reading a 1.1v signal with the Prop?

Comments

• Mike Green Posts: 23,101

  2010-09-12 13:25 edited 2010-09-12 13:25

  Something as simple as an LM339 comparator should work. It can run on the same supply voltage as the Prop (3.3V) and its switching threshold can go down to zero. You'll need some kind of high impedance voltage divider to supply the switching threshold (maybe 0.8V?) The datasheet for your device should tell you what the maximum logic low voltage is.
• Phil Pilgrim (PhiPi) Posts: 23,514

  2010-09-12 14:01 edited 2010-09-12 14:01

  The simplest solution might be to bias the Prop's input so that the 0-1.1V signal evenly straddles the Prop's 1.67V threshold. This circuit will do that:
  
  
  
  When the device outputs 0V, the Prop will see 1.34V; when it outputs 1.1V, the Prop will see 1.99V.
  
  -Phil

  

  

  input_bias.gif

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• $WMc% Posts: 1,884

  2010-09-12 14:11 edited 2010-09-12 14:11

  Mike Green wrote: »

  Something as simple as an LM339 comparator should work. It can run on the same supply voltage as the Prop (3.3V) and its switching threshold can go down to zero. You'll need some kind of high impedance voltage divider to supply the switching threshold (maybe 0.8V?) The datasheet for your device should tell you what the maximum logic low voltage is.

  '
  This is excellent Mr.Green.
  '
  I did a quick check through my organizer drawers and viola, I have 3 LM339's on hand.
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  I think I understand the high impedance voltage divider, Know after reading Your post reply a few times.
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  I'll bread board this circuit and see how it works.
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  Many Thanks!!!
• $WMc% Posts: 1,884

  2010-09-12 15:32 edited 2010-09-12 15:32

  Phil Pilgrim (PhiPi) wrote: »

  The simplest solution might be to bias the Prop's input so that the 0-1.1V signal evenly straddles the Prop's 1.67V threshold. This circuit will do that:
  
  
  
  When the device outputs 0V, the Prop will see 1.34V; when it outputs 1.1V, the Prop will see 1.99V.
  
  -Phil

  '
  Thanks for the reply PhiPi
  '
  This is close to what I started with. I later found that just the touch of my hand on the data or CLK pin would add enough load to shut down the tiny device.
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  The data sheet, gives the CLK speed and the info on the data, two 24bit word LSB first,Thats it.
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  The first word starts with a 55uSec start bit and ends with a 55uSec stop bit. So I see 110 uSec between words.
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  The CLK-out in the data sheet shows 90kHz, I read it as 77kHz with the Parallax O-scope.
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  I have pulled the parts to set-up Mr.Greens circuit. I'll give this a try,And report back with my findings
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  Thanks for all the help so far!
• Dr_Acula Posts: 5,484

  2010-09-12 18:36 edited 2010-09-12 18:36

  I wonder if a voltage follower might help? Ideally one that can handle rail to rail - I've found the CA3140 is a good chip - very reasonably priced. Just connect the negative to the output and feed the input into the +ve.
  
  Though maybe I misunderstand the problem - are you reading digital or analog signals? If digital, then any logic gate would work.
  
  It seems odd that the signal is so fragile. ? another cause for it not working, eg a noisy power supply.
• $WMc% Posts: 1,884

  2010-09-13 12:36 edited 2010-09-13 12:36

  Dr_Acula wrote: »

  I wonder if a voltage follower might help? Ideally one that can handle rail to rail - I've found the CA3140 is a good chip - very reasonably priced. Just connect the negative to the output and feed the input into the +ve.
  
  Though maybe I misunderstand the problem - are you reading digital or analog signals? If digital, then any logic gate would work.
  
  It seems odd that the signal is so fragile. ? another cause for it not working, eg a noisy power supply.

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  Thanks Doc.
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  If the LM339 doesn't do the trick it give it a try.