Ok this is off arduino... anyone here who build model bridges? I'm looking to get started and wondering whats the best way. I have some balsa with me.. or should i be looking at tooth picks for a start? or are there any nice lego like kits for building different types of bridges?
Toothpicks or matches tend to be the cheap material of choice; popcicle sticks or straws can work well, too. You could even use cardboard or paper. Straws can be stuck together using pipe-cleaners. Suspension-style bridges would need twine or string, of course, and the towers could be built from anything.
I know that at one time they used to make "bridge modeling" kits (look up "panel and girder" kits - these were made back in the 50's and 60's for model skyscraper building, and there were similar kits for bridges); they won't be cheap (but will be cheaper than lego).
Something I used to have fun with as a kid was building "structures" from origami-like folded paper; a really strong structure of that nature can be made by multiple valley folding a piece of paper (turn it into a corrugated-like sheet), then with all the folds done (thin strip), folding "legs" back and forth, then opening the sheet up and valley folding those legs in - you get a "quonset-hut" style arch structure with multiple compound valley/mountain folds; I found that a single sheet of copy paper folded this way could support a small book (provided you anchored the outer "walls" to keep them from collapsing outward).
I always wanted to replicate it in another material, like thin plywood or coroplast, and scale it up, perhaps for a shade structure or other shelter. Never got around to it, though...
The girder and panel system wasn't very strong. I never had one of the bridge-building sets, but, interestingly, another manufacturer has picked them up since Kenner dropped them, and google pooped up multiple online sellers. They have a diagonal brace system that would look more realistic than trying to build with panels, or with Lego.
But, if I were trying to build something functional, for anything other than Hot Wheels-type models, I'd be more inclined toward Lego or Rokenbok. Admittedly, this is partly because I have more Lego on hand than many toy stores. ;D But it also offers a lot more design flexibility, in exchange for sacrificing some cosmetic realism.
You can solve the problem of needing parts that aren't common in today's sets by shopping at Bricklink, where lots of people sell parts from their collections and from cannibalized new sets.
google pooped up multiple online sellers.
Those poor sellers!
this is partly because I have more Lego on hand than many toy stores
how did u acquire them? I have 2 mindstorms nxt sets on me... thats all the lego i got... I think Popsicle sticks seem to be the cheapest option... the problem seems to be with understanding how to put the first bridge together... the process of actual construction 
I've got literally hundereds to thousands of legos from when I was a kid (OK, I'm still a teen, but not a kid any more).
Have you thought about using Meccano. Now that's real engineering!
Still available in France, I believe, and lots advertised on flee-bay
jack
Praccas,
Considering where you're from, there might be an opportunity for one of your local enterprises to market a near "copy"
jack
how did u acquire them?
A combination of good luck and poor mental health 
I had the good fortune to have a high-paying job, and no heavy family responsibilities, back in the late 1990s. At that time, many stores were over-stocking Lego sets, especially at Christmas time, and selling them at deep discounts when the season ended or new lines came out. So I was able to buy way more Lego than I probably should have for prices that were 50-90% below retail.
This thread reminded me of a cool old game called Bridge Builder (links at the top on that page). One of the earlier physics-based games I think. I remember playing the 2000 version a while back from time to time (which seems offline now). But the 2006 version, basically the same but improved I guess, is there and free. Even for linux! I didn't know that until now Only 2D, but still great fun while it lasts.
As for cheap material, how about spaghetti strands (no, not after you cook them :P) and a glue gun? I used that many years ago one time to build a model bike, but other than that I haven't used it really. Also I don't build model bridges.
I've been sitting on the sidelines watching this thread to see what might surface and felt that it was time to add my 2-cents worth.
The best job I ever had was as a professional model builder (20 years ago) for a consulting engineering firm http://www.rwdi.com/. My job was to build scale models of bridges and buildings for use in wind tunnel and snow flume analysis prior to the expensive construction of full scale (real world) ones.
Most of the study buildings were made of sheet acrylic or polycarbonate with some small scale details made from sheet styrene.
The bridges where made from various combinations of balsa, thin aviation grade plywood, acrylic, polycarbonate, styrene, brass and aluminum; with the choice of material based on the type of the part, its load bearing requirement and the level of scale detail required.
Truss work was usually balsa . . . sometimes laser cut in a truss-like pattern, whereas roadway surfaces might be thin (1/32 - 1/16” thick) plywood. Piers might be acrylic, polycarbonate of in some cases machined aluminum. Suspension cables were made from monofilament line, very small gauge wire or in one case human hair . . . yes human hair because of its tensile strength vs diameter. We also made extensive use of CA glues and epoxy for bonding.
Pracas, your choice of material should be based on a number of factors:
*your desired aesthetic (a bridge made from popsicle sticks looks a lot different than one made of toothpicks or balsa)
*your materials budget and materials availability
*your capabilities as a model builder
If you do not have much experience working with tools and these types of materials, then choose a simple project made from simple inexpensive materials so that you do not get frustrated and give up midway through.
I've been sitting on the sidelines watching this thread to see what might surface and felt that it was time to add my 2-cents worth.
Well so have I 
been loaded with work ever since i posted this.
I do have some balsa sheets, CA and some other material. As for a budget, not much this is more of pursuing a childhood 'want to do' things.
I was alway a fan of using match sticks or balsa. But I hate doing the same thing twice so used to build the second half of the bridge to a different design.
i have a hobby making model Godzillas to destroy model bridges
i have a hobby making model Godzillas to destroy model bridges
Reminds me of Mr.Bean (Rowan Atkinson) @ the christmas shop.... lol! ;D