Easy question for someone.

Feb 15, 2009 9 Replies
J

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?



Thanks in advance, James


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A

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 searchable Google Groups.

Whoops :-)

Adrian C
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U

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.

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J

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.

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J

If you MUST post the *same* question to MULTIPLE groups, put **all** the groups on the To line the first time you post.

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J

Thanks for that prompt reply. 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.

Thanks again for your help. Regards James

Thanks

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D

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 dave@davenoise.co.uk London SW To e-mail, change noise into sound.
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J

UCLAN wrote:

OP: Follow the link and ignore UCLAN's bad advice.

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