Hi TJH, and welcome to English Forums.
The problem you're having is that "LED" is not a word, but an abbreviation, and is pronounced "ell-ee-dee." Because the pronunciation starts with a vowel sound (ell), you use "an" instead of "a."
Abbreviations will use "an" if they start with F, H, L, M, N, R, S, or X, as well as with the vowels.
An FBI agent came to my house asking about you.
We have an MOU with the purchasers.
You'll need an X-ray.