I hope it is ok to post this basic 'newbie' question. I'm a food scientist / psychologist and am exploring the possibility of using the arduino uno + sensors in food science & psychology research. At the mo, I'm trying to measure weight via the Arduino. My end objective is to post a blog for fellow 'no electronic skills' scientists so others can build such devices. I'm also linking up the arduino to this research software I am writing (www.xperiment.mobi).
After googling, although I could not find a clear guide how to build this, I settled on this guide http://forums.netduino.com/index.php?/topic/1284-connecting-a-beam-load-sensor-to-netduino/. Specifically this post:
This is the IC that I have INA128P. Look at the schematic on the first page. 1. Connect S- to pin 2 of the INA128, connect S+ to pin 3 of the INA128. 2. If you have a bread board connect the netduino +5v and ground pin to the power rails on the bread board. 3. Connect E- and E+ to the breadboards power rails. 4. Connect pin 7 on the INA128 to the +5v rail. 5. Connect pin 4 and pin 5 of the INA128 to the ground rail. 6. Either use a resistor for the gain you want or do what I did and use a pot as a variable resistor. Use a multi meter to determine which two pins of the pot will show a resistance change when you turn the pot and connect those to pins 1 and 8 of the INA128. 7. Connect pin 6 of the INA128 to the analog pin of the netduino.
I bought these: -cheap electronic weighing scales from tescos http://www.tesco.com/groceries/Product/Details/?id=262683786 -INA128 amplifier. Here's info on it: http://www.ti.com/lit/ds/sbos051b/sbos051b.pdf
I dismantled my electric scale and soldered the 4-wires from the strain gauge to thicker wire (to let me more easily interface to the bread board). Luckily, the wires were appropriately labelled on the existing circuit board:
As shown in the image (slightly photoshopped to make things more clear, such as only one wire interfaces with each of the pins of the amplifier), I then linked up the breadboard as advised in that post (I've used a 100ohm resister, but also tried using a variable resister). Running an Arduino loaded with Firmata Standard, and running the Firmata test program (http://firmata.org/wiki/Main_Page), I get nothing - the analog pin just fluctuates around '400', and to the same degree as the unused analog pins do. Pressing down on the sensor does nothing :( . To check that I had not blown the strain gauge I resoldered up the weighing scale and it still works (rather incredibly).
I would very much appreciate your help on this, and your patience with my very limited knowledge of electronics. Hopefully I am doing something silly. With many thanks, Andy.