Hi, I want to connect a scale to a Arduino to automatically upload the weight to a website to create a graph.
I bought a $20 digital scale, pulled it apart, connected the Wheatstone bridge strain gauge to +5V and and ground and measured the voltage on the sensor wires while using the scale. A weight of 0 to 125 Kg is equal to 0 to 50mV. I then fed the sensor wires to Vin+ and Vin- on a LM324, in the hope of creating a (perhaps crude) differential amplifier.
Now when I measure 2.5V on Vout and when I put my full weight on the scales, it goes down to about 2.37V.
What I want to do is use the Arduino Analog in and measure the voltage on LM324 that will go from 0 to about 3.75V (or from 3.75 to 0).
What did I do wrong and what should I change on the schematic to achieve that? I don't know too much about electronics but I do have a copy of "the art of electronics".
Remove R7 you don't need it. Also R6 is not doing anything for you so replace it with a short.
What resistance values are your bridge resistors, it could be that R5 is too low and presenting a low input impedance into the amplifier. If so you would have to up the absolute values of R5 and R8 but keep the ratio the same.
I'm trying to do the exact same thing. I've only worked with electronics for a short time, and this diagram seems straightforward enough. I used the same op amp with a cheap digital scale, but no dice. I can't get a change in my reading when I apply weight to the scale. Can anyone provide me with a bit more detail or some troubleshooting hints?
Also, I'm shocked that there is not a lot out there in terms of building cheap and dirty weight scales for our own projects. What approaches can I take for measuring 0-25 lbs. without using expensive load cells or other transducers? Basically, I need to be able to measure the weight of the contents in a bin. When Arduino gets the reading it'll normalize the numbers and send the data serially.
This is for a thesis project and any help with this would be greatly appreciated.
Measure the voltage at the junction of R1 and R3, it should be half way, that is 2.5V.
Remove R7.
Measure the output from the op-amp. Is it at 0V? If so and step 1 was OK then the op-amp is broken or wired up wrong. If it is 5V then check the bridge is OK or try reducing the value of R8, putting it down to 100R will give you only a X1 gain.
Measure the change in voltage at pin 3 of the op-amp. If that doesn't change the output is not going to change.
Thanks so much for the troubleshooting tips. I'll let you know how it goes. I also found this video: NerdKits - Digital Scale Strain Gauge Weight Sensor which could also help...just need to get it to work with Arduino instead.
Sparkfun sells single units of the Flexforce pressure sensors for $17 that look perfect for this task. They have a 25 lb version in stock that looks like it would be easier to interface to than something you took out of a kitchen scale.
I'm interested in using them to build a pet scale that can weigh animals from 20-100 lbs. It would be easy to use four 25 lb sensors at the corners of a board, but that quadruples the cost. I'd appreciate any thoughts on a mechanical design that would concentrate up to 100 lbs of weight onto a single sensor without being unstable.
Remove R7 you don't need it. Also R6 is not doing anything for you so replace it with a short.
What resistance values are your bridge resistors, it could be that R5 is too low and presenting a low input impedance into the amplifier. If so you would have to up the absolute values of R5 and R8 but keep the ratio the same.
Can you please explain to me why this works. I'm interested in building one of these but before I dive into it I want to figure out whats going on here. By removing R6 & R7 aren't you grounding V+ and the wheatsone bridge intersection? I've neer seen a wheatstone bridge with an interesection grounded, what happens in this case?
Also by removing the 2 resistors don't you make the op amp into a non-inverting one instead of a differential? Again I've never seen a wheatsone bridge connected to anything other than a differential op amp. what happens in this case?
As an electronic scale technician for 25 years, I can give you a couple tips:
If you are unsure which leads of the loadcell get the excitation voltage, its typically the pair with the highest resistance.
Your voltage to the loadcell needs to be regulated well or expect to see your weights drift or jump.
Typically the op amps used are low offset units, like an OP07, but I think for your purpose the LM324 should work ok.
Most loadcell amp circuits utilize a split power supply of +/- 5 to 15 volts. Its more difficult to get a linear voltage output from a single ended supply and amplifier circuit. But I've seen it done in cheap bathroom/kitchen scales. I'll look around for some schematics.
You'll typically need to trim the output of the loadcell to compensate for the deadload (scale top plate) weight. Just add a resistor from one signal lead to one power lead in the range of 40k-200k, depending on the amount of voltage you need to offset. You dont want the a/d converter to read exactly zero with the scale empty. Have it read +20 or so counts and then have a zero routine in your code to subtract that value from your reading.
If you add/replace a potentiometer inline with R8 you can adjust the gain of the amp so that you can span the full range of the a/d converter when you apply full load to the scale.
If the weight reading is jumpy, there are two solutions. Take several readings and average them before you process the data, or add a 2 to 10uF electrolytic capacitor accross the signal leads of the loadcell and that will smooth out (and slow down the response) of you scale signal. I use the cap trick on crane scales where theres a 30,000lb coil of steel bouncing in the air while being lifted/moved. Works like magic.
I'll be glad to answer any other questions you may have.
