can I use a photoisolator?

Discussion in 'General Electronics Chat' started by mlaymance, Dec 6, 2008.

  1. mlaymance

    Thread Starter New Member

    Nov 22, 2008
    7
    0
    I have a small project that I am working on, a system monitor for the HVAC system in my home. I want to be able to monitor AC input voltage status (on or off) and thermostat status (fan on, compressor on, etc.....)

    I would like to eventually interface this to a microprocessor (PIC), but for now would like to be able to indicate status with LEDs. Most of the circuits I understand, however, I have never used optocouplers/photoisolators.

    when I look at the spec sheets they don't give voltage values, only current.

    is this possible to monitor these circuits this way, or do I have to use relays. I was really hoping to use all solid state components if possible.

    Thanks for the assistance

    Michael
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    First consider that working with the mains supply is dangerous.
    However, you can do it easily with relays. Why do you want to use solid stahe devices?
    More information about what you want to sense would be helpful.
     
  3. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    "when I look at the spec sheets they don't give voltage values, only current."

    That's because the light emitter inside a optoisolater is an LED type device and such devices are operated by current not voltage. So it's up to the circuit designer to install an external series resistor such that the expected voltage when present is limited to the specified operating current for the opto device, typically 10ma or so. Also if monitoring AC voltage one has to look at the maximum reverse voltage that the opto can work with. If the monitored voltage is higher then that then a series diode is also need to limit current in one direction only.

    Lefty
     
  4. mlaymance

    Thread Starter New Member

    Nov 22, 2008
    7
    0
    Basically I want to monitor whether there is Voltage present on a circuit (I.E. fan, heater circuit, etc.

    The reason i want to do all soild state circuits is because of size. The monitor panel will be very small therefore big honker relays take up too much room.

    I am very careful around the mains power as always.

    Thanks,
    Michael
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You can use small relays with a coil voltage rating equal to the mains voltage in parallel with the device you want to check. When the device will be turned on the relay will turn on and power a led. If you want to use opto-couplers, you will need to use current limiting resistors on their inputs because they will fry if you connect them directly to the main supply. Read this article to see how you use a diode at the input of an opto to protect it from reverse voltages:

    http://www1.jaycar.com.au/images_uploaded/optocoup.pdf

    Or you can use an AC input opto like this:

    http://www.nteinc.com/specs/3000to3099/pdf/nte3089.pdf
     
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