Question about capacitor function in the circuit??

samy555 · Mar 25, 2013

Hi
For the following motion detector circuit:

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

keith1200rs · Mar 25, 2013

Filtering. C3 will limit the high frequency response, C2 will limit the low frequency response.

Keith.

samy555 · Mar 25, 2013

keith1200rs said:
Filtering. C3 will limit the high frequency response, C2 will limit the low frequency response.

Keith.

lucbra · Mar 25, 2013

I would assume the first stage acts as an AC amplifier, thus C2 removing the DC component. Otherwise stated - by C2 you don't need an offset voltage at the input of this amplifier because the opamps are not powered with + and - voltage.

IC1b, IC1C and IC1D operate as a window comparator, IC1B introducing the 1/2 power DC offset.

pplus · Mar 25, 2013

samy555 said:
Hi

I know that the first IC1A operate as an amplifier,

IC1A and IC1B operate as an filter f=1.59Hz

IC1C and IC1D operate as a window comparator U/2+-0.6V

samy555 · Mar 25, 2013

Thanks to pplus for help
When someone moves in front of the PIR, the frequency of the output signal will be small, about 1 Hertz or so.
But the resulting amplitude of that output signal will be up to 3 volts peak- as datasheets said.
Let's focus on the first stage, is it operate as a filter, amplifier or both?
What made the designer compelled to put that first stage?
Are my calculations in post # 3 are true and what is your comment?
Thank you

keith1200rs · Mar 25, 2013

Your calculations are correct. I am not convinced the circuit is. I would normally expect a difference between the high and low frequency filters - usually a factor of 10 or more. Typical values I have used in the past have been 0.1H to 1.6Hz. That is similar to the values shown in the datasheet www.onsemi.com/pub/Collateral/NCS36000-D.PDF

Keith.

samy555 · Mar 25, 2013

Thank you Keith
your Datasheet file was useful
I want if possible to direct me to a site talking about using op-amp as a band pass filter to see how to design mine for other voltage amplification, low cutoff freq and high cutoff freq
thanks a lot

- - - Updated - - -

Thank you Keith
your Datasheet file was useful
I want if possible to direct me to a site talking about using op-amp as a band pass filter to see how to design mine for other voltage amplification, low cutoff freq and high cutoff freq
thanks a lot

- - - Updated - - -

Thank you Keith
your Datasheet file was useful
I want if possible to direct me to a site talking about using op-amp as a band pass filter to see how to design mine for other voltage amplification, low cutoff freq and high cutoff freq
thanks a lot

pplus · Mar 25, 2013

samy555 said:
When someone moves in front of the PIR, the frequency of the output signal will be small, about 1 Hertz or so.
But the resulting amplitude of that output signal will be up to 3 volts peak- as datasheets said.

maybe it's sensors with integrated controller?

Let's focus on the first stage, is it operate as a filter, amplifier or both?
What made the designer compelled to put that first stage?

First and second stage are identical, but first is non inverting.Gain define R4/R3 or R8/R5 ~100
Characteristic first stage we are on image as Out1(green)

Are my calculations in post # 3 are true and what is your comment?

Yes, it is true.
P.S. Use LTspice - it is easy and free.

samy555 · Mar 25, 2013

thank you very much
please point me to a site talking about using op-amp as a band pass filter to see how to design mine for other voltage amplification, low cutoff freq and high cutoff freq
thanks a lot

samy555 · Mar 25, 2013

samy555 said:
thank you very much
please point me to a site talking about using op-amp as a band pass filter to see how to design mine for other voltage amplification, low cutoff freq and high cutoff freq
thanks a lot

I found one:
https://www.physics.unlv.edu/~bill/PHYS483/op_amp_filt.pdf

Without you I did not know that the first stage is Bandpass filter, I thank you very much.

I now move to the second stage, consisting of IC1B. It is also a band pass filter but an inverting type, where the first (IC1A) was a noninverting type.
my questions are:
1) Are D1 and D2 for voltage reference?
2) why the first stage was noninverting and the second inverting?
thanks

pplus · Mar 26, 2013

samy555 said:
my questions are:
1) Are D1 and D2 for voltage reference?

Yes. It is in detail described in manual. Post #7

2) why the first stage was noninverting and the second inverting?

First stage is non inverting for high impedance, required by sensor.
Second stage is inverting for removing the DC component by first stage, and set output DC=U/2 by window comparator.

I shall tell at once - D3 and D4 are logical OR for high level comparator output .

samy555 · Mar 27, 2013

pplus said:
First stage is non inverting for high impedance, required by sensor.

Ok, thanks

Second stage is inverting for removing the DC component by first stage,

I think C4 removes the DC component by first stage.
What would happen if interchaning pin5 and pin6 of IC1B?

and set output DC=U/2 by window comparator.

If the power supply was 12 volts, what will be the output voltage at pin7?

I shall tell at once - D3 and D4 are logical OR for high level comparator output

What can happen if you remove D3 &D4?
thank you very much

pplus · Mar 28, 2013

samy555 said:
I think C4 removes the DC component by first stage.

Yes, of course, but do not forget that it is also the part of the filter. Two in one.

What would happen if interchaning pin5 and pin6 of IC1B?

IC1B will have positive feedback and potencial output 7 will be near ground.
Guess why?

If the power supply was 12 volts, what will be the output voltage at pin7?

At pin7, pin5 and at pin6 voltage will be Usupply/2=6V/. It is defined to divider R6, D1, D2, R7.

What can happen if you remove D3 &D4?

Outputs IC1C and IC1D will compete among themselves and on input IC2 will be not defined voltage.

Here is files for simulation in LTspice.
View attachment Motion Detector PIR325.zip

samy555 · Mar 28, 2013

Thank pplus, has helped me a lot and made the effort to help me
Attached file fails and I get the following message:

I still have a lot of questions, but I'll try to discover answered through simulation
If I could not, I;ll ask help
I repeat my sincere thanks

pplus · Mar 29, 2013

samy555 said:
Attached file fails and I get the following message:

Sorry. I am forget.

samy555 · Mar 29, 2013

So far I could not Carry simulation, the same error massage appear, I hope you give me the specific steps that I must do so runs the program.

Secondly, I want to tell me what's wrong with using the following simple circuit?

Thanks pplus

pplus · Mar 30, 2013

samy555 said:
So far I could not Carry simulation, the same error massage appear, I hope you give me the specific steps that I must do so runs the program.

Put on
???\Program Files\LTC\LTspiceIV\lib\sym\EXTRA\CD4000 -SYM (*.asy)
???\Program Files\LTC\LTspiceIV\lib\sub\ - LIB (*.lib)
Change J1 to U309

Secondly, I want to tell me what's wrong with using the following simple circuit?

I do not understand what in the square "PIR sensor" attached base of the transistor.

samy555 · Mar 30, 2013

pplus said:
I do not understand what in the square "PIR sensor" attached base of the transistor.

the square "PIR sensor" is the PIR with 3 pins, it is as in the schematic Post #1
my questions are:
1- It is without any bpf or signal amplification?
2- the resistor (1k) connected the output signal (pin 2 of PIR) to +ve 12V,,, Would it not be better to choose a greater resistance (100K) and connect it to the negative terminal of the battery 12 like the design in the schematic Post #1

What I want to know is whether the filter is necessary and why?
Is resistance 1 kilo enough and the difference between connecting it to positive or negative terminal of the 12V batt.?
thank you very much

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