please help me with my infrared tx circuit

Discussion in 'The Projects Forum' started by Atif Abdulhady, Apr 2, 2011.

  1. Atif Abdulhady

    Thread Starter New Member

    Mar 30, 2011
    3
    0
    I am using an infrared tx-rx circuit as an obstacle avoidance for my mobot. the equipment used are infra red led, an infrared receiver module and resistors and lm555. the fact is that when i use the the circuit, i only get a high output from the infrared rx irrespective of whether when the IR led is on or off.i think the ir sender doesn't send with 38khz coz it's works with the remote control please help me to get the ir diode to send with 38khz frequency . this is my circuit http://www.mediafire.com/?w1n9aurjyz6ee75
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,647
    631
    That circuit should be tunable to 38 kHz -when the pot set in the middle if all values were spot-on, it would oscillate at about 36 kHz. You might want to try tuning the pot slowly through the range. If you have a means of measuring the frequency on pin 3 of the 555 that might tell you something.

    If the LEDs are being driven at a 50% duty cycle, a DVM would indicated approximately 0.9 volts across each LED.

    On problem that I would look for is a .001 uf capacitor that is way out of range. Be sure you aren't using one of those 20% ceramic decoupling capacitors!

    Although it should go without saying, I will remind you to double check all connections and assumptions about package pin assignments, especially if they are unfamiliar packages.
     
    Atif Abdulhady likes this.
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The horrible circuit is missing a very important diode across the relay coil that keeps the very old BC107 transistor from blowing up and is missing an input capacitor on the voltage regulator.

    The IR module has AGC that reduces its sensitivity when continuous 38khz is received from compact fluorescent bulbs and from that transmitter. The 38kHz is supposed to be modulated as described in the datasheet.

    The current in the IR LEDs is about 23 times less than the current in the IR LEDs of a TV remote control.

    Next time please post your schematic here, instead of over there.
     
    Atif Abdulhady likes this.
  4. Atif Abdulhady

    Thread Starter New Member

    Mar 30, 2011
    3
    0
    thank u Audioguru i know about the diode in the receiver but my real problem is with the sender, i faced the problem that u issued about reducing the sensitivity when continuous 38khz is received from that transmitter but can u be more specific about the solution coz i really don't know wt to do. and sorry about posting the diagram over there i was new to this website :)
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The datasheet for TSOPxxx IR receiver ICs describes in detail why they must not have continuous IR and describes the recommended burst of 38kHz pulses and the recommended pause time between bursts.
    Use the correct datasheet and read it.

    If your IR receiver IC is foreign then the datasheet probably does not bother to translate the details into English.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,086
    3,024
    You need to send some data on top of the modulated carrier. The carrier alone will not take the receiver output low - it will stay high as you are observing.

    You could send bursts of 1kHz, maybe on half the time and off the other half, with a cycle time of maybe 1/2 sec. Not sure about these timing details but you need a pulsing signal to pull the output low.
     
    Atif Abdulhady likes this.
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    No.
    Do not simply guess about the timing. It is detailed in the datasheet.
     
Loading...