touch plate

[Dimitree]

hi everyone
while this is not directly stompbox related, I'm sure many of us can help me.
I'd like to know what kind of touch sensor is this mounted on the Suzuki Omnichord:



This is a sort of electronic auto-harp..the touch plate is that on the right, it's composed by 12 "touch sensors" (according to the schematic..it doesn't say anything more than this), each one triggers a transistor that gates its corresponding note/sound to the output.
It can be played with bare hands, or with a plectrum (a rubber one from Suzuki, as far as I know)
Looks like many metal strings.. but I cannot undestand how that work..it's not capacitive I think because there's no capacity sensing circuitry..
My guesses it that it's like many metal bars, alternating between not connected ones and connected-to-ground (or some high level) ones, so when you touch, you short at least 2 of them..
what do you think?

[artifus]

bah! i had some bookmarks for this beast but they've died! (http://www.soundonsound.com/forum/showflat.php?Number=891146)

maybe a google image search?

maybe google 'probe wah schematic' or some such.

[Dimitree]

thank you for the help, btw I have the schematic but unfortunately it doens't help me to understand the kind of touch sensors used.

[defaced]

My guesses it that it's like many metal bars, alternating between not connected ones and connected-to-ground (or some high level) ones, so when you touch, you short at least 2 of them..
what do you think?

That way my first thought as well.  Can you post the schematic?  That'll probably help if there's anything that sticks out to someone. 

[Dimitree]

yes of course:
https://docs.google.com/file/d/0B8-MOD-DNXkHZGE3NmU4NzctZDhkYi00Njg0LWIwMmQtNjQ3ZDJjZDAxNzk4/edit?ups=drive_web&num=50&sort=name&layout=list&pli=1

[g_u_e_s_t]

it definitely looks like some sort of resistive sensing.  there is a bus connected to 12v, and then a bunch of fingers that go to envelope generators.  the odd thing, is that the input resistors to the sensing fingers are really small, such that it wouldnt work well just using the resistance of the hand.  is there a conductive band on top of the fingers?  a lot of the old touch sensors used a third conductive band that was suspended just above the fingers, and pressing on it brought it into contact.

[The Tone God]

Probably just a simple strip with each section going to a resistor. You finger contact connects the sections so as you move your finger along the resistance connected through the resistor network changes. Its basically like a linear pot only your finger is the wiper. There was a short trend awhile back with resistive strip theremins. Some DIY people even made them out of cassette or VCR tape.

Andrew

[earthtonesaudio]

it definitely looks like some sort of resistive sensing.  there is a bus connected to 12v, and then a bunch of fingers that go to envelope generators.  the odd thing, is that the input resistors to the sensing fingers are really small, such that it wouldnt work well just using the resistance of the hand.  is there a conductive band on top of the fingers?  a lot of the old touch sensors used a third conductive band that was suspended just above the fingers, and pressing on it brought it into contact.

I agree.  The impedances on both ends of the sensors are low-ish, and a metal-to-metal contact would make more sense than a resistor.  Although I am not familiar with those resistive strip techniques.  If the resistance range could be from the 100ish to >100k then that is plausible.

Also cool is the weighting resistors R1:R13, which appear to give the "touchpad" four different sensitivity zones.

[Dimitree]

thanks for the help..

it actually looks like resistive sensing:
https://www.dropbox.com/sh/kfsz747sr0yu7nv/D0o6B3S86w#f:IMG_8653.JPG

I tried to simulate the envelope geneators with Falstad and with ISIS, but no luck

[g_u_e_s_t]

thats interesting, it looks like a series of parallel conductive strips, which means that it would be just using the finger's resistance.  but, the pull down resistors are around 10k, which means you wouldn't get much meaningful voltage change unless the finger's resistance was less than 50k or so.  i just measured my fingers, and they were 1M.  ive seen anywhere from 100k to 1M when doing these sorts of tests in the past.

can you measure the resistance between 2 adjacent tracks both with your finger on it, and without your finger (power off)?  id be curious to see what its like.  perhaps there is something else going on that is not visible in those pictures.

[Dimitree]

unfortunately I don't have the device only those pics and the schematic..
here there is another schematic (the one used by the Omnichord in my pictures)..pretty similar (the biggest differences are in the tone generation circuit..)

https://docs.google.com/file/d/0B8-MOD-DNXkHNzY0ZWE4YjAtMDNlZi00YTY0LWI5YzYtNzNjMDhiYTI4MzJm/edit?num=50&sort=name&layout=list&pli=1

[g_u_e_s_t]

no, that one is very different.  they use 1M pulldown resistors followed by buffers.  that makes a lot more sense, and would work well with a finger.  perhaps the other schematic was for a device that had a special wand you used to make contact.

[Dimitree]

thank you
now my question is..why some transistor have 10k resistors, some 33k, some 24k and some 18k? what's the difference?
how can the amplitude be modulated with the transistor? It doesn't only act as a on-off switch?

[artifus]

How Do Transistors Work? NO, HOW DO THEY REALLY WORK?

1:

http://amasci.com/amateur/transis.html

2:

http://amasci.com/amateur/transis2.html

tl;dr:

http://amasci.com/amateur/trshort.html