There are lots of 9V flashlights instructables. Mine only takes advantage of the perfect fit of 5mm LEDs in the holes of the pieces of the classic brick of LEGO and the fine and "I paid for it" look of the brick.

Also thanks for the ones explaining how to reuse the connectors of exhausted 9V batteries.

Thanks for your time!

Step 1: Elements:

You need:

1 2x4 plain brick of LEGO
1 2x4 "normal" brick of LEGO
3 5mm 3V LEDs
1 Connector of an exhausted 9V Battery

You may (will) need a solder an lead.

Of course you need also a working 9V Battery...

Step 2: Drilling the Holes.

Sinply drill the holes of the brick. This will house the LEDs. Finish it whith a dremel's trimmer

Remember that theLED s must be 5mm dia, exactly the inner diameter of the factory brick's holes.

Step 3: Trimming the Inside.

You need to trim the inside of the brick to make room for the bottom part and the pins of the LEDs

Step 4: Soldering.

Now you can cut, bend and twist the LEDs' pins and... solder it.

You must connect it anode to catode; if you don't know, simply try and try until they light..

I didn't use resistors because 3V (white) LEDs can work even at 4,5V so there is no fear.

One more thing: You must insulate contacs, with tape or paper as I did, leaving the main contacts out.

Step 5: Contacts and Finish.

Drill and trim the 2x4 plain brick to house the contact of the old battery.

Mount the block one way and try the other if it not work. LED's will not be fused if you mistake.

And it is done!