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Unintentional Theramin... — Parallax Forums

Unintentional Theramin...

HughHugh Posts: 362
edited 2010-02-14 16:35 in Propeller 1
Hi,

I have a Protoboard with the Prop connected to an LCD, an SD card and three switches. Each switch brings a Prop pin to ground when pressed - otherwise pull-up resistors keep the pins high. Today I noticed that one of the switches appears to activate if I hold my hand near the protoboard (the number on the LCD counts down as it should if the switch is pressed). As part of troubleshooting I cut the wire close to the protoboard but it still happens.

The other switches/pins are unaffected and a 0.1uF capacitor soldered from the Prop-pin to gnd didn't make any difference. There are quite a few long messy wires (it is a ptototype!) but its got me stumped. Any thoughts?

Hugh

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Hugh - the thinking woman's Geoffrey Pyke.

Comments

  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-02-13 07:27
    I used to get wierd effect like that on AVR lash-ups. Usually it went away when the reset lines had pull-ups and the right amount of decoupling caps were put on. you could try to put the whole thing on top of some grounded aluminium foil, separarted with a couple sheets of paper of course, to prove it. That shielded a lot of the electical fields.

    I notice that the Prop ccts always seem to favour the pull-down res and the switch on the high side.

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    Style and grace : Nil point
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-02-13 09:32
    Hugh,

    The pull-up on that switch is either open or not connected to Vdd.

    -Phil
  • ElectricAyeElectricAye Posts: 4,561
    edited 2010-02-13 14:12
    Of course you can fix the problem orrrrr.... you can join the Clara Rockmore fan club and get rockin!

    www.youtube.com/watch?v=WPZQi2m7i9Y
  • HughHugh Posts: 362
    edited 2010-02-14 14:51
    Cutting back a few wires and wriggling other wires so they were better laid out seemed to have fixed it. Strangley, it wouldn't happen as much with other people - its ether my magnetic personality or the wires holding my chest together acting as antennas! wink.gif

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    Hugh - the thinking woman's Geoffrey Pyke.
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-02-14 16:35
    Thats a tough one. Resets from your pacemaker? If you were truly commited to to Prop path ....

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    Style and grace : Nil point
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