Here is a simple sound activated circuit assemble this circuit the transistor is directly driven from the speaker output , a capacitor connected with a relay in parallel to avoid some small gape ( intervals) connect it with your mobile phone ring output ,save a continuous tone (beep) in your mobile , set this tone for specific contacts (family members)now assemble it with your house main gate lock ,keep your mobile in silence profile just set continuous beep for specific family members by calling the mobile output will be appear from mobile ring output it will trigger the relay this relay will drive another relay it will trigger the lock on, Only known contacts will access the main gate other unknown will not be entertained where there will be no output if unknown will call.lock will be remain open while ringing continues.
