# Easy question for someone.

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Hi,

I have a 5mm red flashing LED, I've had it for probably 15 years, I
connected some others that I bought at the same time to my house
burglar alarm. They are are on the bell box and on a warning panel on
the garden shed. They make the alarm system look very scary, I think
any potential burglar would move onto another house after seeing
these.

Now I have moved to another house and I want to do the same again, the
problem is that I can't remember how to calculate the resistance that
I need to put in series.

A similar looking item that is for sale here has a specification as
follows,
Forward voltage:    2.5V
Forward current max.:    55mA

I think that this has something to do with V = I x R but I didn't pay
proper attention at school.

Can somebody tell me what a suitable resistance would be please?

James

Re: Easy question for someone.

<http://ourworld.compuserve.com/homepages/Bill_Bowden/led.htm

I think most villians are on to these LED flashers by now, especially
yours having given your name and geographic location by IP via the very

Whoops :-)

--

Re: Easy question for someone.

You'll have to tell us your circuit voltage: +5v? +12V?

R (min) = (V? - 2.5) / .055
R (typ) = (V? - 2.5) / .035

The higher your forward current, the brighter the LED.

Re: Easy question for someone.

Sorry I omitted that vital bit of information. The voltage is 12v. So
I presume that the sum is...
R (min) 3D% (12 - 2.5) / 0.055 3D% 172 ohms
R (typ) 3D% (12 - 2.5) / 0.035 3D% 271 ohms

I realise that I will have to hunt through my box for something of a
standard size but 330 would be appropriate.

Regards
James

Thanks

Re: Easy question for someone.
In article

If it's an LED which flashes on its own - ie has the circuitry built in -
they normally operate off a 9-12 volt DC supply with no series resistor
needed. I'd guess that's what you bought before.

--
*He who laughs last, thinks slowest.

Dave Plowman         snipped-for-privacy@davenoise.co.uk           London SW
We've slightly trimmed the long signature. Click to see the full one.
Re: Easy question for someone.

Assume that you have a 12 volt battery or supply some where.

R = (12-2.5)/0.055 = 173 ohms, ~ 0.5 watts

P.S.
I wouldn't operate the LED at max current.

http://webpages.charter.net/jamie_5 "

Multi-posting sucks (was: Easy question for someone.)

If you MUST post the *same* question  to MULTIPLE groups,
put **all** the groups on the To line the first time you post.
http://en.wikipedia.org/w/index.php?title=Cross-post&oldid24%0748598

Re: Multi-posting sucks

Re: Multi-posting sucks

OP: