Hi . why we use zener diode and resistance with the detector circuit?

Question

FIRE DETECTOR

Answer 1

Your question is rather cryptic, as it does not give us much information to work with.

Generally whenever you see a zener and resistor used in conjunction with a detector circuit, it is generally there to establish some voltage reference for the detector to be compared to.  

I would also expect that the detector would have some method for calibration relative to the zener, because zeners, while good, are not exact regulation devices.  Low cost is the main benefit of using a zener as a regulator.

Answer 2

What detector circuit?

need more details!

.