Please explain the action of these capacitors

Discussion in 'The Projects Forum' started by haxxx, Mar 16, 2009.

  1. haxxx

    Thread Starter Active Member

    Feb 14, 2009
    This is a part of a guest arrival detection circuit i found online.
    Could anyone explain the action of the capacitors in the circuit on the right when the path between the led and the tsop1736 is broken.
    Also the datasheet for the tsop suggests using a rc network across pin 1
    and 2 of the tsop to suppress power supply disturbances,
    Is it unecessary in the design of this circuit or inadvertently omitted.
    If so is there any harm in me adding it.

    Last edited: Mar 16, 2009
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    I would suggest adding the capacitor from pins 1 to 2 would certainly do no harm. Is it necessary - from where are you obtaining Vcc at the receiver? Is it likely to be a source of noise.

    The capacitors (1uF and 0.1 uF) are there to filter the transmitted signal - the TSOP1736 is intended for more complex duty as a PCM receiver, but I guess is used here to respond only to a narrow frequency band - around 37KHz. It is then less likley to get false indications from sources other than the IR beam being interrupted by a guest arriving. That's why you have a frequency adjustment pot on the transmitter to tune the frequency to the receiver's band centre frequency.

    Your remote TV controller may upset it ...?:rolleyes:
  3. Audioguru


    Dec 20, 2007
    As I said on another website, both circuits are missing supply bypass capacitors.
    I fixed and posted a fixed circuit but I can't link to it since I can't remember which electronics chat site.
    The IR receiver does not need the two or three resistors and filter capacitors since its ouput is digital, not analog.

    With the continuous 36kHz the receiver IC's AGC will reduce its gain which will reduce its range. It expects bursts of data with pauses in between for its gain and range to remain high.
  4. haxxx

    Thread Starter Active Member

    Feb 14, 2009
    I just responded to ur post on another site, but let me repost here
    in case u didn't see it.
    If i understand u correctly and based on the graph in the datasheet, the
    555 delivers the square wave output with necessary
    pauses so the tsop can distinguish its signals from ambient light, my question is, wouldn't the resulting high and low output cycle from the tsop equal a false trigger to to the transistor, clearly it doesn't i just want to know why.

  5. Audioguru


    Dec 20, 2007
    The 555 delivers 36kHz continuously, exactly like an interfering compact fluorescent light bulb. The TSOP detects the continuous carrier and turns down the gain and range to reduce the interference.
    You need a second 555 to turn the carrier 555 on and off making bursts of 36kHz with pauses. Then it is like remote control data.
    As I said on the other website.