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Help reusing a PIR sensor

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Evangelopoulos Panagiotis:
Greetings,

I found in my junk box an old security motion sensor that uses one of those Passive IR (PIR) sensors. I was in the mood of making a some kind of meter to detect changes of temperature in a room.
The whole sensor circuit switches it's output high or low to if it detected movement. But what I want is a more analog output that will swing a small volt meter or a digital one made with an LM3914.

So i pulled the sensor bit out of the board and wired it up to a 5V supply and my scope. The output of the sensor goes to about 1V 1.5V and then swings around that value if it senses any change in IR.
I measured the swing to be from as low as a couple of mV all the way up to a few hundred mV.

I noticed that the sensor seems to be doing some sort of self calibration procedure. When you power it up it is trying to match 1 or 1.5V to the ambient room IR and when you move your hand near it the voltage swings down. But if you hold your hand near it then it will eventually reach the 1 or 1.5V value again. 
A note here, the swing is really slow so I don't know if a DC blocking cap can work here to isolate the swing.

My question is: Can a simple circuit be designed to take that few mV input and turn it into an output that swings from 0V to 5V?

My guess is that a differential amplifier can be used here and It will probably need an accurate voltage reference but I don't know how to design that.

An other note, I found a 10k thermistor in the enclosure. Was it used to subtract the ambient temperature from the PIR sensor?

I know micro controllers seem easy and it's tempting to use a 10bit ADC but I'd like to keep this a complete analog design if possible.

Thank you all in advance,
E.P.

RoGeorge:
Some PIR sensors are digital, some are analog.

Remove that white plastic support, read the marks printed on the metal case, and find its corresponding datasheet.

Evangelopoulos Panagiotis:

--- Quote from: RoGeorge on March 10, 2020, 02:34:46 pm ---Remove that white plastic support, read the marks printed on the metal case, and find its corresponding datasheet.

--- End quote ---

There are no markings anywhere on the sensor.  :-\ It's just a metal can with a window and three pins just like the photo. It's response seems analog to me.

RoGeorge:
AFAIK there are 2 major categories:
- sensor only (with only a JFET inside), i.e. D203S, datasheet good to see what to expect from a 3-pins sensor only PIR https://www.futurlec.com/PIR_D203S.shtml
- sensor + amplifier (looks like yours might be like that, since there are no amplifiers on the PCB)

Evangelopoulos Panagiotis:

--- Quote from: RoGeorge on March 10, 2020, 03:20:52 pm ---(looks like yours might be like that, since there are no amplifiers on the PCB)

--- End quote ---

I forgot to take a picture of the board from the underside. There are two ICs which I can't see their markings (8pin and 14pin packages) and one 78L05A regulator. I'm sure one of them must be some sort of op-amp.

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