this is a lpg sensor which is used for sensing lpg gas .
it has 4 pin 1.ao 2.do 3.vcc4.gnd
but i dont know how to use as lpg sensor and its circuits
please send me circuit for lpg sensor
This page has the spec for a module using the MQ135 sensor (which it seems yours has).
I'm guessing the pins are Analogue out, Digital out (TTL compatible), Vcc (+5V) and Ground.