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filter capacitor question and so on . — Parallax Forums

filter capacitor question and so on .

yarisboyyarisboy Posts: 245
edited 2010-01-07 03:17 in Propeller 1
Hi,
As suggested by members I'm using an H11L1M optoisolator to protect the input on my Propeller. The test circuit in the technical data sheet shows a resistor and capacitor in parallel between the signal source and the opto input. For good measure I also have a zener diode between the opto input and ground. I understand how to size my resistor for a given source voltage to keep from damaging my zener diode and my opto input but I've forgotten which type of capacitor I should use. Also, is the size frequency dependent? My fogged-up old memory suspects the RC time should have a relation to the desired input pass band frequency. The output side of the H11L1M test circuit shows a pull-up resistor between Vcc and the output. Because I'm using a Propeller my Vcc should be hooked to the 3.3 volt rail right?

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MOORE'S LAW: The capabilities of electronics shall double every 18 months.
cloyd's corollary: Hardware is easy, software is hard.

Post Edited (yarisboy) : 1/6/2010 8:36:33 PM GMT

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-01-06 18:37
    yarisboy said...
    For good measure I also have a zener diode between the opto input and ground.
    The zener really does no good right at the LED (since the LED's forward voltage will be less than the zener breakdown voltage), except to protect the LED from reverse voltages. If reverse voltages are possible, a regular silicon diode, in reverse polarity to the LED, will suffice. If you're still worried that the LED's forward current rating could be exceeded, a zener and a second series resistor is what it will take to protect it.
    yarisboy said...
    Because I'm using a Propeller my Vcc should be hooked to the 3.3 volt rail right?
    Yes.

    -Phil
  • yarisboyyarisboy Posts: 245
    edited 2010-01-06 20:41
    When I built a BS2 board I had to put little ceramic disk capacitors on my Tx and Rx lines. In this application I don't know if they would still fill the bill. Good point about the zener. Thanks

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    MOORE'S LAW: The capabilities of electronics shall double every 18 months.
    cloyd's corollary: Hardware is easy, software is hard.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-01-06 20:50
    The cap you cite in your original post is a "speedup" cap, which helps the device achieve its rated response time specs. I've never used one with the H11L1. Depending on the frequency of your signal, you probably won't need it, either.

    -Phil
  • yarisboyyarisboy Posts: 245
    edited 2010-01-07 03:17
    I ran the numbers and you are right about the frequency too. The OBD II tachometer wire won't be putting a signal out above 300 Hz nor will the vehicle speed sensor (VSS) wire. The square wave that comes from my dash light dimmer control hasn't been checked with the scope yet. Now I can re-configure my layout with two resistors in series with a zener to ground at their connection point. That way anyone putting this kit together can easily adapt it to different voltage inputs by resistor selection.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    MOORE'S LAW: The capabilities of electronics shall double every 18 months.
    cloyd's corollary: Hardware is easy, software is hard.
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