PIR sensor need help

Discussion in 'The Projects Forum' started by ericyeoh, Jan 21, 2011.

  1. ericyeoh

    Thread Starter Active Member

    Aug 2, 2009
    58
    0
    Guys i still dun understand the band pass design in below attrached circuit.

    the 1st IC1A and IC1B opamp hv , FL and FH at 1.59hz.

    FH = (1/[2(pie) 10mic x 10K]) = 1.59hz
    FL = (1/[2(pie) 0.1mic x 1M]) = 1.59hz

    how come both cut off frequency at 1.59hz? if this is the case, the bandwidth =0 hz?

    Please help me pls, i'm lost
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I'm no expert on filters, but they're not like a brick wall. Wouldn't that arrangement give a peak transmittance at 1.59 Hz and taper off on both sides?
     
  3. ericyeoh

    Thread Starter Active Member

    Aug 2, 2009
    58
    0
    But as i studied about PIR sensor, the radiance produce by human carry around 0.1-10hz with 1mV-20mV waveform.
     
  4. frpr666

    Active Member

    Feb 2, 2010
    37
    6
    Use free simulator LT Spice. It is really great.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Perfectly consistent then. Filter out everything that's not near 1Hz ± factor of 10
     
  6. ericyeoh

    Thread Starter Active Member

    Aug 2, 2009
    58
    0
    wayneh, i cann;t catch what u meant. Could you please explain further?
     
  7. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    The IR radiation is not what this circuit is about. These sensors are typically dual (opposing) elements. The circuit senses a rate change across the elements. The filtering aspects of the circuit are designed around this rate of change. Motion from a radiant object has a very low frequency compared to circuit noise, which the latter must be filtered out.
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,032
    Combining a low pass and a high pass filter gives you a band-pass filter. Frequencies too high or too low are attenuated. An audio amplifier is sort of a band-pass filter, since it allows frequencies in the 20-20kHz range to pass, but not much outside that range. Of course that's a 1000X range, quite wide and a challenge to achieve.

    In your case the band allowed to pass is centered at 1.59Hz, and the width of the band allows a range of nearby frequencies to also pass. But frequencies too high or low - things not related to a human - are filtered out.
     
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