AnalogPackrat

Need help with a simple circuit design - touch switch
« Reply #20 on: October 20, 2008, 12:35:58 PM »
I found my copy of the Serge TKB--it was in my thick Synth DIY file.  I don't have a scanner, so I took a digi pic and diddled with it a bit.  Sorry about the poor quality.  The source paper was not that good.  



Obviously this isn't the whole circuit which is a two page schematic including all the synthesizer specific bits (multi-channel sequencer, etc.).  The important part is on the left and upper part of the image.  The touch pads themselves are the little numbered ovals directly above the 74C150.  They are of the same design as the previous schematic I linked in the message above.

The four inverters with RC networks form an oscillator which drives the touch pads.  The CD4520 is a binary up counter which is clocked from the same oscillator and in conjunction with the 74C150 multiplexer allows the pad array to be scanned sequentially.  The two inverters driving RC networks in the upper center look like a variation on the Don Lancaster idea posted by PRR.  The difference is that the shape of the pulse, and hence the "delay" of synchronization, is independent of the touch pad capacitance.

I don't understand what the sensitivity adjustment thing at the top left is doing.  Maybe someone else can explain it to the rest of us...

A P
If it is to be, it is up to me.


CJ

Need help with a simple circuit design - touch switch
« Reply #21 on: October 21, 2008, 04:56:56 AM »
This looks different than that kit:

http://vacuumbr.ipower.com//The_Lab/Radio_Shack/555_Timer/555_5.jpg


Not to lurking terrorists:
Please do not use circuit 9 for the construction of explosive devices.


Actually, people are a lot cheaper than 555 chips in Iraq.

And more reliable.
If I can't fix it, I can fix it so nobody else can!
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sahib

Need help with a simple circuit design - touch switch
« Reply #22 on: October 21, 2008, 07:46:56 AM »
My apologies. Wrong thread.


 

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