simple torch: idiot check

[kris]

hi,

This easter I'm planning on building a torch with my daughter (5 years)
using an aa battery in holder, a resistor, a button and an led.

i'm going to use conductive glue and build it on a paddle pop stick.

Can someone check my values:


battery is 1.5v

led has 20mA forward current

so the resister should be 75 Ohms? is that right?

20mA is 0.02Amps, R=V/I so

x = 1.5/0.02

x = 75


Kris

[Patrick Barnes]

Nope, sorry.

Subtract Vfd from the circuit (forward LED voltage drop at rated current), THEN calculate the resistor value.

A red LED has Vfd of about 1.8V, so with an AA cell won't need any resistor and will be quite dim.

A 3V button cell with no resistor works quite well.
(The internal resistance is enough)

-Patrick

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[kris]

ah thanks! glad i checked.

i'll use the button battery solution and pot it in hot glue.

Kris