5 volt sensors interface

Comments

• Martin Hebel Posts: 1,239

  2006-04-09 18:44 edited 2006-04-09 18:44

  As an output, the Propeller can drive 5V devices fine since it is above the threshold.
  
  As an input, a 1K-10K resistor will provide needed current protection from a 5V device.
  
  -Martin
  
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  Martin Hebel
  Southern Illinois University Carbondale - Electronic Systems Technologies
  
  Personal Links with plenty of BASIC Stamp info
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• TimC Posts: 77

  2006-04-09 19:33 edited 2006-04-09 19:33

  Martin your solution works for one way data but not two way, such as I2C data.
  
  For 1 and 2-way traffic a real level shifter could be used. Like one of these 3 chips:
  
  SN74LVC8T245
  GTL2010
  ADG3308
  
  NO DIP format that I can find. This world is not getting hobby friendly
  
  Regards
  Tim
• Larry Posts: 212

  2006-04-09 20:17 edited 2006-04-09 20:17

  Jeff,
  
  You'll have to do a little NEWTHINK, but it's really not such a problem. For example,
  
  I2C works fine at 3.3 v. Just make sure the pullups go to +3.3v, and maybe use slightly smaller resistors.
  Proper I2C device interface with the clock and data lines as open collector (thus the pullups). They don't power the lines.
  
  
  Many CMOS devices likewise will work just fine at the lower voltages. I use 8 Pin Pic chips al the time at 3 v. with no problems,
  Likewise Sharp IR sensors and lots of other stuff. A good example is the 24LC256 used for the Propeller memory- It'll run at either voltage.
  
  In fact, circuits are more forgiving with regards to overcurrent at those levels. I'm not saying you should do it, But it is possible to run an
  LED right off a PIC pin with no resistor and not sustain any damage at 3 v.
  
  The transition is pretty easy. Datasheets for devices will get you started, and if you are stumped, here's the place to ask..
  
  I look foreward to running projects off of just a single lithium battery.
  
  Larry
  
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• El Paisa Posts: 375

  2006-04-09 20:22 edited 2006-04-09 20:22

  This issue is a touchy one.
  Some 3.3v devices are 5v tolerant, other not.
  The chip designer is the only one that should specify the max specs.
• Paul Baker Posts: 6,351

  2006-04-09 20:23 edited 2006-04-09 20:23

  Actually the limit resistor works for bidirectional communications as well. The resistor is there to limit the current through the diode clamp in the propeller. For a bi-directional line they are almost always configured with pull up resistors, so you use a pullup resistor to 5V on the side of the limit resistor the 5V device is. The limit resistor will have to be scaled to be roughly an order of magnitude (10x) smaller than the pull up resistor so when the propeller sinks its pin to 0V the 5V input will be under the logicl low threshold. If you use a 10kΩ resistor for the pullup and a 1kΩ resistor for the limit, the low voltage seen by the 5V device would be 0.45V which is safely into the low voltage threshold of the 5V device.
  
  Now for 5V analog signals, a resistor divider must be used to maintain the linearity of the signal.
  
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• Jon Williams Posts: 6,491

  2006-04-09 22:21 edited 2006-04-09 22:21

  The I2C spec shows how to connect a 3.3v device to 5v devices -- using fets in the lines with pull-ups on each side of them.
  
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  Jon Williams
  Applications Engineer, Parallax
• JED1 Posts: 17

  2006-04-09 22:25 edited 2006-04-09 22:25

  Thank you everyone for your input.
  
  Jeff
• JED1 Posts: 17

  2006-04-15 19:01 edited 2006-04-15 19:01

  Thank you Jon, but it is just that, I was wanting to avoid any extra components needed. Any chance an I/O
  module to minimize this interfacing will be provided by parallax?
• Jon Williams Posts: 6,491

  2006-04-15 19:09 edited 2006-04-15 19:09

  Not likely, but you never know.
  
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  Jon Williams
  Applications Engineer, Parallax