GB2312312A - A mains powered alarm sensor unit having a primary battery backup - Google Patents

Abstract

A smoke alarm unit 1 has a cover 2 and a base 3 forming a housing 4 of an alarm sensor and circuit. Extending rearwardly from the base 3 there is a shroud 5. The shroud 5 forms a housing for power supply leads and terminals both for mains and battery power when it is pushed over a mounting plate (20, Figs 3 and 5) which is mounted on a wall or ceiling. A battery drawer 6 allows insertion and removal of a battery from this housing and contact between the battery and the circuit is made by mechanical snap-fitting contacts. The drawer cannot be closed unless a battery is contained therein.

Description

"A Mains Powered Alarm Sensor Unit having a Primary Battery Backup" The invention relates to a mains powered alarm sensor unit having a primary backup, such as a smoke, gas, or heat sensor unit. The invention relates particularly to such a sensor unit which includes both a condition sensor and a circuit so that it can be used either in a stand-alone manner, or when connected with other similar units by an interconnect line.

It is known to provide such an alarm unit which is mounted on a junction box for connection to mains power. Because a junction box must be provided for each unit, this arrangement becomes expensive, particularly where a large number of units is to be installed such as in a large office building. It is also time-consuming to mount both the junction box and the unit and the electrician must be extremely careful to ensure that they are mounted in a safe place.

The invention is directed towards providing an alarm sensor unit which is of simpler construction and may be more easily installed for safe operation.

According to the invention, there is provided an alarm sensor unit comprising: a mounting plate comprising means for mounting on a wall or ceiling, input mains terminals for connection to a mains supply, and output mains terminals; an alarm sensor and associated circuit mounted in a housing; circuit mains terminals and circuit battery terminals mounted at the rear of the housing; a battery compartment at the rear of the housing comprising a battery drawer having drawer output battery terminals for connection to the circuit battery terminals when the drawer is closed, and safety means for preventing closure of the drawer without a battery; an engagement means at the rear of the housing for engagement of the housing with the mounting plate; and a shroud extending rearwardly from the housing and being configured to surround the mounting plate to form a power supply housing surrounding the input, output and circuit mains terminals, the circuit and drawer output battery terminals, and the battery compartment.

Preferably, the engagement means comprises undercut tabs for engagement upon mutual rotation.

In one embodiment, the mounting plate comprises a catch for snap-fitting engagement with a slit in the shroud.

In one embodiment, the shroud comprises a snap-off section for side-access of mains cable.

In another embodiment, the output mains terminals comprise a female plug connected by mains leads to the input mains terminals.

Preferably, the safety means of the battery compartment comprises a metal tension spring mounted in a base of the drawer and biased to a closure-prevention position in a manner whereby insertion of a battery moves it to a closure-allowing position.

In one embodiment, the drawer comprises drawer input battery terminals comprising means for mechanical snapfitting engagement with a battery.

Ideally, the drawer and the alarm sensor housing comprise inter-engaging alignment formations to maintain alignment of the drawer output battery terminals during opening and closing of the drawer.

In another embodiment, the circuit comprises a switch to connect mains-derived power to a battery level sensing device, and control means for closing the switch upon opening of the drawer.

In a further embodiment, the switch comprises a resilient movable contact biased to the open position, and the control means comprises a cam in the battery compartment which slides with the drawer to close the switch during opening of the drawer.

Preferably, the drawer has imprinted indicia at an outer extremity which are covered when the battery is fully inserted.

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which: Fig. 1 is a perspective view from above of an alarm sensor unit of the invention; Fig. 2 is a perspective view from above showing a battery drawer of the unit when opened; Fig. 3 is a front perspective view of a mounting plate of the unit; Fig. 4 is a rear perspective view of the unit without the mounting plate; Fig. 5 is a rear view showing the manner in which the mounting plate engages the sensor housing of the unit; Fig. 6 is a side view of a torsion spring of the battery drawer; Figs. 7 and 8 are diagrammatic cross-sectional side views showing some of the parts of the battery compartment to illustrate the manner in which the battery drawer may be closed.

Fig. 9 is a partly cut-away rear view of the housing showing construction of the battery compartment in more detail; Fig. 10 is a circuit diagram of part of the alarm circuit; Fig. 11 is a further partly cut-away rear view showing construction of the battery compartment when the drawer is closed; Fig. 12 is a perspective view from above of a switch of the circuit; Figs. 13(a) and 13(b) are diagrammatic side views illustrating operation of the switch; Fig. 14 is a plan view showing a panel used in production of the circuit boards; and Fig. 15 is a perspective view showing circuit boards for a single unit.

Referring to the drawings and initially to Figs. 1 to 5 there is illustrated a smoke alarm unit 1. The unit 1 has a cover 2 and a base 3 which form a housing 4 for the alarm sensor and its associated circuit.

In addition, the unit 1 comprises a shroud 5 which extends rearwardly. A battery drawer 6 is flush with the shroud 5 in the closed position and the drawer 6 has a handle 7 and a printed warning indicia 8 which is located such that the warning is visible when the drawer has not been fully pushed home.

Referring in particular to Figs. 3 and 5, the unit 1 comprises a mounting plate 20 which has screw holes 21 for mounting on a wall or ceiling. The mounting plate 20 also has a circular cable opening 22 for entry of mains cables from behind. A block of input mains terminals 23 is mounted on the plate 20 and is connected by mains leads 24 to a socket 25 which provides output mains terminals.

The mounting plate 20 also comprises a resilient safety catch 27.

Reference is now made to Figs. 4 and 5 in which parts connected to the base 3 and the shroud 5 are shown in more detail. The shroud 5 includes lines of weakness 30 which allow a part to be snapped off for side-entry of mains cables. Undercut tabs 31 are connected to the base 3 for engagement with corresponding undercut tabs 26 on the mounting plate 20. The shroud 5 includes a narrow slit 32 for engagement with the safety catch 27. Circuit mains terminals are provided by pins 33 which protrude through an opening in the base 3. The battery drawer 6 forms part of a battery compartment 38 mounted on the base 3 and surrounded by the shroud 5.

To mount the unit on a wall or ceiling, the mounting plate 20 is secured to the wall or ceiling by use of the screw holes 21. If the mains cable comes through from behind, it is routed through the opening 22. However, if it is on the surface, it is routed through the side as indicated by the arrow A in Fig. 3. In either event, the mains leads are connected to the terminal block 23. Before mains power is restored, the socket 25 is connected to the pins 33 to connect the circuit to the mains supply. The housing is then pushed over the mounting plate 20 and is rotated anti-clockwise in the direction of the arrows B of Fig. 5 to the extent that the undercut tabs 26 and 31 inter-engage and the safety catch 27 engages the slit 32 in the shroud 5. It will be appreciated that this task is very simple. The open construction of the mounting plate 20 allows the mechanical and electrical connections to be easily made. It is then a very simple task to secure the housing to the mounting plate to provide an effective and long-lasting engagement. Tampering with the housing is resisted by the fact that it is necessary to use a tool such as a screwdriver to press the safety catch 27 out of engagement with the slot 32 to allow rotation of the housing to disengage it from the mounting plate and expose the mains leads and terminals. It will also be appreciated that construction of the unit is very simple as there is no need for a junction box. The construction is also more compact and it provides for inclusion of all of the power supply parts including the mains terminals and leads and the battery compartment and terminals in a general power supply housing formed by the mounting plate 20, the shroud 5, and the base 3 of the sensor housing 4.

Referring now to Figs. 6 to 15 inclusive, construction of the unit is described in more detail. The battery drawer 6 receives a battery 40 and comprises a base 41 which forms an anchorage for a torsion spring 42. The spring 42 comprises a base 43, a torsion arm 44, and a prong 45 which projects upwardly as viewed in Figs. 7 and 8. The prong 45 extends through a housing 46 in the battery compartment 38 and its outer end engages in a slot 47 in the base 3. By virtue of the resilient nature of the spring 42, the prong 45 is urged upwardly into engagement with the slot 47 which terminates after approximately 1 cm of travel of the drawer to prevent further closure of the drawer.

To insert a battery 40, it is firstly electrically connected by use of terminals of the snap-fitting type and indicated generally by the numeral 60 in Fig. 9. It is then physically inserted into the drawer, which action causes the torsion arm 44 to be pushed downwardly so that the prong 45 moves out of engagement with the slot 47.

Thereafter, the drawer may be pushed closed as illustrated in Fig. 8. It will be appreciated that this is an extremely simple and reliable arrangement. A steel torsion spring such as that illustrated has a very long life and does not suffer from creep to any significant extent.

The battery compartment 38 also comprises a locking arm 50 which extends from the drawer 6 and terminates in a tab 51 having a through-hole 52. When the drawer is in the closed position, the hole 52 is in registry with a corresponding hole in the base 3 so that a screw may be inserted to lock the drawer in the closed position. This feature is of benefit in situations where a long-life battery such as that of the Lithium type is used and there is no need to use a drawer. It will be appreciated that this is an extremely simple and effective way of keeping the drawer locked.

As shown most clearly in Figs. 9 to 13, the drawer 6 has a pair of drawer output battery terminals or contacts 62 mounted around a plastics support 63 on the drawer. In addition, the drawer 6 has a U-shaped recess 64 which moves with the contacts 62. When the drawer is being closed, the contacts 62 slide into engagement with a pair of circuit battery terminals or contacts 65 mounted on the base 3. The contacts 65 are resilient and have re-entrant formations at their ends to allow them to slide outwardly and snap in behind the contacts 62 for a positive mechanical engagement. This is best illustrated in Fig.

11. In this way, it is not possible that there is electrical contact between the battery and the circuit when the drawer has been partly or fully opened. Further, this mechanical snap-fitting arrangement helps to ensure that the battery remains in contact, even if the housing is moved or knocked. A rail 66 is mounted on the base 3 and this is in alignment with the U-shaped recess 64 so that as the drawer 6 is pushed in they inter-engage and ensure that the contacts 62 and 65 are in registry. This is an effective way of ensuring reliable operation.

As illustrated in Fig. 10, the alarm circuit includes a positive rail Vec and the battery 40 is connected to the rail by a diode D1. A sensing pin 100 of a sensing integrated circuit is connected to the battery 40 via a diode D2 and a resistor R3. A pulse is drawn on this line by a pulsing pin 101 which causes an intermittent flash of a light emitting diode LED as it draws a pulse through the diode D2, the LED and a resistor R1. These flashes act to indicate that battery power is still available and also used by the sensing pin 100 to detect the battery level.

To avoid nuisance alarms indicating that the battery level has dropped below a threshold (such as low-battery "beeps"), a switch 71 is mounted in the circuit to connect the sensing pin 100 to the rail Vcc so that it receives a mains-derived voltage when the battery is removed. The switch 71 is illustrated in Fig. 12 and it comprises a resilient arm 73 on which there is an upwardly-directed contact 74, and an anchorage 75. When the switch 71 is mounted on the circuit, the two parts of the anchorage 75 surround an electrically isolated wire 76. The switch 71 is operated by a cam 70 which is integral with the drawer 6. When the drawer 6 is closed, the natural resilience of the arm 73 maintains the contact 74 spaced-apart from the wire 76. Thus, the switch 71 is open. However, when the drawer 6 is opened the cam 70 as illustrated in Fig. 13(b) pushes the resilient arm 73 upwardly so that the contact 74 moves into contact with the wire 76 to close the switch. It will be appreciated that this is an extremely simple and effective way of connecting mains-derived power to the sensing pin when the drawer is opened, thus preventing nuisance low-battery indicating outputs from the unit 1.

Some of the process steps for production of the unit 1 are illustrated now with reference to Figs. 14 and 15. A panel 80 of circuit boards is soldered and assembled in conventional manner. The panel 80 comprises four circuit boards 81 which are the main sensor circuit boards housed within the housing 4. In addition, there is a contact circuit board 82 connected by a line of weakness to each circuit board 80. The contact circuit board 82 is for mounting within the battery compartment 38. During production, the panel 80 is soldered and assembled in the usual manner and leads 83 are soldered to inter-connect each contact board 82 with the associated circuit board 81. The boards are then separated along the lines of weakness to provide four sets of boards, each comprising a circuit board 81, a contact board 82 and interconnecting soldered leads 83. During production, the contact board 82 is pushed through an aperture in the base 3 and is screwed into position behind the base 3, the leads 83 extending through the aperture. The circuit board 81 is then screwed into position in front of the base 3. Thereafter, the housing of the battery compartment 38 is mounted over the contact board 82. It has been found that this is an extremely simple and efficient way of producing the unit, which is contributed to by the construction of the unit whereby there is an alarm circuit and associated housing and a separate power supply housing formed by the base 3, the shroud 5 and the mounting plate 20.

The invention is not limited to the embodiments hereinbefore described, but may be varied in construction and detail.

Claims (12)

1. An alarm sensor unit comprising: a mounting plate comprising means for mounting on a wall or ceiling, input mains terminals for connection to a mains supply, and output mains terminals; an alarm sensor and associated circuit mounted in a housing; circuit mains terminals and circuit battery terminals mounted at the rear of the housing; a battery compartment at the rear of the housing comprising a battery drawer having drawer output battery terminals for connection to the circuit battery terminals when the drawer is closed, and safety means for preventing closure of the drawer without a battery; an engagement means at the rear of the housing for engagement of the housing with the mounting plate; and a shroud extending rearwardly from the housing and being configured to surround the mounting plate to form a power supply housing surrounding the input, output and circuit mains terminals, the circuit and drawer output battery terminals, and the battery compartment.

2. An alarm sensor unit as claimed in claim 1, wherein the engagement means comprises undercut tabs for engagement upon mutual rotation.

3. An alarm sensor unit as claimed in claim 2, wherein the mounting plate comprises a catch for snap-fitting engagement with a slit in the shroud.

4. An alarm sensor unit as claimed in any preceding claim, wherein the shroud comprises a snap-off section for side-access of mains cable.

5. An alarm sensor unit as claimed in any preceding claim, wherein the output mains terminals comprise a female plug connected by mains leads to the input mains terminals.

6. An alarm sensor unit as claimed in any preceding claim, wherein the safety means of the battery compartment comprises a metal tension spring mounted in a base of the drawer and biased to a closure prevention position in a manner whereby insertion of a battery moves it to a closure-allowing position.

7. An alarm sensor unit as claimed in any preceding claim, wherein the drawer comprises drawer input battery terminals comprising means for mechanical snap-fitting engagement with a battery.

8. An alarm sensor unit as claimed in claim 7, wherein the drawer and the alarm sensor housing comprise inter-engaging alignment formations to maintain alignment of the drawer output battery terminals during opening and closing of the drawer.

9. An alarm sensor unit as claimed in any preceding claim, wherein the circuit comprises a switch to connect mains-derived power to a battery level sensing device, and control means for closing the switch upon opening of the drawer.

10. An alarm sensor unit as claimed in claim 9, wherein the switch comprises a resilient movable contact biased to the open position, and the control means comprises a cam in the battery compartment which slides with the drawer to close the switch during opening of the drawer.

11. An alarm sensor unit as claimed in any preceding claim, wherein the drawer has imprinted indicia at an outer extremity which are covered when the battery is fully inserted.

12. An alarm sensor unit substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.