A security alarm system that may be selectively armed or disarmed when monitoring a protective premises is described which utilizes the position of an entry door to determine whether the security system should be armed or disarmed. The security system comprises (i) an entry door for permitting ingress to the protected premises from the outside of the entry door and egress from the protected premises from the inside of the door; (ii) means for selectively closing and opening the entry door; and (iii) a switch having a first state indicative of the entry door being in a closed position and a second state indicative of the entry door being in an open position wherein, when the switch is in its first state, the security system is armed and, when the switch is in its second state, the security system is disarmed. The disabling of inadvertent (false) alarms is easily, yet securely achieved by activating a first user-controlled disarming means consisting of a kill switch, a keyfob, a telephone keypad coupled to a DTMF decoder, a keypad, an electronic coded digital key, or other means.