I will attach a PIR sensor to a motor and i want the motor to stop turning when the output of the PIR sensor ( http://www.rapidonline.com/pdf/61-1466.pdf ) is below a threshold. I also want the motor to start turning again when it is above the threshold.
Here is an outline of one of the ways of meeting the requirement. The motor can be connected through the contact leads of the relay.
As the question does not indicate a specific problem arrived at after due research, this schematic is merely a pointer to start off a direction of study. Do not take any component values or choices as final.
I think the major clues are in figure 2 of the spec: -
I've circled in red an amplifier circuit that you should consider using prior to triggering a comparator circuit (in blue)
The amp has a gain of 72.5dB - this is what the PIR sensor manufacturer use as a test circuit for the device and this means it is worth implementing for these simple reasons: -
1) It will likely produces a signal of sufficient amplitude to trigger a comparator
2) It will filter the noise that might otherwise false trigger the comparator.
One resistor value is marked (47k) and if you want a gain of 72.5 dB (210 in ratio) you'll need to make the feedback resistor 209k and the resistor from -Vin to ground 1k. This gives the circuit a gain of 210 BUT please do consider putting a 10k pot in series with the 1k in order to reduce sensitivity.
The capacitor that reduces noise is across the 209k resistor and the diagram says it has a 2.7Hz 3dB point. This means that C = 0.28uF.
OK choose a 220k resistor and let C = 270nF. However, if you expect to have a more rapid response, you might want to make C lower by some factor.
The op-amp.... depending on what power rails you have will dictate your choice of device. Assuming you only have 0V and 5V means you should use a rail-to-rail op-amp. It doesn't have to be extraordinarily fast but you should consider a reasonably low noise one given the gain you have. An AD8605 springs to mind as fairly usable generally and it will give you some headroom to increase gain or open up the high frequency response.
For the comparator YOU WILL need hysterisis built around it to prevent the relay and motor chattering when the trip point is close. Maybe in the order of 1 to 5% but again, if you don't know you can use a pot to make it variable until you are happy then swap out with a normal resistor.