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Newbee: Optocoupler to BasicStamp

Discussion in 'Electronic Basics' started by Sam, Oct 5, 2005.

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  1. Sam

    Sam Guest

    I am new to all this stuff an could use a little help.

    I am trying to determine the state of a MOC3010 Optocoupler using a

    I have tried both reading the pin state as well as using the POT command to
    read the resistance of the 3010.
    I have tried pullup and pulldown (10K) resistors.

    Could someone help me design a simple interface circuit to read the state of
    the 3010?

    Also, I want to power the LED portion of the optocoupler from the computer
    USB port. I have been using a 100ohm resistor. Is this a good value to use?

    Thanks for your help
  2. Driving the IRLED (about 1.2 volts drop) in the coupler with 5 volts
    through 100 ohms will allow a current of about (5-1.2)/100=.038A which
    is lower than the maximum rating of .06A (60mA) and above the maximum
    trigger current of 10mA, so it is reasonable.

    But why are you using a TRIAC output coupler to communicate anything
    to a microprocessor? It is designed to control line voltage, AC
    loads. Once the TRIAC is triggered, it will stay on as long as its
    current stays above the holding current (100uA, typical), regardless
    of the input current.
  3. Sam

    Sam Guest

    Thanks for your reply.
    I did not realize some of the things that you said regarding the triac.

    Is there a optocoupler that I can use that will just act like a switch based
    on the LED status (preferably available at RS). What would the interface
    circuit look like.

  4. You need a coupler that has a photo transistor as the output device.
    Adding a 10k pull up resistor to that output should give you a nice
    digital output. Radio Shack is a very limited source of such things,
    but you should be looking for something like 4N25:
  5. Lets start a couple steps back form here. What are you trying to
    accomplish with the optocoupler? Is the Basic Stamp powered from the
    same source as that driving the LED in the coupler?
  6. Sam

    Sam Guest

    Thanks again for your help.

    The Fairchild site has 12 optocouplers. I dont really understand the specs.
    Anyone specifically I should be looking for?

    The Stamp will NOT be powered by the same source as the LED.
    The Stamp needs to detect when several computers are turned on or off (each
    computer will be on its own input pin).
  7. All 12 are slight variations on a theme, any of which would do what
    you want. They just have slightly different current transfer transfer
    ratios (amount of output current relative to the LED input current.
    If you find a source for any of these or anything similar, they will
    work for your application.
  8. Sam

    Sam Guest

    Thanks again.

    It's not as easy as you would think to find small quantity electronic
    components in NYC. Once a store wanted to charge me $12 for a 7400 logic
    chip! Probably will go mail order but have to place a minimum order.
  9. Jasen Betts

    Jasen Betts Guest

    MOC3010 is designed for switching AC power, use 4n28 instead.
    it can be done, you need a way to control the current flowing through it.
    (or supply it with AC) and also you need to accept a 3v drop across
    the device while it's on...
    no.. usb has 5v, the LED wants [email protected] max, so you need a resistor for just
    over 3.5V 3.5V / .015A = 233 ohms, so use 270 Ohms.

    ....|.... USB
    . 2 1--[270R]--+5V
    . .
    . .
    +---------4 6------[10K]------------+---> to pic input pin
    | ......... |
    | | |
    | optional power section -----
    9VAC---+--->|-+----[7805]-+--> power to pic |/ \ 4.7V
    | | | | zener
    ----- | --- |
    ----- | --- |
    | | | |
    Ov -------------------------------------------+--> to pic ground pin

    it's way easier to just use a 4n28, you only need a pull-up resistor.
    (and the LED resistor)

  10. Sam

    Sam Guest

    Thanks :)

  11. ehsjr

    ehsjr Guest

    Sad but true. Don't overlook
    It's usually cheaper than others, but they are not a supply house.

    You can make a list of parts you need and keep adding. When it gets
    big enough, use Mouser, Digikey, Newark, whoever and the s/h
    charge doesn't irritate as much single orders with 6 dollars shipping
    for one 29 cent part.

    Also, don't overlook investment in learning. For example, you
    indicated that you are unsure about optos. You could buy 5 different
    optos from Allelectronics for under 3 bucks plus s/h, and experiment
    with them to learn a bit more.

  12. Sam

    Sam Guest

    I just ordered some 4N28s. While I am waiting for them to arrive I thought I
    would play around a bit.
    I just cracked open a mouse and removed the photo transistors, but cant get
    the circuit to work.

    I have the center leg (base?) going to the input on my Stamp it is also
    pulled up by a 10K resistor.
    I have tried grounding both (not at the same time) other legs of the photo
    transistor, straight to ground and also through various resistors.

    I cant get the Stamp to detect on/off states. Any suggestions on where my
    circuit went wrong?

    Thanks all again.

  13. For most sensitivity, you leave the base unconnected, so the charges
    generated by light build up the maximum base voltage to turn the
    transistor on. But every photo transistor I have seen in a mouse
    doesn't even have the base connection brought out to a pin. They are
    usually two pin devices, just collector and emitter, NPN devices.

    I would connect the emitter to ground and use a 10k pull up resistor
    to the positive supply. Then when you shine light on the transistor,
    it should pull down the resistor voltage to within about a half volt
    of zero. When kept in the dark, the resistor voltage should approach
    the positive supply voltage. Something like this:

    | |
    | |

    (created by AACircuit v1.28.4 beta 13/12/04

    Once you get that to work, you can make an opto coupler out of that by
    sticking the infrared LED part of the mouse, lens to lens, against the
    photo transistor, with a drop of 5 minute epoxy.

    drive the LED through a 470 ohm resistor from the 5 volt supply, and
    you should be able to swing the photo transistor on and off via that
    light source. Then paint the whole thing black, to keep out stray light.
  14. Ralph Mowery

    Ralph Mowery Guest

    Offhand I would say you leave the base open and connect the other two leads
    to the Stamp. The collector (probably need a pullup resistor) goes to an
    input and the emitter goes to the ground or common of the circuit.

    The input to the base of a transistor is what normally turns it on. In the
    case of a phototransistor the base is left open (in most cases) and its
    input is the light (photons ?) .
  15. Sam

    Sam Guest

    This photo transistor does have 3 legs.
    I will try your circuit tomorrow, so tired cant focus to even write this

    I will report back after trying it.

    Thanks :)
  16. Sam

    Sam Guest

    Just a quick report back...

    I think your circuit works, but the photo transistor is so sensitive that it
    is picking up ambient light an triggering. I could not get a stable reading
    even with the lights turned out and the circuit covered by foam, I don't
    want to start gluing and painting until everything is stable.

    Anyway, I am going to wait for the optocoupler in the sealed package and try
    again in a few days.

    Thanks again
  17. You can also lower the sensitivity by using a lower value collector
    resistor, like 4.7k or 2.2k. But I suspect you have something else
    connected wrong. Were you testing the output with a meter or with the
  18. Rich Grise

    Rich Grise Guest

    Not even with a NYC yellow pages?

    Just curious.

  19. ehsjr

    ehsjr Guest

    The chances of one of John's circuits not working right are extremely
    close to 0. Suspect bad part/wiring error/noisy supply/poltergeist/etc.
    before thinking the circuit is wrong.

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