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IR Light Curtain Circuit Questions

  1. Sep 30, 2009 #1
    These questions relate to an IR Light curtain i am trying to build.
    The light curtain is going to be used to cut the power to an electrical testing bench when any of the curtains IR light beams are broken.

    The project will consist of IR transmitters ( IR LED's(zd1946)- data sheet-http://www.jaycar.com.au/products_uploaded/ZD1946.pdf), [Broken] IR Receivers
    (Photodiodes (zd1952)-data sheet- http://www.jaycar.com.au/products_uploaded/ZD-1952.pdf [Broken]), and the circuitry needed to connect them to the mains power and trip the power to the bench when an IR beam is broken.

    My questions relate to the connection of the IR receiver to a power supply and the wiring parameters i need to be aware of.

    1. does there need to be a current limiting resistor between power supply and photo diode?

    2. does the IR led need to be pulse width modulated, if so how?

    3. what is the general wiring layout to power a photodiode that has a TTL level output, how can i use this TTL output to send a stop signal to the mains power supply of the bench?

    4. any information that you think could be helpful in this situation would be great!


    I thank you for taking the time to read this rather long thread, and any help would be greatly appreciated.
     

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    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 30, 2009 #2
    hello brooksENG-
    Your receiver module apparently wants (needs?) a carrier frequency for the IR signal. Use a NE555 to modulate the light emitting diode. Do not exceed about 1.5 volts across it. Use a series resistor to limit current.
    Bob S
     
  4. Sep 30, 2009 #3

    berkeman

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    Staff: Mentor

    I only skimmed the datasheets so far, but here are a few thoughts.

    You wouldn't current limit the input to the receivers. They look to be monolithic detector/amplifier modules. They will only consume what current they need.

    You will want to set the TX current within the datasheet limits for those emitting diodes. You do that with the drive voltage and series current limiting resistors.

    It is generally good to use IR filter plastic windows at the detectors to cut out ambient interfering light.

    It's not so much PWM drive of the IR drive, but more some modulation frequency drive of the IR emitters and a bandpassed detection of that modulation at the IR receivers.

    Quiz Questions -- Why would such a system help the ambient noise rejection? How should you choose the modulation frequency? What lessons can you use from TV IR remote control standards to help you in your IR curtain design?
     
    Last edited by a moderator: May 4, 2017
  5. Oct 1, 2009 #4

    vk6kro

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    Science Advisor

    My questions relate to the connection of the IR receiver to a power supply and the wiring parameters i need to be aware of.

    1. does there need to be a current limiting resistor between power supply and photo diode?
    YES. This is a 100 mW diode and it has a forward conducting voltage of about 1.3 Volts, so the maximum current before the dissipation limit is reached will be about 76 mA. Start off with less than half of this, though.


    2. does the IR led need to be pulse width modulated, if so how?
    It has to be switched on and off at a 38 KHz rate. Just a square wave is OK.

    3. what is the general wiring layout to power a photodiode that has a TTL level output, how can i use this TTL output to send a stop signal to the mains power supply of the bench?

    An IR receiver will produce a 0 Volt output while it receives a signal and 5 volts when it loses the signal. (This is not the same as a photodiode).
    You could use these signals to switch a relay in the power supply if you feel qualified to do that safely. When you talk about mains power, you ring alarm bells. Can you do it safely?


    4. any information that you think could be helpful in this situation would be great!

    You would want to design it so that any cutting of the IR beam cuts off the power and leaves it off so it has to be reset.
     
  6. Oct 21, 2009 #5
    I am not going to actually wire it to the mains i just need to know how it could be done (design)
     
  7. Oct 21, 2009 #6

    berkeman

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    Looks like vk6kro gave you lots of useful information. What have you done with it so far?
     
  8. Oct 21, 2009 #7

    Firstly thank you for your help.

    Can you please explain what you mean by TX current?
    can you please elaborate on the modulation frequency drive of the IR emitters and bandpass detection?

    As you can probably tell im out of my depth here. its like trying to learn several topics i have had no exposure too.

    thank you.
     
  9. Oct 21, 2009 #8
    I have been on holidays for a couple of weeks and have only just returned to the task.

    I have been trying to take the very useful information given to me and, firstly, understand it, second try use it to my advantage.
    I have not had much experience with cct design but i am thankful for everyone's help on this subject.
     
  10. Oct 21, 2009 #9

    berkeman

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    Staff: Mentor

    The TX current is the transmit current that you drive each IR LED with. It looks like vk6kro gave you some info on how to set the TX current with a current-limiting resistor in series with your drive signal.

    You will need to drive the TX LEDs with a signal around 39kHz (see the receiver module datasheet for the frequencies for the different modules). This is done to reject DC IR background noise, and noise at 50/60Hz and its harmonics from AC lighting nearby. The receiver module only responds to IR light that is being switched on and off at that 39kHz frequency.

    So you will need to find/make a source of 39kHz square wave voltage, and drive a number of LEDs with that in parallel (each LED has its own current-limiting resistor). Make the curtain with a row of the TX LEDs on one side, and a row of the receivers on the other. Combine the outputs of the RX modules with logic, and use that overall logic signal to trigger a flip-flop that drives a relay for the AC power to the bench to cut it off. Put a reset button on the FF to reset the whole thing.
     
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