Building A Digital Scale From Scratch

June 12, 2013 by Brian Benchoff 13 Comments

duplo

[Raivis] was given a particular task at his university – find a way to measure how many Duplo bricks are stacked together. There are a number of ways to do this, everything from computer vision to using a ruler, but [Raivis] chose a much more educational method. He built a digital scale from scratch out of a strain gauge and a Wheatstone bridge. The build log is immensely educational and provides some insight into the challenges of weighing things digitally.

A strain gauge is a simple piece of equipment, just a small force sensitive resistor. When attached to a metal bar and a force is applied, the resistance inside the strain gauge changes, but not by much. There’s only a few micro Ohms difference between the minimum and maximum of [Raivis]’ load cell, so he needed a way to measure very slight changes in resistance.

The solution was a Wheatstone bridge, or four resistors arranged in a square. When one of the resistors in the bridge is replaced with a strain gauge, very small changes in resistance can be measured.

With a custom ‘duino amplifier shield, [Raivis] can measure the resistance of his load cell with 10-bit resolution, or a maximum weight of 1.32 kg with a resolution of two and a half grams. A single duplo block weighs about 12 grams, so we’ll call this one a success.

Kitchen Scale Key Transplant

January 14, 2013 by Mike Szczys 7 Comments

kitchen-scale-key-transplant

[Markus] is quite happy with his kitchen scale. It’s one of the tools he uses most frequently when cooking. But recently the button has begun to give him problems. He figures the years of spilling a little bit of this and that has mucked up the contacts. His solution was to bypass the button using a Cherry MX switch.

Really any replacement should do since the switch merely completes an electrical connection. But there’s a subset of hackers who swear by the Cherry MX switches that come in some keyboards. [Markus] had just such a keyboard on hand, which he was already using for parts, so he pulled out the switch and cut a hole in the scale’s case where he could mount it. After temporarily super gluing the switch in place he completed the task by filling the gap on the outside with hot glue, then running another bead of it along the inside. The addition of the ‘T’ key finishes the hack. The plastic key is easy to clean and will help shed flour, oil, or anything else he might spill during his culinary adventures.

This hack was fast and easy and may have convinced [Markus] to roll his own controller board for the device. We’ll keep a lookout for a follow-up post detailing those alterations.

Firing Rocket Engines In The Wrong Direction — This Is Only A Test

July 16, 2012 by Mike Szczys 28 Comments

LVL1 has a new rocketeering group. This rocket engine testing platform is the first project to come out of the fledgling club. The purpose of the tool is to gather empirical data from model rocket engines. Having reliable numbers on thrust over time will allow the team to get their designs right before the physical build even starts.

The rig uses a pine base, with a PVC frame, threaded bolts, and a PVC cuff for mounting the engine in place. It is set to fire up in the air, directing the thrust down onto a scale. The flex sensor in the scale is monitored by an Arduino, and should be able to hold up to the 5000 ~~pounds~~ grams of thrust max which this type of engines can put out. The data is pushed via USB to a laptop computer where it is stored in a spreadsheet.

Calibration would be an issue here. But as long as they’re always using the same strain sensor the numbers will be accurate enough relative to each other.

Connecting A Dumb Scale To Your Smartphone

February 13, 2012 by Mike Szczys 6 Comments

[Casainho] wanted to track his body weight using an app on his Android phone. He just needed a way to get the weight readings onto the device automatically. He ended up adding Bluetooth to a bathroom scale and hacking the app to grab data from it.

The scale which he hacked is a digital model, which makes it possible to read the weight data if you know what you’re doing. [Casainho] already completed a weight logging scale hack which stored the data on an SD card. So this was a recreation of that project but with a Bluetooth module for the output rather than the card for storage.

Now you can buy WiFi enabled scales, but that’s not nearly as fun as a hack like this. Plus one of those will cost you around $200 and the hardware for this version came it at only $75. It includes an LPC2103 dev board, $6 Bluetooth module, character display, batteries, and misc. supplies. The software end of the hack was helped greatly by the fact that the Android apps which [Casainho] is using are both open source.

ATtiny Hacks: BEES! An Electronic Scale To See Who Brings In More Honey.

September 20, 2011 by Nick Schulze 19 Comments

[MakingThingsWork] wanted an accurate way to keep track of the weight of his beehive, so he decided to build himself a data logging electronic scale. First he ripped the strain gauges from an old electronic scale which he then fitted to his home made beehive base. He then went about designing and building the control board which is based about the Attiny 85 (if you hadn’t guessed by the banner). An instrumentation amplifier was used to amplify the signal from the strain gauge, which is then read by the ADC on the Attiny. It looks like he had some trouble getting consistent results from the scale, so to eliminate the error caused by temperature variations he set up a fixed voltage divider for reference. With this setup the scale can produce results at +/- 0.5lb accuracy, sounds just fine for a system that cost less than $50. The V-usb project software has been used to connect the scale to his PC which he uses to collect and graph the data. All in all a very neat project and by the looks of it, some very productive bees.

Zigbee-powered Wireless Scale

March 11, 2011 by Noah Dunker 5 Comments

[joe] wanted to make it easy to record his weight every day, and added a few bits to decode the weight and send it to his computer. The end result is a ZigBee-powered wireless scale. Additionally, his scale can track more than one person’s data simply by knowing whose previous weight the new measurement is closest to. Now [joe] and his family can spend more time working out, and less time messing with spreadsheets and data entry.

[joe]’s build is not only elegant, but well-documented, too. He walks through the reasons he chose this specific floor scale, reverse engineering it to decode the weight, then provides links to his schematics, source code, and pretty much everything else you would need to play along at home.