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Easy question for someone.

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

Reply to
jhleslie
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Almost there, we need to know from what voltage you plan to power the LED so we can help you. But this is more of a electronics.basics question really. I liked the story though.

Reply to
a7yvm109gf5d1

But it scare (or annoy) the wife and friends too.

What supply voltage?

Reply to
linnix

This question has been answered MANY times in the BEGINNERS' group:

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Reply to
JeffM

See my reply to your subject in alt.binaries.schematics.electronic where I have posted an Excel spreadsheet to accomplish this easy task.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Use only Genuine Interocitor Parts" Tom Servo  ;-P
Reply to
RFI-EMI-GUY

Unlike a normal LED, you don't usually need or want a resistor in series with a flashing LED. Check the data sheet for usable voltage range.

Reply to
Spehro Pefhany

A spreadsheet was posted in alt.binaries.scematics.electronic

The poster needs a real ISP with access to a real news server, however, to gain access to that group.

The resistor depends on the input voltage of the circuit, not the voltage you want to put onto the LED The series resistor limits the current in the circuit.

Reply to
Archimedes' Lever

Yes, that's exactly it. However, it lists Imax as 55 mA - that's the point where the LED becomes reliable - I'd run them at more like 20 or even 10 mA.

The formula for R is: R = (Vsupply - Vf) / .02 (or .01, to taste.) Vf here is 2.5V.

And yes, this is a .basics question so I've crossposted this, to share with the others.

Have Fun! Rich

Reply to
Rich Grise

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