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Drew Fustini

Biosensor Q & A

Posted by Drew Fustini May 19, 2011



Our biosensor array team was recently featured on the Make blog:


I really liked the answers that team member Shawn Blaszak wrote up but unfortunately they didn't make it into the Make post, so I figured I would post them here.


1. What was it like working on the GGHC challenge?


ANSWER:  Working on the challenge has been a lot of fun for me.  My mom is a teacher, so I've always had an affinity towards k-12 education and am familiar with many of the difficulties educators face in this country (to say nothing of what teachers in third world countries must face).  I've found it really rewarding to know that I'm working on a project that could help teachers all over the world lower the cost of entry to something that can help students learn about how the human body works and, possibly, about electronics.    Also, we've seen a huge showing of support from the membership of our PS:One.  As the project progressed towards the contest deadline, even members that were not, originally, signed up for the contest team have pulled together to help advance the project.  In all honesty, I believe that this may constitute the largest number of PS:One members that collaborated on a single, organized, project.   Considering that, as I’m sure is common at other spaces, organizing hackers can, often, feel like herding cats, I find this to be an amazing accomplishment that serves to help bring out space community together while, simultaneously, producing something to make the world a better place.


2. How did you split up roles with your teammates?


ANSWER:  Our roles, for this project, have been split in, what I would describe as, an organic manner.  There has never, really, been a “leader” for the project (though, Avner Shanan did take lead on ensuring timely blog posts and most communications with the event organizers).  Major decisions were discussed, amicably, in groups.  This, usually, happened in person after our weekly space meetings but, also, happened on the mailing list we created, specifically, for this project.  Personally, I believe that our ability to have regular, in person, meetings really helped to improve communication amongst the group as well as group cohesion.  As for the work itself, we agreed to split it amongst the group members based on the interests and skill sets of the individuals involved.  We had a large enough number of members interested in participating that we were able to have a wide range of skills available to us.  Members (such as Dan Dumitrescu, Drew Fustini, and Bill Mania) that were better with electronics took “ownership” of individual sensors; others took responsibility for documentation (such as Avner Shanan); etc.  In the end, our system was flexible enough that when, as is inevitable with a volunteer based group project like this, some members had to stop participating, we were able to compensate for the loss.


3. In coming up with a project, did the educational requirement of the contest pose a special challenge for you?


ANSWER:  I don’t know that I would say it posed a “special” challenge for us, but I would say it was definitely a challenge.   Having a teacher for a parent I, personally, feel I had an advantage in having some basic knowledge about the mindset of teachers; their average technical competency; and what kind of environment they have to deal with on a daily basis (at least in the U.S.).  On the other hand, it’s still pretty hard to put yourself in the shoes of a real-world teacher and come up with something totally new that would significantly improve their ability to perform their job and, also, be cheap enough/easy enough to build that they can put it together.


4. How did you come up with your idea?


ANSWER:  Our first team meeting consisted of a brainstorming session for project ideas.  As I remember it, one of our other members (I don’t remember who) suggested doing something with DIY biology.  From there, we fleshed out the idea of doing a low cost, unified, biosensor array.  This lead to further discussion of things like what sensors we thought we could do in the time allotted; what kinds of teachers might find it useful; what the target age groups might be; etc.  


5. Tell me about the sort of student who you think would learn a lot from the Biosensor Array project.


ANSWER:  Right now, our primary target age group is in the Junior High and High School age range.  It was our intent that it might appeal to biology, health, and gym teachers for giving their class a direct view of what goes on in the human body.  With a more simplified/colorful software interface and/or a simple game, it might be possible to extend the target range into younger groups.  A secondary group that might find the project appealing would be junior high and high school technology classes interested in building the project in class as a way of teaching the kids basic electronics.  An ideal situation might be where a school’s technology teacher builds the project as a class exercise and then passes the finished unit(s) along to the school’s biology/health/gym teacher for use in their class.


6. What was it like working with all those sensors?


ANSWER:  If I had to put it in one word, I think I’d say “challenging”.  We learned, early on, that each sensor seemed to have it’s own “personality”.  For example, some sensors (such as temperature) were very stable while others (such as Galvanic Skin Response and ECG/EKG) were inherently instable because of how sensitive they are and how reliant they are on a stable mechanical connection with human skin.  Much of the challenge was in figuring out how much filtering each sensor needed to overcome things like ambient noise.  It was really rewarding seeing the final, clear, signal once we got each sensor working.


7. One requirement of the contest is that the project be easily reproducible, what sort of skill level is needed to build the Biosensor Array? ANSWER:  We’ve tried to keep the difficulty of constructing and using our project as low as possible.  Early on, the decision was made to base the whole thing on Arduino protoboard shields and through-hole components.  While we would, eventually, like to see a custom PCB version of the system, using shields, protoboard, and through-hole components means that anyone with basic soldering skills, and a minimum of hand-eye coordination, should be able to follow our build instructions and put together their own functioning unit.



Our biosensor team is honored to be selected a finalist and humbled to be in the company of fellow finalists, Hackerspace Charlotte's Feltronics & BuildBrighton's Phonicubes.







I had a total "WOW!" moment when I realized, while watching the Feltronics video, that the circuit on the white board was a radio transmitter!  I think Feltronics will be a big help to electronics instruction everywhere including at hackerspaces.  And how can one not love a giant felt multimeter!








Phonicubes is both adorable and innovative.  I really appreciated how they integrated different technologies into one smoothly operating device.  I think it will serve children learning to read very well and would love to see a kit or even retail version produced.







Beyond the finalists, I had my eye on the Big Board project by the Hack Factory team in Minneapolis from the start.  A 10x breadboard and working components is just so darn cool!  I can't wait to pick up a giant LED and stick it into a breadboard someday.






The OpenLab by the Melbourne Hackerspace "down under" really impressed me with their professional design & top-notch documentation.  Having used an awful, proprietary classroom lab system in the past, I think OpenLab fills a big need for electronics education.  Also, I think their "Team moment of the week" posts on element14 really exemplified what the Challenge was all about: coming together as a team to solve problems and build cool stuff for eduction!







Another project that amazed me was the TileBot from Artisans' Asylum in Boston.  It's like a 21st century version of the classic LOGO and its turtle!  I think their simple-to-use, yet open-ended system will lead to endless learning & fun for children.







Workshop 88, our hackerspace neighbors just a few miles West of Chicago, built the incredibly reconfigurable Educubes.  These IR networked, touch-sensitive blocks can help kids practice math on the fly or just about anything else that be dreamed up in the software.


Also, I can't say enough how much the Workshop 88 folks embody the true essence of the hackerspace community.  Our biosensor team decided late in the game that we wanted to add a local display, but the best option was the sold-out Adafruit 2.8" TFT LCD.  We emailed Workshop 88, and they offered to give us an extra LCD.  When the spare proved faulty, they even offered to sacrifice one of their Educubes and give us a LCD known to be working.  I can't overstate how gracious they were.  Although being relatively close to each other, our two spaces have not had that much interaction previously, but I think now, prompted by the competition, we are getting to know each other better!







Finally, I'd like to highlight a kindred DIY biology project in the challenge: the BioBoard by Noisebridge in San Francisco.  They developed an amazing set of sensors like pH & dissolved oxygen along with a fantastic web interface with a database backend.  Their system can be used by students to investigate biological processes like fermentation.  In true spirit of the hackerspace movement, I hope that our two teams will collaborate in the future to make a most excellent DIY biology platform for students to use.






On behalf of the entire Pumping Station: One Biosensor Array Team, I'd like congratulate all our fellow teams in this challenge.  There may be only 3 teams selected as finalists, but, as Mitch wrote, all the teams are truly winners.  I have no doubt the hard work that was poured into each project will provide great benefit to anyone who seeks to learn & discover our wonderful world for years to come!





The Pumping Station: One Biosensor Array Team



The primary sensor I've been prototyping for the Pumping Station: One Biosensor Array is the galvanic skin response, or GSR, sensor.  GSR is a measurement of the electrical conductance of the skin.  Sweating causes changes in the conductance, which offers an indication of emotional excitement, because the sweat glands are controlled by the sympathetic nervous system.  GSR does not offer a fine grained indication of emotion, since it does not distinguish between the many different emotions which elicit a change in skin conductance.



     GSR graphed on SparkFun LCD shield


If you have a multimeter on hand, then you can measure the resistance between two fingers by pressing one probe against each finger.  The range for the resistance of skin is typically 50k to 10M Ohm (per Sean Montgomery).  Skin conductance is the reciprocal of that resistance.  Thus 50k Ohm is 20 uS (micro Siemens, not Seconds) and 10M Ohm is 0.1 uS.  To summarize, skin conductance is inversely proportional to skin resistance.


However, if one just looks at their skin resistance on a DMM, then it will be very difficult to determine when their skin is responding to emotional changes.  Thus, a circuit is required which produces an output voltage proportional to the change in skin resistance.  The circuit needs to filter out noise and amplify the GSR signal.  This output voltage can then read  by any microcontroller (an Arduino for this project) and graphed over time in software.


I tried many different approaches when prototyping GSR, but I found a modified version of the Truth Meter circuit by Sean Montgomery worked the best for our application:




Sean built this circuit so that no microcontroller is needed as it employs both a low & high pass filter to create a 0.5 Hz to 5 Hz band pass which works well for GSR since it is a slow 1 to 2 Hz signal.  The low-pass filter cuts off high frequency noise above 5 Hz such as 60 Hz noise from AC power.  The high-pass filter cuts off frequencies below 0.5 Hz, or 2 seconds.



Truth Meter uses a 9V for power and an LED for output. The brightness correlates to the GSR magnitude. (Photo by Jordan Bunker)


In effect, the high-pass filtering subtracts the baseline average skin resistance leaving the only change in skin resistance within the 1-2 second range of GSR.  The result is that the GSR sensor is able to auto-calibrate for each user regardless of the the baseline skin resistance.  Essentially, the Truth Meter circuit produces just the GSR events, or spikes, where an emotional response occurred which is illustrated by this graph on Sean's website:




Sean's Truth Meter circuit worked really well for our project as the voltage output of the circuit is already filtered and can be simply read by disconnecting the LED and instead connecting the output resistor to the Arduino via an analog input.  The complexity of the Arduino code is reduced as no additional filtering needs to be done in software.



Biosensor GSR circuit prototype based on the Truth Meter connected to Arduino analog input.


Arduinoscope, a Processing sketch, is running and graphing the output the Arduino is sending via serial link over USB.  The spike pictured below is the result of me poking myself in the leg with the leads of an LED to illicit pain:


I found slapping myself also worked well too:



Safety Tip:

As an aside, always have your laptop unplugged from AC power when doing biosensing prototyping.  First and most important reason is safety.  Second reason is that the analog inputs will have 60 Hz noise imposed on their waveforms.  I found this to to be the case even with the bandpass filter in the GSR circuit.  The above screenshot was taken when the laptop was plugged in AC and the waveform looked much smoother when laptop is running off battery.  For our actual biosensor array, a Bluetooth module & 6xAA batteries are used so that there is no connection to laptop or AC power.


After prototyping on breadboard with good results, I made the Biosensor Array GSR shield which is pictured below.  It is stackable with the ECG shield and has header for the BlueSMiRF bluetooth module.  It is essentially the Truth Meter circuit but with pots for the important resistors to enable maximum calibration by the user: 1MOhm (skin voltage divider), 10kOhm (non-inverting reference voltage divider, substituted for the diodes) and 100kOhm (gain of final op-amp):


These simple finger straps below connect to header on the GSR shield and are extended for actual usage by our biosensor array cabling system.  Based on the Truth Meter instructions, I built the straps from copper foil & velcro tape I got at Jo-Ann Fabrics.  After building several pairs, I went out and got some stranded wire as it makes a much better solder joint than solid core wire on the copper foil.  Also, I made sure to use lead-free solder as the solder joints are in direct contact with fingers during usage.


And finally, if your interested in building your own Truth Meter, the circuit is featured in Make Magazine Vol 26 (Spring 2011) in the "Biosensing" article by Ira M. Laefsky & Sean M. Montgomery:



  • Truth Meter kit assembly & usage video by Make:



    In my next blog post, I will share the other circuits I prototyped and challenges encountered along with how I plan to improve the GSR sensor in the future, like refining the LCD display:







    Working feverishly down to the wire, we posted our final project status over on our own hackerspace blog and emailed our final documentation to the organizers in the nick of time.


    Thanks so much to Mitch Altman, Mark and the rest of the Silverfox crew, and element14 for this wonderful opportunity!


    Jordan Bunker, PS:One founding member & ReMade producer, was most excellent to stay up late with our biosensor team and took some great photos during our final integration demo (he also brought Red Bull to share!):



    Photo by Jordan Bunker

    Below is the final biosensor array configuration outside of the enclosure & panel jack cabling.  The bottom shield contains Galvanic Skin Response electronics, while the top is the ECG shield which also has a socket for the pulse oximeter IR sensor (not inserted during this picture).  The electronics for CO2 saturation, respiratory flow rate & temperature are all integrated onto the actual sensors, so their cables connect directly into the Arduino input pins (or via jacks & cabling when in the enclosure).  On the right-hand side of the photo is the BlueSMiRF, a Bluetooth modem, which connects to the top shield as a seamless wireless replacement for the USB serial data link - an important safety consideration given the sensors attach to one's body.



    Photo by Jordan Bunker


    Dan, who designed the ECG shield, yet again played our intrepid test subject:



    Photo by Jordan Bunker


    Tonight (oh gosh - it's actually morning ) might have be the end of the competition build, but we plan to continue our work developing DIY biology technology to further education.




    The Pumping Station: One Biosensor Team!



    P.S. Hope all our fellow teams are getting some well deserved sleep now!

    Our biosensor array team worked late into the night on Saturday at Pumping Station: One.  With the hackerspace challenge build deadline on Tuesday night, it's definately crunch time!



    Thankfully we've got a nice array of working sensors (in the order of signal speed):


    • EKG
    • Pulse oximeter
    • GSR (Galvanic Skin Response)
    • CO2
    • Temperature



    Dan yet again played the role of test subject:



    The QRD1114QRD1114 sensor from SparkFun contains an IR LED and a compatible phototransistor.  It dectects pulse via the change in the amount of infrared light reflected by finger as blood pumps through it.


    We observed a great correlation between EKG and pulse oximetry signals: