# Measure weights with arduino

I want to measure weights with arduino. After the measurements, i want to analyze the weight data of the measured objects.Precision is not very important, but it should be work in the range of 10 to 100 kilograms.

Is there a force-sensing socket, weight-sensor or something similar for Arduino that i can use?

• have you heard about transducers??? – perilbrain Oct 1 '12 at 19:54
• Of course precision is important, or would you accept a 40 kg reading for a 100 kg weight? – stevenvh Oct 1 '12 at 21:08
• Have you though of hacking some digital scales of the size you need. Reading the scales LCD with the Arduino? – Hellonearthis Oct 10 '12 at 16:08
• I just found this Patented method to do that with a piezo element. The circuit is not as difficult: google.com/patents/US20020076253 What do you think? – user59932 Dec 7 '14 at 14:57

PIEZO speakers can be used as a sensor to force. I guess they could be placed on a point to become a scale. Very DIY.

• A piezo sensor converts physical force to charge. That's why they give out only AC signals. In order to measure a static weight we'd need to integrate the output with ridiculous accuracy. – Jonny B Good Oct 1 '12 at 20:47
• I'd hardly call the output of a piezo sensor AC. It will always die away, eventually, but the time constant could be very long. It's the shunt resistance of the piezo element that bleeds away the charge, and that can be 10^12, and time constants on the order of 0.5 hours aren't hard to achieve. They'd be fine for something like weighing trucks at a weigh station, and are too good a tool to rule out thinking they're only good at "AC". They are more difficult, though, and maybe not for beginners. – Scott Seidman Oct 1 '12 at 22:06
• Jonny your right, there would be ridiculous complexity to using a Piezo as a scale. There are better solutions available. – Hellonearthis Oct 10 '12 at 16:05

You can either buy one of these expensive transducers, or go via a more creative route.

The only cheap alternative to buying a transducer would be finding an analog weight on ebay and interfacing it electronically.

You could use the good ol' technique of the dynamometer: $$m.g=k\Delta x$$ Where x is the displacement of a spring , k its stiffness coefficient, g the gravitational acceleration (approximately, at the Earth surface) constant 9.81m/s², and m the mass of the weight. Basically place the weight on a spring and read its displacement to get the mass.

You can read the linear displacement however you prefer, the easiest would probably be using a linear potentiometer (e.g. at sparkfun) with the slider tied to the tray. Just measure the voltage of a voltage divider with the potentiometer in it. For more accurate readout, you should make sure the tray is only free to move in one direction (compression of the spring), which is easy to do by attaching flanges to the tray, sliding along walls.

If the maximum displacement of the potentiometer is 20mm and you want to measure from 10 to 100kg, then you should buy a spring with a stiffness of approximately 44.1N/mm.

There is a good webpage that tells how to use weight sensors with arduino. Most of these techniques utilizes ADC for weight transducers to connect to arduino.