edit: see this commercially available $19 opensource arduino weight shield!
- arduino, breadboard, wires (just buy the latest Arduino starter kit,~£25), computer:
- <10 ohm resister, e.g.
- an amplifier:INA128P (~£10), e.g.
- electronic kitchen scales, e.g. £10 scales (Salter, 1066BKDR08) from Tescos:
- A really tiny Phillips screwdriver, and a regular Philips screwdriver.
- Dismantle your scales. The ‘load cell’ (what measures weight) is the metallic block firmly attached to the plastic encasing. Notice the four wires connecting the load cell to the circuit board have been conveniently labelled on the back of the circuit board. Best note which wire goes with which label! For me it was:
- E-= black wire.
- E+= red.
- Either use a soldering iron or cut these four wires close to the circuit board (if you do the latter, strip some plastic from the ends of the wires exposing say 1cm of wire – best to then fold each exposed wire end over and twirl it around to give each some strength).
- Remove the old circuit board as we don’t need it any more.
- To make sure everything is in working order it is best to connect up all your components on a breadboard. I decided to solder the ends of the wires from the load cell onto thicker wire, to make them easier to insert into the breadboard.
- For completeness, here is where I placed the components and wires on the breadboard and the Arduino uni:
- place the left pin next to the dimple on the INA128P amplifier in hole E8 on the breadboard. Put the pin farthest away from it in F11
- E-(black wire) in one of many negative holes on the left hand side of the breadboard(see pic above).
- E+(red) in one of the many positive holes on the left hand side of the breadboard.
- S-(green) in B9.
- S+(white) in A10.
- Link +5v on the Arduino to one of the many positive holes on the left hand side of the breadboard.
- Link a GND (ground) pin on the Arduino to one of the many negative holes on the left hand side of the breadboard.
- Link A5 (an ‘analog in’ pin) on the Arduino to I10.
- Use a wire to link G11 to a negative hole on the breadboard.
- Use a wire to link G9 to a positive hole on the breadboard.
- Use a wires to connect D11 to a negatibe hole on the breadboard.
- Connect the resister to E8 and G8 (does not matter which way around).
- Hopefully no steps have been missed out above! If all has gone to plan, using the Firmata software with the Arduino plugged into the computer (see this tutorial on how install the Firmata software on the Arduino and to run the Firmata software) you will see the Analog pin 5 reading vary as you press down on the load cell.
- installing plugs in your weighing scale to make a professional build.
- interfacing with an Arduino Nano (~£10 ebay).
- interfacing all this with research software.
- calibration via research software.
—thanks to the Arduino Forum for helping with the above circuitry—