In this article we will be discussing about a home security alarm circuit. This is a LPG gas leakage sensor circuit that detects the outflow of LPG gas and alerts the user via audio and visual indications. The heart of this simple engineering project is a gas sensor module SEN 1327. QM 6 gas sensor is used in this SEN 1327 module. The output sign
al from this gas sensor module is used to drive a 555 timer astable multivibrator. Here 555 timer works as a tone generator, the frequency of tone can be altered by varying the preset VR1. A preset employed in the module is used to set the threshold or reference level. Interfacing with the SEN 1327 sensor module is done through a 4 pin SIP header. High voltage at the 4th pin (Reset pin) enables timer IC 555, which is wired as an astable multivibrator. The multivibrator essentially works as a tone originator.