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[on:]

hi, bit new at this,
simple question

i have this 7 segment display
http://www.datasheetcatalog.com/datasheets_pdf/T/O/S/5/TOS5121AS.shtml

its got a common cathode.
i'd liek to drive it with my arduino eleven clone through a shift register (74hc595). which is currently light 7 seperate led's (which i want to replace with this 7 segment display)
but the data sheet says forward voltage is 3v and i think the arduino runs off 5v
will it work? i'm using some 470ohm resistors to help add a bit of resistance.

thanks,

[Reply #1 on:]

You should use a resistor for every segment. If I google for it, I see that 470 Ohm is used most of the times.
Scroll down on this page for a similar schematic, http://arduino.cc/forum/index.php?topic=20201.15

[Reply #2 on:]

thanks, will do, and the voltage will be ok even though it says 3v on the data sheet?

[Reply #3 on:]

They need a certain current, and the resistor reduces the current.
Just don't apply a voltage to them, and you are fine.

[Reply #4 on:]

yay, plugged it in, all good.
i used a 470ohm resister to each segment.
I'm not sure what you mean about not applying a voltage to them. I have to apply a voltage to create a high or low signal. Or in other terms, to have electricity 'flow' through a circuit i need voltage don't i?
sorry if im' not making myself clear. its been a long while since i did basic electronics at school )

[Reply #5 on:]

If you apply a voltage with unlimited current, the leds of the 7 segment display will get overheated very quickly if the voltage is too high.
If you apply a current, there a a voltage over the leds. But that voltage can be called a 'voltage drop'.

Use any led you have, and try a few things. If you connect a led to a 5V power supply without resistor, you might see a flash or not. But the led is broken. If the voltage is too low, the led doesn't do anything. Connect the led with a resistor to 5V and it is okay. Perhaps you should wear safety glasses.

[Reply #6 on:]

but the data sheet says forward voltage is 3v and i think the arduino runs off 5v
will it work? i'm using some 470ohm resistors to help add a bit of resistance.

http://led.linear1.org/1led.wiz

You enter your source voltage (5V) your forward voltage (3V) and the number of milliamps (from the datasheet) and it gives you a resistor value.

From that page, 470 ohms would give you a current of 5 mA.

[Reply #7 on:]

ah!
right that makes sense.
thanks very much