ok i need to find a good LED, must be very bright and blue that will run on a 9V circuit for a school project
Look for extreme brightness LED's. You don't say where you live, so thats all I can help you with. If you live in the US you could get hold of a Luxeon Star... damn, when will they be available in the UK
i live in the UK, i need them because i have an acrylic base and i need to shine a light through it, as it is only a small box i can use one LED
Maplin sell some half decent extreme brightness leds for £2.99, although I'm sure I've seen blue LEDs in a retail store with a higher brighness than 3cd, I just can't remember where. I'm quite sure if you looked you could find some that were better though.
ok well the idea is that i have a lamp that comes on when it gets dark enough, i have the sensor and everything built but the black acrylic box looks very boring and i thought of the idea of having an led shining thru the bottom but i have a 9v bulb to supply the lighting, would a extreme brightness led drain this and become dim?
Although you've got 9v, you still need to drop that down so the led only sees 3.7v (or whatever the spec is), so you need a series resistor of around 270R. (with the Maplin 4v 30mA you could squeeze the most out of it with a 180R resistor, or use 2 in series with a 33R resistor)
Two in series is much better since you're wasting less of the power from your battery. (essentially you're getting double the light output from no more power drain from the battery). The only negative point is as soon as the voltage drops to around 7.5V the led's will cease to light up, whereas the battery could drop to 4V before it would stop illuminating with a single LED and resistor.
i'm not using a battery, i have one of the 9v sockets for a 9v adapter so i can plug it into mains power
