 'old' clock radio unit and buy my own large 7 segment LED and build my own clock? Replace the original LED display with my large ones? Blow this outa of the water if you all want, but gotta ask here.
there is no reason why not however you have to take into account that clock radios may use multiplexed displays so it may take you a while to figure out which wire goes where
driving 7 segment

Thanks PStevenson....I'm not too worried about deciphering what wires feed what. The clock I picked up uses a nice flat ribbon style to connect to the LED display....with a little trial and error I should be able to figure out which wire controls which segment of each number. I just wasn't sure what voltages my unit will have and if a larger LED number will work with it. A large 3" 7 segment display I found online for $16. I suppose I could buy just 1 to start with and go from there...
well as long as the LEDs used in the big display are just normal LEDs (which they probably are) then it should all be good otherwise you may need some kind of driver to power it - but the best thing to do is just try it out
Link ot the display you found. Large LED displays often have multiple LEDs per segment and require a higher voltage.



¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
If you read the datasheet you'll see that these require in excess of 12V to illuminate the segment. That compares with 2V for a normal display with a single LED per segment.

It is unlikely that a normal (read designed at minimum cost) clock radio is going to use voltages high enough to drive this. At a minimum you'll have to add additional driver circuitry to allow the use of a higher (external) voltage source.