Instrumentation Amplifiers and How to Measure Miniscule Change

These days there a large number of sensors and analog circuits that are “controller friendly” meaning that their output signal is easily interfaced to the built-in Analog to Digital Convertors (ADCs) often found in today’s micro-controllers. This means that the signals typically are already amplified, often filtered, and corrected for offset and linearity. But when faced with very low level signals, or signals buried in a larger signal an Instrumentation Amplifier may be what’s needed. The qualities of an Instrumentation Amplifier include:

  • A differential amplifier with high impedance and low bias current on both inputs.
  • Low noise and low drift when amplifying very small signals.
  • The ability to reject a voltage that is present on both inputs, referred to as Common Mode Rejection Ratio (CMRR)

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How to Weigh an Eyelash

So you’re a boxer, and you’re weighing in just 80 micrograms too much for your usual weight class. How many eyelashes do you need to pluck out to get back in the ring? Or maybe you’re following the newest diet fad, “microcooking”, and a recipe calls for 750 micrograms of sugar, and you need to know how many grains that is. You need a microgram scale.

OK, we can’t really come up with a good reason to weigh an eyelash, except to say that you did. Anyway, not one but two separate YouTube videos show you how to build a microgram balance out of the mechanism in a panel meter. You know, the kind with the swinging pointer that they used to use before digital?

Panel meters are essentially an electromagnet on a spring in the field of a permanent magnet (a galvanometer). When no current flows through the electromagnet, the spring pulls the needle far left. As you push current through the electromagnet, it is attracted to the fixed permanent magnet, fighting the spring, and tugs the pointer over to the right. More current equals more pull.

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Fixing a Broken Postage Scale

[Starhawk] had an old Pitney Bowes G799 postage scale that wasn’t working as it should. After years of faithfully measuring packages and cooking ingredients, the scale stopped working. At first it fell out of calibration. Then the power up sequence stopped working. The scale normally would turn on, light up the entire display, then change to dashes, and finally set itself to 0.0 lbs. In this case, it would get stuck at the dashes and never change to 0.0.

[Starhawk] ended up purchasing another duplicate scale from eBay, only to find that when it arrived it had the exact same power up problem. Using deductive reasoning, he decided that since the scale was broken during shipping the problem would likely be with a mechanical component. He turned out to be correct. The cheap momentary power button was at fault. When pressing the button, the contact would get stuck closed preventing the scale from zeroing out properly. [Starhawk] easily fixed his problem by replacing the switch.

Next [Starhawk] replaced the old scale’s LCD module with one from the new scale, since the old one looked to be on its way out. The scale still had a problem correctly measuring weight. [Starhawk] tried swapping the load cell from the new scale to the old one, but he found that the new load cell had some kind of problem that prevented the scale from zeroing out properly. The solution ended up being to use the newer “analog board” as [Starhawk] calls it.  The end result was the old scale with two newer circuit boards, an older load cell, and a new power switch. Next time it might be easier to just build his own scale.

Reverse Engineering a Bathroom Scale for Automated Weight Tracking

[Darell] recently purchased a fancy new bathroom scale. Unlike an average bathroom scale, this one came with a wireless digital display. The user stands on the scale and the base unit transmits the weight measurement to the display using infrared signals. The idea is that you can place the display in front of your face instead of having to look down at your feet. [Darell] realized that his experience with infrared communication would likely enable him to hack this bathroom scale to automatically track his weight to a spreadsheet stored online.

[Darell] started by hooking up a 38khz infrared receiver unit to a logic analyzer. Then he recorded the one-way communication from the scale to the display. His experience told him that the scale was likely using pulse distance coding to encode the data. The scale would start each bit with a 500ms pulse. Then it would follow-up with either another 500ms pulse, or a 1000ms pulse. Each combination represented either a 1 or a 0. The problem was, [Darell] didn’t know which was which. He also wasn’t sure in which order the bits were being transmitted. He modified a software plugin for his logic analyzer to display 1’s and 0’s on top of the waveform. He then made several configurable options so he could try the various representations of the data.

Next it was time to generate some known data. He put increasing amounts of weight on the scale and recorded the resulting data along with the actual reading on the display. Then he tried various combinations of display settings until he got what appeared to be hexadecimal numbers increasing in size. Then by comparing values, he was able to determine what each of the five bytes represented. He was even able to reconstruct the checksum function used to generate the checksum byte.

Finally, [Darell] used a Raspberry Pi to hook the scale up to the cloud. He wrote a Python script to monitor an infrared receiver for the appropriate data. The script also verifies the checksum to ensure the data is not corrupted. [Darell] added a small LED light to indicate when the reading has been saved to the Google Docs spreadsheet, so he can be sure his weight is being recorded properly.

Small-Scale Projects Use Snail-Mail Mail-Scale scale offers a free USB scale when you sign up for their service. The first versions of this scale did not have a display. In order to find the weight of an object the scale had to be connected to a computer running the software. If you happen to have one of these old scales or are able to pick one up cheap, you may be interested in using it outside of the service. There are several options on how to do that.

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Hacking Digital Scales for the Disabled


[Jan] works with both physically and mentally disabled individuals, some of whom cannot read, making many of their tasks more difficult. Although [Jan] is not in a position to teach reading or writing skills, he was able to build an add-on device for the scales used in repackaging sweets to provide simple feedback that the user can interpret.

The device has three LEDs—red, green, and yellow—to indicate the package does not weigh enough (red), weighs too much (yellow), or lies within an acceptable range (green). The industrial scales at [Jan’s] workplace each have a serial output to connect to a printer, which he used to send data to the device. An ATMega8 controls the lights and an attached LCD, with the usual trimpot to change the display’s contrast and a rotary encoder to adjust the device’s settings. Everything fits snugly into a custom-made frosted acrylic enclosure, laser-cut at a local hackerspace.

[Jan] provides a rigorous guide to approaching each step on his Instructables page, along with source code and several pictures. See a video overview below, then enjoy another scale hack: building one from scratch.

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Posterior Posture Videogame Controller

Normally we see some crazy mad science projects coming from [Ben Krasnow’s] laboratory. This week [Ben] changes gears a bit and hacks his Xbox controller to interface with his bathroom scale and function as a posture controlled input device. You may want to take a moment for that to tumble around in your noggin before we trying to explain. What this means is you sit catawampus on a bathroom scale and when you lean forward your game character moves forward, lean back your character backs up and lean side to side for strafe left and right.

A modern digital bathroom scale has four pressure point transducers — one in each corner — which are read by the central controller and summed to generate the weight of the object setting on the scale. To use the scale as a controller input [Ben] removed the central scale controller and created two amplified Wheatstone bridge differential circuits, one for each diagonal axis between load cells. After adding an offset potentiometer to fix the resting point at 0.8 volts, the amplified differential voltage signals are fed directly into an Xbox controller’s thumb stick input for game control.

Additionally, to add rotation to his new game controller he hacked a an old ball type mouse and added a bit of rubber tubing that contacted and tracked the base of a  Lazy Susan platter. The scale sits on the Lazy Susan and allows for the partial rotation of your torso to controlled game rotation. However, [Ben] still needed a regular mouse interfaced with the game for full 360° rotation control.

There is more after the break, plus the build and demonstration video.

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