resistance problem?

[yaiy]

hi great forum. first i want to tell all the guys  that helped me on my last troubleshooting accident. hope its the right place.

my problem now is not midibox specific,  but as this problem troubeled me in an earlier midibox project i ask.

i made a "pressure sensitive" grid of digital switches, made from 2 pecies of thick aluminium foil with double sided sticky window sealing foam (see pic bottom). my problem is i am getting slow response from my "buttons". i am using a usb keyboard as logic.

the keybaord has a matrix type logic so i am running 5 rows over five columns of plastic sheets with the aluminum foil sprey gload on the inside. my frst attmpt didnt work as the conductive aluminium tape i used was too resistive, long areas and not enough current running i guess.

this prototype works mechncly but i was hopeing to get it fast enough to respond to thrown objects, like slingshot stubs. now it works only for touch. any other problems in the design?

the usb provied a current of 5v thru the usb interface.

The Data and Clock lines are both open-collector with pullup resistors to Vcc.  An "open-collector" interface has two possible state: low, or high impedance.  In the "low" state, a transistor pulls the line to ground level.  In the "high impedance" state, the interface acts as an open circuit and doesn't drive the line low or high. Furthermore, a "pullup" resistor is connected between the bus and Vcc so the bus is pulled high if none of the devices on the bus are actively pulling it low.  The exact value of this resistor isn't too important (1~10 kOhms); larger resistances result in less power consumption and smaller resistances result in a faster rise time.  A general open-collector interface is shown below:

    Figure 1: General open-collector interface.  Data and Clock are read on the microcontroller's pins A and B, respectively.  Both lines are normally held at +5V, but can be pulled to ground by asserting logic "1" on C and D.  As a result, Data equals D, inverted, and Clock equals C, inverted.

Note: When looking through examples on this website, you'll notice I use a few tricks when implementing an open-collector interface with PIC microcontrollers.  I use the same pin for both input and output, and I enable the PIC's internal pullup resistors rather than using external resistors.  A line is pulled to ground by setting the corresponding pin to output, and writing a "zero" to that port.  The line is set to the "high impedance" state by setting the pin to input.  Taking into account the PIC's built-in protection diodes and sufficient current sinking, I think this is a valid configuration.  Let me know if your experiences have proved otherwise.

from http://www.computer-engineering.org/ps2protocol/

this is a montage of the work so far (500k pic)

http://img.photobucket.com/albums/v315/pixelpusher/slingshotMake.jpg

[TK.]

I think, that this is not an electrical, but a conceptional issue. Computer keyboards are usually polled with a rate of 50 times per second (-> 20 mS), this is not fast enough for your project.

With the sm_example2 project you could do the same at a rate of 12500 times per second (-> 80 uS)

Best Regards, Thorsten.

[stryd_one]

I was thinking earlier that a S&H circuit might help ... But if I were you, I'd take TK's advice. He knows stuff  ;D

[yaiy]

TK, i couldnt find a referance to a sm_example2 anywhere in here or ucapps.

i will try to connect a midibox interface i have and check i want it super low cost for handing out to kids that cant efford smashtv prices ;).

i still hope i can implement stryd idea. ill look into it.

http://www.soundonsound.com/sos/mar04/articles/synthsecrets.htm

to show my love for the forum, here is a mini controller i made from a cigar box (very good material to work with)

couldnt find a better place to post this(?)

[stryd_one]

TK, i couldnt find a referance to a sm_example2 anywhere in here or ucapps.

Keep looking, it's there :)

I dig the cigar box!!! Raw  :D