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Question about capacitor function in the circuit??

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samy555

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Hi
For the following motion detector circuit:

**broken link removed**

I know that the first IC1A operate as an amplifier, but I can not guess the function of capacitors C2 and C3.
Can you help me please
thanks
 
C3, at high frequencies shorts out R4 and sets the gain at 1.
C2, has two functions.
1. At DC (low frequencies) is open to the signal and sets the gain at 1.
2. (I don't know what the output of PIR looks like so assume a signal)
....The average voltage at PIR is a voltage. That voltage will be across C2.
....If the signal is a square wave at 50% then this voltage is at 1/2 the supply.
The gain of the amplifier is 1 at low frequencies, approx 100 at some frequency and 1 at high frequencies.
 
it's a bandpass filter (a narrow one) with a peak gain of near 100 at 15hz. lower frequencies are rolled off to keep the slow movement of sunlight, changing shadows, etc... from triggering it, as well as higher frequency transitions... some engineer figured this response was very close to the optimum response to detect most motion of animals or people walking or running... if you saw the mythbusters episode about defeating motion detectors, the best way is to move very slowly, and now you can see why. a single large object moving faster than what would produce a peak response will still produce a transition within the response of the filter, and likely be detected, but smaller objects in motion that fast may get filtered out.
 
...I know that the first IC1A operate as an amplifier, but I can not guess the function of capacitors C2 and C3....

Frequency response.
 

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The peak is +33dB compared to the low band and high band. That means the peak has 50x the amplitude of the low parts.
 
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