When I lived in Bangladesh a young entrepreneur used to come to our house with his mother in tow, selling freshly roasted peanuts. He used a balance of the kind I had only seen in illustrations about courts of law: "scales of justice." He had a rock on one side and he scooped an equal weight of peanuts onto the other side.
You can make a simple balance that is surprisingly accurate. It hinges on the coincidence that either one United States nickel or two pennies weigh almost exactly 5 grams.
Note that what follows is a 3 minute, crude design to get you on your way fast. Feel free to make it nicer or adapt the design to what materials you have available.
*2 (U.S.) nickels or 4 pennies (or anything else that
weighs 5 grams)
*3' or 4' dowel,or a yard stick or meter stick, or any long stick
*thin string, thread or dental floss
* tape, scissors and an envelope
Seal the envelope, then cut the envelope in half to make two pouches (or think of them as pockets: closed on three sides, open on one. These will hold the weights on either end of the balance.
Tape the two halves of the envelope to either end of the dowel or stick. Using just one piece of tape might make them easier to empty.
Tie the string or thread in the middle of the dowel or stick. Tie it loosely enough so you can slide it back or forth. Add the 5 gram weights to each envelope and slide the string until the stick balances by becoming horizontal.
Take one of the weights out of one envelope half and put into that envelope some modeling clay that you think is 5 grams. Keep adding or taking away clay until you attain balance again.