help to interpret a PIR sensor circuit

Hi!
I'm new to electronics and are about to use the Murata's IRS-B210ST01 PIR pyroelectric sensor. Provided to the sensor is schematics of typical application circuit for human detection:
**broken link removed**

or


Shortly the circuit is a filter/decoupling, a load resistor, a non-inverting amp with gain around 100, an ac-coupled biased inverting amp with gain 100 and a window comparator and after that is not of interest..
However I've some questions regarding it:
1: Why are the window comparator's hi and low voltage reference levels not equal level from the midpoint between GND and Vcc? Is it possible that they have forgot a 260k resistor? It makes no sense as it is. In other words if Vcc is 3V why are not 1.5V used as midlevel (equal to the offset of the output signal from the inverting amp) and as an example hi level is 0.3V above and low 0.3V below?

2: What are the capacitors in the feedback of the op amps used for? Stabilization or some kind of filtering?

3: What purpose do the diode in the first amplifier have?

4: Why do they use capacitors between the interting and non-inverting inputs of the op amps??

5: Why do they use two resistors of equal value on both the non-inverting and the inverting inputs of the op amps?

I need answers quick and therefore I post here because I believe that it will take longer to mail the company.


And some questions regarding simulation of the sensor that is not as important:
is there any PSPICE simulation models of PIR sensors using dual elements available? I've searched for it but I can't find any..
I've tried to make a simple model my self using the equivalent circuit but because there is almost no specifications provided to the sensor I find it not that easy. What can typical values be of the equivalent circuit of a ceramic pyroelectric sensor? I believe the parallel resistance is 1G-10Gohm but what can the parallel capacitance be? How to choose an appropriate FET for simulation? How is the equivalent circuit affected of the sensor using two sensor elements being connected together? Either PSpice or Multisim like two current sources to be connected in series.. Is it correct to use two in parallel and the current from either is the differences (between the two in series in the real circuit) where one generates positive pulses and the other one negative pulses?

I really appreciate answers
thanks!

1 diffrent resistors used to introduce hysteresis
2 to reduce high frequency noise

The PIR detector circuit schematic is correct and used in millions alarm devices. The first preamplifier has a low-frequency response, and capacitors in opamp feedback determine the filter response. The diode in opamp feedback prevents lockup under an overdrive.
Equal resistors in opamp inputs are needed for their function- please read a textbook on opamps. Many questions you have presented also indicate that you need to study basic circuits. They cannot be answered in a short entry.

picgak said:

1 diffrent resistors used to introduce hysteresis
2 to reduce high frequency noise

Click to expand...

thanks for your answer!
I now understand 2.

But I don't really understand 1. Cannot the window comparator itself be considered as a kind of hysterisis where the "dead window" is the hysteris?
It cannot be that the positive and negative response (output signal) of the sensor is unbalanced somehow and therefore requires different reference levels (difference in voltage from midlevel to high/low reference level)?

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jiripolivka said:

The PIR detector circuit schematic is correct and used in millions alarm devices. The first preamplifier has a low-frequency response, and capacitors in opamp feedback determine the filter response. The diode in opamp feedback prevents lockup under an overdrive.
Equal resistors in opamp inputs are needed for their function- please read a textbook on opamps. Many questions you have presented also indicate that you need to study basic circuits. They cannot be answered in a short entry.

Click to expand...

thanks for answering

I agree that I should have figured out the purpose of the use of feedback capacitor and the equal input resistors myself. I've read more now and I understand how it works.
thanks for the information about the diode.

The only thing that I cannot figure out now is why the voltage divider for the window comparator isn't equally balanced? In all other similiar application circuits I've seen the voltage divider is equally balanced.
I fully understand (at least I think I do) the function of the window comparator so that's not the problem on this question.

berres said:

thanks for your answer!
I now understand 2.

But I don't really understand 1. Cannot the window comparator itself be considered as a kind of hysterisis where the "dead window" is the hysteris?
It cannot be that the positive and negative response (output signal) of the sensor is unbalanced somehow and therefore requires different reference levels (difference in voltage from midlevel to high/low reference level)?

- - - Updated - - -



thanks for answering

I agree that I should have figured out the purpose of the use of feedback capacitor and the equal input resistors myself. I've read more now and I understand how it works.
thanks for the information about the diode.

The only thing that I cannot figure out now is why the voltage divider for the window comparator isn't equally balanced? In all other similiar application circuits I've seen the voltage divider is equally balanced.
I fully understand (at least I think I do) the function of the window comparator so that's not the problem on this question.

Click to expand...

There are two reasons: - one is a practical adjustment, dependent on typical installation situation. For instance, a small shift prevents false alarms due to PIR detector response to small animals, moving curtains, and the like,.
The other reason is that the circuit is "single-supply", while most opamps and comparators are powered by two +/- DC power supplies.
For an optimum setting, some PIR detector circuits had a potentiometer for local optimization of alarm output.

It is not for noise filtering actually. These are basic band pass active filters. You can see details below link.
https://www.electronics-tutorials.ws/filter/filter_7.html