Module tamper detection circuitry

A detector tamper/trouble delay circuit incorporates a sensor which senses the presence or absence of a cover for the detector. When the cover is removed, the sensor triggers a delay element preset for predetermined time interval. If the cover is replaced during the time interval, no trouble or fault indicator is generated. If the cover remains off the detector for the entire time interval, a trouble or fault indicator will be generated. The indicator can be used by an alarm system control element or a stand alone trouble or fault indicator for producing a humanly perceptible representation of the existence of the fault or trouble condition.

FIELD OF THE INVENTION 
The invention pertains to supervisory circuits for use in monitoring 
systems. More particularly, the invention pertains to supervisory circuits 
for use in establishing an operational status of ambient condition 
detectors such as intrusion, thermal, fire or gas detectors. 
BACKGROUND OF THE INVENTION 
Monitoring systems for supervising the status of various conditions in a 
selected region are useful in protecting property and individuals in the 
region. Representative systems include premises mounting systems such as 
burglar detection or fire detection systems. One such system is disclosed 
and claimed in Tice et al. U.S. Pat. No. 4,916,432 entitled Smoke and Fire 
Detection System Communication. 
In addition to monitoring the region, it is desirable for the system to be 
able to supervise its constituent components. Where ambient condition 
detectors are present, detector supervision circuits are useful. It would 
be preferable, however, if routine maintenance could be distinguished from 
a fault condition. 
SUMMARY OF THE INVENTION 
An electrical module includes tamper detection apparatus to sense the 
removal of a portion of a housing of the module or alternatively the 
removal of other structures associated with the module. The apparatus 
includes a sensor for detecting removal of the relevant element. One 
exemplary form of the element is a cover for module. 
A delay circuit is provided which is coupled to the sensor. The delay 
circuit generates an output signal at a predetermined time interval after 
removal of the element. The output signal is indicative of the fact that 
the element was removed and not replaced on the module during the time 
interval. 
In one aspect, the apparatus can be incorporated into an ambient condition 
detector with a removable cover. If the cover is removed during 
installation or maintenance, such as cleaning or testing, the delay 
circuit is triggered. In the event that the cover is not replaced during 
the predetermined time interval after the delay circuit has been 
triggered, an output status indicator is generated. This status indicator 
is indicative of the cover not having been replaced on the detector during 
the time interval. 
In another aspect, the sensor incorporates a manually operable switch, or 
alternately, reed relay. Removal of the cover permits the switch to change 
state thereby opening or closing metallic contacts. 
In yet another aspect, a solid state sensor could be used. In one form, the 
supervisory sensor can include a radiant energy sensor. Removal of the 
cover exposes the sensor to a source of radiant energy thereby providing a 
trigger to the delay circuit. Alternately, removal of the cover could open 
circuit a portion of the sensor thereby triggering the delay interval. 
In one aspect, the detector can be implemented as a smoke detector. In one 
embodiment, the smoke detector can be a duct mountable detector which 
incorporates a removal cover.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
While this invention is susceptible of embodiment in many different forms, 
there are shown in the drawing and will be described herein in detail 
specific embodiments thereof with the understanding that the present 
disclosure is to be considered as an exemplification of the principles of 
the invention and is not intended to limit the invention to the specific 
embodiments illustrated. 
FIG. 1 illustrates an electrical module 10 which includes a housing 12. The 
housing 12 defines an interior region 12a wherein are located a transducer 
14a, control circuitry 14b and a sensor 16. Electrical signals 18 are 
coupled to the module 10 for purposes of providing electrical energy 
and/or communicating information to or from the module. 
The module 10 is closed by a removable cover 20 in a normal operation. For 
purposes of installation or maintenance including checking or cleaning the 
transducer 14a or testing the transducer or control element 14a, the cover 
20 could be removed. 
When the cover 20 is in place on the housing 12, sensor 16 which is coupled 
to the control element 14b exhibits a first state. Removal of the cover 20 
results in the sensor 16 entering a second state which can be detected by 
the control element 14b. 
The sensor 16 can incorporate for example a reed relay. Alternately, a 
manually operable spring loaded switch or a contactless sensor such as a 
photodetector could be used. 
As will be appreciated by those of skill in the art, there are times when 
as a result of installation or routine maintenance, the cover 20 is not 
properly reinstalled on the module 10. In such an instance, the module 10 
generates a supervisory fault or trouble indication a predetermined 
interval of time after the cover 20 has been removed. In normal 
installation or maintenance, once the module 10 has been connected and is 
operational or has been serviced the cover 20 will be reattached thereby 
resetting the control element or circuit 14b and blocking generation of 
the fault or trouble indicator. 
As discussed subsequently, the control circuit 14b can incorporate a solid 
state programmable timer which is driven by a clock. The timer can be 
preset to a predetermined time interval, for example 20 minutes. 
The timer can be triggered by removal of the cover 20. The timer can be 
reset by replacing the cover 20 on the module 10. In the event that the 
cover is not replaced, after the delay interval, the timer and control 
circuitry 14b will generate a detectable fault or trouble signal. 
As illustrated in FIG. 1A, control circuitry 14b can incorporate a normally 
open relay 24. The relay 24 includes a coil 24a and normally open contacts 
24b. 
Sensor 16, in FIG. 1A, is a normally closed manually operable switch or 
reed relay. In the presence of cover 20, the switch or relay is open which 
in turn causes control circuitry 14b to continuously energize coil 24a 
thereby holding normally open contacts 24b closed. 
When the cover 20 is removed and the contacts of sensor 16 change state, 
solid state timer 14c is triggered. After a predetermined delay interval, 
assuming cover 20 has not been replaced in the interim, coil 24a is 
de-energized. This in turn enables contacts 24b to open providing a fault 
or trouble indicator. Alternately, instead of de-energizing coil 24a, 
circuitry 14b could provide a signal on communication lines 18, which 
signal would be received by a remote control unit and would indicate the 
presence of a trouble or supervisory condition. 
FIG. 2 illustrates an alternate module 10-1. The module 10-1 includes a 
housing 12-1. 
Transducer 14a is carried within the housing 12-1 along with control 
circuitry 14b-1. The housing 12-1 incorporates a radiant energy sensor 
16-1 which can be energized by radiant energy R generated by a source 
16-2. While in the configuration illustrated in FIG. 2 the radiant energy 
R is directly incident on the sensor 16-1, it will be understood that 
other arrangements could be used without departing from the spirit and 
scope of the present invention. 
The module 10-1 includes a cover 20-1 which closes the housing 12-1. In 
normal use, with the cover 20-1 attached, a shroud or enclosure 28 
(illustrated in phantom in FIG. 2) blocks incident radiant energy R or 
radiant energy from any other source from falling upon sensor 16-1 and 
generating false indications that the cover 20-1 has been removed. 
FIG. 2A illustrates one form of a sensor 16-1, a phototransistor which 
changes state in response to incident radiant energy R. It will be 
understood that other types of radiation sensitive elements could be used 
in lieu of the phototransistor of FIG. 2A. 
FIG. 3 illustrates an exemplary system 30 which incorporates a plurality of 
electrical modules 32. In one embodiment, at least some of the modules 32 
could be ambient condition detectors. For example, typical detectors would 
include smoke, thermal or intrusion detectors. Such detectors can be used 
to supervise a region of interest. 
In the exemplary system 30 of FIG. 3, the plurality of modules 32 is 
coupled by a communication link 34 to control/supervisory circuitry 36. 
The circuitry 36, which could if desired include a programmable processor, 
includes a supervisory loop 38. The loop 38 is coupled to a plurality of 
normally open relay contacts, comparable to the contacts 24b. 
The contacts on the loop 38 are connected in series. It will be understood 
that a variation of the loop 38 could incorporate parallel connected 
contacts without departing from the spirit and scope of the present 
invention. 
So long as all of the contacts 24b-1 . . . 24b-n are held closed by the 
respective member of the plurality 32, the supervisory circuitry 36 will 
not produce any module fault indicators. In the event that a cover is 
removed from one of the modules 32 and that cover is not replaced before 
the predetermined time interval has run, the respective pair of relay 
contacts 24b-i will open circuit thereby providing an immediate indication 
to the supervisory circuits 36 that one of the members of the plurality 32 
is exhibiting a trouble or fault condition. 
It will be understood that instead of a separate loop 38, the link 34 could 
be used by the plurality of modules 32 to communicate a trouble or fault 
in the indicator to the supervisory circuitry 36 without departing from 
the spirit and scope of the present invention. In such an instance, it 
might also be possible to identify the particular member of the plurality 
32 which has transmitted the fault or trouble indication. 
FIGS. 4A and 4B illustrate an exemplary system 40 which includes a 
plurality of modules 42. In the system 40, the modules 42 could, for 
example, be implemented as ambient condition detectors such as smoke 
detectors. In such an event, the members of the plurality 42 would each 
incorporate a smoke sensor such as the exemplary sensors 44-1a . . . 
44-na. Each of the members of the plurality 42 includes control circuitry 
such as the circuitry 44-lb which includes delay circuitry 441c. 
Each of the members of the plurality 42 includes a cover removal detection 
sensor or switch corresponding to the switches 46-1 . . . 46-n. As 
discussed previously with respect to sensor 16, removal of the respective 
cover causes the respective switch to change state, close or open circuit, 
and trigger the respective delay circuit 44-1c . . . 44-nc. 
If the respective cover is replaced on the respective module before the 
delay interval terminates, energy will continue to be supplied to the 
respective supervisory relay such as the relays 48-1 . . . 48-n. This in 
turn continues to hold the respective contacts such as the contacts 50-1 . 
. . 50-n closed. 
In the event that the respective cover is not replaced within the 
predetermined delay interval, the circuitry 44-1b and 44-1c, for example, 
will disable the respective supervisory relay coil 48-1. In response 
thereto, the respective contacts 50-1 will assume their normally open 
state thereby interrupting flow of current in the loop 38-1, -2. Hence, 
the supervisory circuitry 36 will become aware of the fault or trouble 
indicator. Where the respective cover is replaced on the respective unit 
during the delay interval, the respective delay element, such as 44-1c is 
reset thereby causing the respective supervisory relay such as 48-1 to 
continue to be energized, hence holding the respective contacts 50-1 in 
the closed state. 
For exemplary purposes only, the system 40 has been illustrated as a four 
wire-type detector system as would be known to those of skill in the art. 
It will be understood that the particular form of communication link 34 
and supervisory link 38-1, -2 are not limitations of the present 
invention. The detectors could without limitation be smoke detectors. 
FIG. 4B illustrates the system 40 after a delay circuit, such as a delay 
circuit 44-1c has timed out in response to a respective cover having been 
removed. As illustrated in FIG. 4B, relay contacts 50-1 have assumed an 
open state in response to the current through the respective supervisory 
relay coil 48-1 having been interrupted by the module control circuitry 
44-1b and delay circuitry 44-1c. Hence, the supervisory notification 
device 36' which could correspond to the supervisory circuitry 36 or could 
correspond to another relay with an output device such as a horn or a 
visible alarm is now able to produce a trouble or fault indicator. 
The system 40 can be used with alarm system control panels, such as the 
supervisory and control circuitry 36. Alternately, the system 40 can be 
used in the stand alone mode with a separate fault or alarm indicator 36'. 
It will be also understood that the plurality of modules 32 could 
correspond to two wire ambient condition detectors which are capable of 
communicating signals to the control and supervisory circuitry 36. In such 
an instance, a solid state switch can be used to provide a fault or 
trouble indication on a separate loop. Alternately, a message can be 
transmitted to the control supervisory circuitry 36 via the link 34. 
It will be understood that neither the form of communication with the 
supervisory circuitry nor the use of a common control element, such as 
control supervisory circuits 36 or stand alone supervisory circuitry 36' 
are limitations of the present invention. 
From the foregoing, it will be observed that numerous variations and 
modifications may be effected without departing from the spirit and scope 
of the invention. It is to be understood that no limitation with respect 
to the specific apparatus illustrated herein is intended or should be 
inferred. It is, of course, intended to cover by the appended claims all 
such modifications as fall within the scope of the claims.