The present invention relates to door security systems. The invention is particularly useful as part of a system which enables controlled access to premises but the invention may have other applications.

A typical door security system includes a door contact switch, often magnetic, having its respective contacts mounted on the door and the door frame so that the switch is closed when the door is closed and open when the door is open. The switch is typically linked to an alarm system which gives a warning signal when the door is opened and means are provided for preventing the generation of the warning signal in the event of an "authorised" opening of the door. For example, on entry to the building, an electronic "key" or a key pad might be used to signal the alarm system not to generate a warning signal.

Since exit from the building should be unrestricted a sensor is often provided on the inside of the building adjacent to the door to detect the approach of a person and signal the alarm system not to generate a warning signal. The sensor might be a passive infra-red (PIR) movement detector for example. Instead of or in addition to a door contact switch, an electromagnetic lock may be provided from which power is removed on receipt of an "authorised opening" signal. A relay operating the lock might bypass a door contact switch on receipt of an "authorised opening" signal to avoid opening of the door contact setting off an alarm.

A problem with these known systems is that they usually operate for a fixed amount of time, either to disable generation of an alarm signal or to unlock the door in the case where a lock is provided. This amount of time must be sufficient to allow the person sufficient time to exit and has to be set to the maximum time taken for a person to exit. This can often result in the door being left unlocked and/or the alarm system being bypassed for a period after the person has gone through the doorway during which unauthorised entry to the building would be possible.

US-A-4412356 discloses a door security system in which a door is provided with a contact switch for indicating whether the door is open or closed. The switch is closed whenever the door is opened. The door is opened by an operator using a transmitter which produces encoded light signals detected and decoded by circuitry within the system. On

CONFIRMATION COPT

2 receipt of an acceptable request to pass through the doorway, from the transmitter, a solenoid is activated to retract a lock bolt on the door. The purpose of the contact switch is to maintain the supply of power to the solenoid .after the door has been opened and until it is shut again, so that the lock bolt does not hinder the re-closing of the door. When the door is closed the contact switch opens causing the solenoid to be deactivated allowing the lock bolt to re-lock the door.

The present invention provides a door security system for use with a door having a status indicator providing a signal indicating whether the door is open or closed, the system comprising: detector means for generating a signal indicating an acceptable request to pass through the doorway; access means activated in response to said signal from said detector means for enabling unhindered passage through the doorway; and means responsive to said door status indicator for de-activating said access means when the door has been closed following said signal from said detector means; characterised in that the access means is operable when activated to disable an alarm associated with the door status indicator.

Thus, the system of the invention ensures that the access means is deactivated as soon as the door is closed by monitoring the door status indicator. Furthermore, the system ensures that the alarm is not set off when the door is opened.

The detector means may comprise a passive infrared movement detector or any other movement detector. The movement detector would typically be mounted on the inside of the door to operate the access means and allow free passage out of a building.

Additionally or alternatively, the detector means may comprise the key pad or card reader of an electronic lock which would generate the said signal in response to the correct code being entered either via the key pad or the card being inserted. Other alternatives for the detector means include a simple push button switch, key operated switch or flip switch or any other means which provides a signal indicating a request to pass through the doorway. The access means may operate to open an electronic lock, for example by removing power from a solenoid which holds the lock closed.

The access means preferably comprises means such as a switch positioned in parallel to the door status indicator to bypass the door status indicator when the door is opened following an acceptable request to pass through the doorway. Then, for example, an

alarm connected to the door status indicator would not receive a "door open" signal due to the bypass switch and the alarm would not be operated.

The monitoring means preferably comprises an inductance which is positioned in series with the door status indicator. The inductance preferably comprises the secondary of a transformer which is connected to the detector means.

The monitoring means could equally well comprise a resistor positioned in series with the door status indicator with the voltage across the resistor being monitored to determine whether the door was open or closed.

The system preferably additionally includes a timer which is started in response to the signal from the detector means and determines the maximum time for which the access means is activated. Thus, if the maximum time expires before the door is closed, the access means is deactivated and, for example, an alarm may sound if the door has been propped open.

It will be appreciated that the thus far may be conveniently provided as an "add on" to an existing door security system. The invention also provides a complete system including the items noted above as well as the door status indicator and, optionally an electromagnetically operated door lock.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawing which is a schematic block diagram of a door security system according to the invention.

The illustrated embodiment of the invention comprises a detector 10. The purpose of the detector 10 is to provide a signal indicating an acceptable request to pass through a door (not shown). Thus, the detector could comprise a passive infra-red movement sensor or an ultrasonic object detector mounted on the inside of the door, or a card reader or key-pad on the outside of the door, for example. The detector 10 outputs a signal on line 11 to a microprocessor 12 which includes a software timer.

Microprocessor 12 has a first output connected via relay control line 13 to relays to be described below, an enable/disable output provided on line 14 and a further input received on line 15. The door has a contact switch 20 connected to an alarm system 30 The alarm system would typically sound a warning if the contact switch was open indicating that the door was open.

Connected in parallel to the door contact switch is a first relay operated switch 21. The relay operated switch 21 is connected in series with the secondary winding of a

transformer 22. The door is provided with a lock 23 to which power is supplied via a switch 24. Both switches 21 and 24 may be relay operated switches and may in fact be operated by the same relay. Switch 24 is normally closed so that the lock 23 is powered and operative. Switch 21 is normally open. Under certain conditions, to be described below, microprocessor 12 provides a signal on line 14 which enables or disables square wave generator 31, the square wave signal is then smoothed by smoothing circuit 32 and passed through buffer circuit 33. The resulting a.c. signal appears across the primary of transformer 22 and is monitored by a peak detector 34. The peak voltage is compared with a reference voltage in a comparator 35 whose output is supplied back to the microprocessor.

The operation of the circuit is as follows:

When the detector means generates a signal indicating an acceptable request to pass through the doorway, this signal is passed from the detector along line 11 to the microprocessor 12. In response to this signal, the microprocessor 12 generates a signal on line 13 to operate the relay controlling switches 21 and 24 and an enable signal on line 14 to square wave generator 31 , and starts the timer.

The operation of the relay causes switch 24 to open, removing power from lock 23 and enabling the door to be opened. The relay also operates to cause switch 21 to be closed. Thus, when the door is opened thus opening switch 20 current supplied by the alarm system power supply still flows between the terminals of the door contact switch 20 via switch 21 and the secondary of transformer 22 and thus alarm system 30 is not "aware" that the door has opened.

When the door is closed again, both switches 20 and 21 will be closed and thus a short circuit will appear across the secondary of transformer 22. The resulting change in impedance of the secondary winding will in turn affect the voltage appearing across the primary. This change in voltage will be detected by the peak detector. This will be signalled by the comparator 35 which will provide a signal to the microprocessor 12 indicating that the door is closed.

The microprocessor 12 then removes the signals on lines 13 and 14 and re-sets its internal timer. Thus, switch 21 opens, and switch 24 closes to re-apply power to the lock.

It should be noted that the microprocessor only removes the signals on lines 13 and 14 and stops the timer if the door has been opened and then closed following a signal from the detector 10. This is achieved through suitable programming of the microprocessor. Thus, when the switch 21 has been closed in response to a signal on line 13 and before the

door has been opened, the microprocessor maintains the signals on lines 13 and 14.

If the door is not closed before the microprocessor timer has "timed out", then when a predetermined time set in the timer has expired, the signals on lines 13 and 14 are automatically removed by the microprocessor. However, the timer is restarted each time a new signal from the detector is received to enable repeated exits or entrances through the door.

The microprocessor also includes a "failsafe" feature, whereby if the door is not opened within a predetermined time after receipt of a detection signal, the timer is re-set and the signals on lines 13 and 14 are removed. This feature is useful in the event of a false detection signal being generated, for any reason.

Another optional feature which may be provided in the system of the invention is an audible or visible warning signal which operates at a predetermined interval before the timer has "timed out" if the door has not already been closed. This will warn users that the alarm controlled by the alarm panel 30 will activate if the door is not closed. The predetermined time could be say 5 seconds. The drawing shows a buzzer 40 connected to the microprocessor 12. Through suitable programming of the microprocessor, the buzzer 40 will operate 5 seconds before the timer has timed out.

The dotted line in the drawing separates door lock 23, door contact switch 20 and alarm system 30 from the other components of the system. The purpose of the dotted line is to show that the components to the left of the dotted line could easily be supplied for addition to an existing door fitted with contact switch 20, optionally linked to an alarm system and possibly having an electronic lock. No modification of the alarm system is necessary to provide the extra security which the system of the present invention provides.

Alternatively a complete system could be provided according to the invention, including any of the components shown on the right hand side of the dotted line.