Here is a sensor of liquefied petroleum gas, also called LPG, GPL, LP Gas, liquid petroleum gas or simply propane. The SEN-1327 LPG Gas Sensor Module is designed to enable the LPG detection interface to Microcontroller without ADC Channels. It allows determining when a preset LPG gas level has been reached or exceeded. Interfacing with the sensor module is done through a 4-pin SIP header and requires One I/O pin from the host microcontroller.
The onboard microcontroller provides an initial heating interval after powerup and then starts to measure LPG sensor output. If it found the LPG contents above-set value, it will inform the Host controller by pulling the Output Pin to High and Starts to blink an onboard status LED. The sensor module is mainly intended to provide a means of comparing LPG sources and being able to set an alarm limit when the source becomes excessive.
You can use other manufacture LPG sensor like MQ-5 sensor