you need the tinker toy or block system.. one sensor one power supply very stable and one digital metter that you buy for 9.95 .. the sensor can be many different types based on how you apply it , but you will want to select one based on it's output . most of the time millivolts is pretty standard like 0 to 30 mills for what ever range you want to weigh, sensors come in different ranges of force that can be used on them like 0 to 100 lbs ect ect. some limits are set by the type of sensor ,ring, button,bar ect ect for scales on a small scale button compression rings are used. all sensors of this type have a excitation voltage applied to them most of the time 10 volts is pretty standard. it's a bridge 10 volts across the circuit apply force to the sensor from 0 to the high range end and millivolts out linear to force. then all you need is a digital meter to read the millivolts 0-10 pound sensor with 0 to 30 mills out =3.33 mills to the pound of force applied to the sensor so you can use a mulitiplyer or electronic scaleing for direct reading in pounds .