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:
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...