Testing OpAmp with Pressure Sensor (Part 1)

After arranging all the bits of pressure sensor and dual source OpAmp configuration on the breadboard, I ran a few preliminary weight measurement tests to get some idea of the range of measurement possible with different levels of amplification (i.e. varying the input voltage and gain).

The above chart shows the output voltage for a given weight plotted for the various levels of amplification with the low end registering just about 0.5V for 35lbs while on the high end, the signal was closer to 5V for the same weight.

Note that the weight is also plotted on the secondary y-axis as reference and, taken together with the voltage trends, appears to indicate that the higher output voltage had a higher degree of direct correlation to the weight/pressure while the lower output voltage lagged a bit.

In any event, the two variables (output voltage, weight/pressure) were clearly very strongly correlated regardless of level of amplification indicating (to me at least) that reliable measurements will likely be able to be rendered with even limited amplification.

The key now is in determining:

1. The weight/pressure range required for the application.
2. The influence of the amplification on the stability and repeatability of the measurements.
3. The minimum level of amplification ( ... and input voltage) required to reliably attain accurate measurements within that range.
4. The minimum configuration (i.e. single versus dual source OpAmp, better OpAmp for lower power sources) needed to deliver said accurate measurements.

Below is a list of scatter-plots showing the (obviously) strong correlation between the weight applied to the sensor and the output voltage using LOW, MID-LOW, MID and HIGH amplification settings.

Notice that while all correlations are stronger than an r-squared of 0.90, the higher amplifications yielded near r-squared at or above 0.99.

Vetting OpAmp Options (Part 1)

Since one of the sensors in my project requires amplification, I've been spending a substantial amount of time getting down and dirty with various OpAmps in an effort to find one ( ... and a particular configuration) that meets all my criteria.

Most importantly, the sensor needs to be amplified enough to present a consistent and wide range of load measurement while not drawing/requiring too much power.

OpAmps generally allow for either "dual" or "single" supply configurations meaning that they require either one or two power supplies in order to operate.

By power supplies, I mean power sources which, in the case of my project, ultimately means batteries... and for this project I would like to keep both the overall power draw and number of replaceable or rechargeable batteries to an absolute minimum.

You would think this decision would be simple... just go with the "single" supply configuration and an OpAmp that can operate with a very low voltage.... but it's not quite that easy as there are many trade-offs and nuances.

First, generally speaking... ( ... remember this is all essentially new to me) "single" supply amps can deliver the similar dynamic range of amplification as the "dual" configuration but with the requirement of a higher single voltage.

For example, a "single" source amp might be able to take a single 5V supply and amplify the source signal within a range of 0 - 3V while a dual configuration might take two 2V supplies to create a range between -1.5V and +1.5V ... also a 3V total range.

Further... there is a the possibility of choosing an amp that supports "rail-to-rail" operation which allows either a "single" or "dual" configuration to generate an output voltage very near the input voltage which appears to me to be essential in a low power application.

Ultimately though, the configuration has to work for the application so the first order of business is test both the "single" supply and the "dual" supply configurations with the sensor across a range of voltages in order to establish the minimum voltage needed to make accurate ( ... and distinguishable) measurements across the desired range loads.

My wishful hope at this point is that I can make accurate measurements with an amplified single that ranges from 0V ( ... no load) to 1V ( ... many hundreds of pounds of load)... but I wont really know for sure until I capture and plot the output data.

Currently, I have the sensor in a "dual" configuration with two 1.5V supplies ( ... a tad under the recommended lower bound of voltage for this amp) and it appears to function pretty well but the proof has yet to be established so stay tuned!

Best to all...

Hello World!

For this inaugural post I would like to present a few rough details about my background as well as outline the general purpose of this blog.

I'm a independent software developer who has spent the last 20 years bouncing around the Boston area participating on many software project across many different industries while simultaneously working on various interesting projects of my own.

I love, live breath and eat software development...

Ever since I was about 10 years old, when my older brother started teaching me how to program on his TRS-80 color computer, coding has been both part of my daily activities as well as a steadily more prominent feature of my overall identity.

I have always nurtured many passions and interests, but no pastime has ever been as fundamental as programming and most have involved programming in one way or another.

Now, with that out of the way... the purpose of this blog is to detail a new project I'm working on that involves more than just software engineering.

Without disclosing too many specifics... I've dreamed up a very ( ... to me at least) interesting concept for a wearable device that I hope to take from prototype to legitimate marketable application and then possibly to Kickstarter or some other crowdfunding venue.

The proposed device, like so many wearable/mobile devices today, involves an accelerometer and various other sensors but yet... nothing quite like it exists.

I like to think of this concept as an application of physioinformatics or bodyinformatics or possibly more simply as a "body information" device.

But for now... let's leave it at that.

If you follow along with this blog, I will disclose more and more of the concept as it becomes more real and in due course, you will know everything you ever wanted to know about it.

Why all the secrecy...

Firstly, I'm not entirely sure that I can pull this concept off... my background is squarely in software development and while I have tinkered over the years with various simplistic electrical engineering projects (re-wiring telescope motors, electric guitars and creating pickups, distortion and phasing foot pedals), I am by NO MEANS skilled in this area... in other words, I'd like to save face if I'm in way over my head!

Further, I truly believe in this concept and feel strongly that it represents a clear marketable application with the possibility of lots of new intellectual property, so while I would like to be as open as possible with the development, I still intend on keeping a cloak around some of the better concepts... at least until I have a strong foothold.

At some point, I may even consider open-sourcing both the hardware and the software but for now, I'm planning on just taking things one step at a time.

I'm hoping this blog will serve as both a log of the development, but more importantly, help to focus and collect my thoughts and efforts to better meet the challenges of each task at hand... I have found that there is nothing quite like writing to help channel the dealings of a complicated task into a productive direction.

Also, I need your feedback! ... I'm hoping that there are others out there that will take an interest in this project and possibly even contribute to the effort ... even if just with some terse random comments.

Best to all...