Idea: LEDs of Demoboard as capacitive switch inputs?

Christof Eb.

Hi,
this is just a -perhaps silly- idea:
some time ago I have read about capacitive proximity Switches:
An Output is connected to an Input via a resistor. The Output is driven by a square wave signal.
The time between Change of Levels at the Output and the Input is measured. It will be longer, if there is an additional capacity -Hand or finger.

Could a similar Setup be possible with the existing Hardware of the demoboard? I am thinking about the leds and the resistors connected to them and the vga output.
Perhaps there is a timer mode to measure the very short time?

Christof

localroger

It's unlikely to work. Capacitive inputs work because they are very high impedance. It would depend on the LED voltage drop being large enough not to draw any current at a voltage the Prop sees as logic 1, and whether the LED's have any leakage below that voltage -- it wouldn't take much to make capacitive switching unworkable.

Clock Loop

Forrest mimms shows this could be done exactly because the capacitive falloff rate was measurable using some simple code.

BlackBox uses exactly this to read on the same output leds, the anode and cathode are connected to prop pins, schematic shows this... that is the limitation, if your demoboard won't allow both legs of leds to be controlled by prop, then you can't use your leds as sensors.

http://forums.parallax.com/showthread.php/115258-TheBlackBox-Release-v2.0-Propeller-HSS-FX-Sequencer-with-Digital-Audio-SPDIF

Code is this:

Pub FxRead

dira[FxBase] := 1           'set direction of fx led anode leg.      

Repeat
 
  '----bLINK led
  dira[FxLed] := 1             ' make n pin an output

  outa[FxBase] := 1             ' p - high     LED ON
  outa[FxLed] := 0             ' n - low       LED ON
  waitcnt(blinkwait + cnt)     'wait for time while led on
  
  outa[FxBase] := 0             ' p - low      LED OFF
  dira[FxLed] := 0              ' turn pin output off 

   '---READ led-----
  dira[FxLed] := 1             ' make n pin an output   

  outa[FxBase] := 0            ' p - low      'reverse bias led        
  outa[FxLed] := 1            ' n - high     'reverse bias led 

  dira[FxLed] := 0            ' n - make n pin an input
  waitcnt(readwait + cnt) ' wait for pin to change state
   
  FxStart := ina[FxLed]      ' read state of n pin, and store it.  

Duane Degn

Clock Loop wrote: »

if your demoboard won't allow both legs of leds to be controlled by prop, then you can't use your leds as sensors.

Here's the schematic of the LED section of the Demo Board.



Looks like they can't be used as sensors.

BTW, That BlackBox project is one of the coolest projects I've seen with the Propeller.

Humanoido

Instead of using capacitance, you could try converting the LEDs to inputs and touching one could alter its light sensing characteristics for an input response. Given the closeness of the LEDs on the Demo board, you may need a tiny touch stick instead of using a human finger.

VbGuru

An ITT classmate worked on using individual leds as buttons with the basic stamp 2. He had 6-8 leds about 1/8th of an inch a part and by lighting the led then turning it off and i think doing a bit of r/c time he could detect at each led individually. the final result worked very well for his project (a touch interface for his home automation).


This may or may not be related...

http://hackaday.com/2006/02/21/low-cost-sensing-and-communication-with-an-led/


hope that helps...

Christof Eb.

Thanks, for your input, alltogether!
I have now learned, that the cap will be for example 20pF. Together with the existing Rs this gives only 10ns, which will not be possible to be detected.
Perhaps I will try an additional R in the socket for VGA.
Christof