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Topic: need help with piezoelectric circuit (Read 5330 times) previous topic - next topic


I have ALS and am paralyzed below the neck.  I use a very sensitive piezoelectric switch taped above my right eyebrow and an onscreen scanning keyboard program to control my computer.  With a very slight twitch of my eyebrow and I can do everything I need to do.  The problem is I don't have a good backup in case my current ten-year-old switch box breaks.  I have only found three piezoelectric switches, and only one is sensitive enough and doesn't eat batteries quickly.  Unfortunately, it is very expensive ($450) and I haven't been able to find any for sale.

Anyway, I think the Arduino would be a great platform to make one of these switches, and very inexpensively (<$50).  I have these requirements:

1. 3.5 mm mini audio jack to connect piezoelectric sensor cable (I already have these)  
2. a knob to adjust sensitivity
3. an LED and a quiet beep to indicate switch activation
4. the Arduino should plug into a USB port and emulate a mouse click when the sensor is twitched
5. the whole thing should be powered from the USB port (no batteries)  

Does this seem possible?  I think the most difficult part of this would be amplifying the very small voltages and currents generated by the piezoelectric sensor, without introducing much noise.  Right?  



Hey there,

just a question: do you think these cheap piezos for about $1 could do the trick? Or do you have any special needs in terms of size, sensitivity, etc.

I've been playing around with these for a week now. The easy setup (I didn't even thought about amplification) really impressed me: http://arduino.cc/en/Tutorial/Knock



Richard: the sensors I use are pretty bare.  They must be more sensitive than the ones enclosed in plastic.  Here is a picture of the "wire side" of one of my sensors:  

I usually have a piece of Velcro stuck to this side and this goes against my skin.  I find that this makes it easier to activate the switch.  I think it provides more leverage to bend the sensor.  

Here is a picture of the other side:

This side is just flat copper and my caregiver puts a piece of paper tape over this side and tight across my skin.

I'm not aware of any volunteer groups doing this type of work, but it would be a good idea.  I have had help a few times with electronic projects.  Contact your local ALS Association or MDA chapter if you want to help someone out.

I am getting excited about this project and will publish the instructions if it works.  



Sammy: as you can see, my sensors are pretty big.  I have used smaller ones in the past, but these are easier and last longer.  I have seen the $1 ones you mentioned, but haven't tried them yet.  

I need to find a source for these too, because they charge way too much for replacements ($25 + shipping for two sensor cables).

I'm going to order a Uno and try the example you linked to.  I'll report back my results.  

Any suggestions for a quiet beeper?


p.s.  I have experience programming, but very little with electronics.  


Hello David,  

I do not mean to be rude but does ALS affect the muscle or the nerves?  Reason I ask is that I am trying to make an idea I had 20+ years ago actually work.  
I want to use myoelectric signals, such as those used in prosthesis to generate data rather than to run motors.  I am quite a ways away from getting done but tonight I joined a group of people that want to create open source prosthesis  and they currently use myoelectric input.

My goal is to have multiple myoelectric inputs  to generate a data stream.  However, it could also be used as a single input with the system you are currently using.  I do not know how expensive the board would be to collect the myoelectric impulse but there would not be any moving parts to replace and electrodes are inexpensive.

Paul H

Ran Talbott

You may want to use the Teensy (which is Arduino-ish),  instead.  It's designed to work as a USB device,  and has a mouse examply you could hack.

The new Uno is said to have similar capabilities,  but I don't know whether there are examples available for it yet.

The piezo you showed us looks pretty generic,  except that (nearly??) all the ones I have are made with a hole in the brass disk so that almost all the piezo part is exposed.  Probably because they were made for use as speakers.  The solid brass disk might be necessary to keep sweat and oils from getting on the piezo disk.  Or it may be coincidental.  You should find out before buying more.


A team at Penn State developed a electro impulse device that used muscles in the person's temple instead of a switch.

This might be something to consider.

Using two inputs would give an X and Y axis movement for something like BLINK but I am getting off topic here.  One input to move X, one to move Y and move both to select.



Is this switch a simple on/off, or is some type of signal generated by the piezo element? The small gyroscopic and accelerometer units used in model helicoptors might be of interest for detecting small movements made by an eyebrow.
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Paul: unlike with MS which affects muscles, ALS affects the upper and lower motor neurons (sensory nerves are not affected).  Upper motor neurons go from the brain to the spinal cord, and lower motor neurons go from there to the muscles.  

I don't believe there is any myoelectric activity in the affected areas.  

I am very dubious of systems that claim to pick up myoelectric signals.  It always seems like they require some muscle movement in order to work.  In that case, why not use the much less expensive piezoelectric technology?  I would love to be proven wrong on this.  



Richard: although my mother says I was in training for this eyebrow twitching stuff by watching so much Groucho Marx in my youth, I am unable to move my eyebrows independently.  I might be able to find another spot for a sensor, I don't think it would buy me much improvement.    

The switch I like the best is the P-Switch from this company:


But, I haven't been able to find it this week.  My second choice is this one:


The problems with this are it eats a 9 volt battery every 3 days, and adds noise to the signal if the sensitivity is turned up too high.  I was able to use a wall wart to power it, but it would go crazy after a couple days and starts activating on its own.  It never did this on battery, and I never figured out how to solve the sensitivity issue.  

I tried to use the power adapter with the P-Switch and it went crazy immediately, firing randomly on its own.  This isn't a problem because this switch lasts 3 months on a 9 volt battery.  I tried rechargeable batteries and found they didn't last nearly as long.  

I should mention the only reason I tried another switch is that two years ago I bought a new laptop with a dual band Dell 802.11n wireless networking adapter, which caused the P-Switch to go crazy as described above.  After a year I happened to swap the Dell adapter with an Intel one (for other reasons), and the interference problem went away.  

I use the sensors from the Don Johnston switch box with the PRC switch box because they are more sensitive and last longer.  

See my previous post for comments about neurons.

zoomcat: these switches implement a fixed time momentary signal.  You can't vary the length of the pulse like you would need for morse code.  The accelorameter is a good idea.  I'll look into that if I can't get piezoelectric working.  



Ran: thanks for the Teensy recommendation.  That is an awesome little board!  Also, I think you are right and I was wrong about the large disk on these sensors.  It's brass.  



Hello David,

I am sorry to hear about your nerves being affected.  That will make my approach a poor choice for you.  



Hello David,

an hour ago i've tested my cheap and standard 2.9kHz piezo (which has the same size and looks a lot like yours on the images you posted) with my belt pressing it above my eyebrow. ... kind of Karate Kid ;-)

I used Processing with the sketch from the following tutorial to visualize my eyebrow movement as a first step: http://arduino.cc/en/Tutorial/Graph

The sensitivity was very good! Also slight movements has been triggered.

Hope this helps. Sammy.


sammy: that is great news!  I can't wait until my Uno arrives and I can try it.  If it works, I plan to order a Teensy, a small enclosure, and a USB A to mini B connector.  

What does the 2.9kHz refer to?  The cheap piezoelectric sensors usually don't show specs.



Paul: as Richard noted, my eyebrow muscle still gets a good signal.  If you know of a good, inexpensive myoelectric sensor I can buy or make, I'll try it out.  I'm only interested in a binary switch, but if you have other ideas, let me know.    

Have you heard of ECoG implants?  I think you will find this talk enlightening:



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