Article removal alarm system for use with portable electronic device

An article removal alarm system includes a base unit for generating an ultrasonic signal for transmission in the vicinity of the base unit. A remote unit accompanying an associated article includes a receiver for receiving the ultrasonic signal. A threshold detector provides a threshold signal upon failure of the receiver to receive the ultrasonic signal at predetermined strength. An alarm unit then generates an alarm signal in response to the threshold signal. In a particular implementation the system enables monitoring of the proximity of a stylus or other input utensil to a responsive user interface of an electronic device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to article monitoring devices, and 
in particular to a system for detecting the removal of an article from 
within the vicinity of a base unit. 
2. Description of the Related Art 
Many systems and devices have been developed in an effort to detect the 
passage of persons and articles from beyond predetermined boundaries. For 
example, U.S. Pat. No. 4,593,273 (the '273 patent) describes an 
out-of-range monitor and alarm system that may be used to alert an 
operator or attendant that a supervised person has walked beyond a 
prescribed distance. The system of the '273 patent includes a base unit 
which preferably includes a radio transmitter having an approximately 
known maximum range to a sensitive receiver within a mobile unit carried 
by the supervised person. The receiver is disposed to produce an output 
signal whenever the detected radio frequency (RF) signal from the base 
unit falls below a predetermined level. In response, a transmitter within 
the mobile unit sends a signal to the base unit, which in turn sounds an 
alarm indicating that the supervised person has exceeded the prescribed 
range from the base unit. 
Notwithstanding the existence of this and other RF proximity sensing 
systems, there remains a need for sensing/alarm devices and systems 
configured for particular applications. For example, existing RF proximity 
sensing and other alarm systems would in many ways be unsatisfactory if 
used to monitor small, inexpensive articles, such as hand-held writing 
instruments. 
The expense of equipping a pen or other writing instrument with an RF 
transmitter and receiver may also substantially exceed the cost of the 
writing instrument or the like being monitored. RF systems also require 
utilization of an antenna, which would be impracticable in applications 
involving small writing instruments. In addition, the considerable power 
requirements of RF systems tend to render battery life unacceptably low 
for such applications. Finally, RF systems are subject to regulation by 
the Federal Communications Commission (FCC), thereby further complicating 
the design process. In addition, since RF energy penetrates most walls and 
doors, these monitoring systems are not disposed to monitor the removal of 
articles from an enclosed area such as a room. 
U.S. Pat. No. 4,870,405 provides an alternative to RF-type systems for 
monitoring objects such as writing instruments. Specifically, removal of 
an object from a base receptacle initiates a timing cycle. At the 
conclusion of the timing cycle, an alarm sounds at the base receptacle 
unless the object has been replaced. However, the fixed duration of the 
timing cycle precludes accommodation of different intervals of use of the 
object or writing instrument. In this regard the setting the timing cycle 
to be of a relatively low duration (to ensure prompt notification of 
unauthorized object removal) may result in undesired sounding of the alarm 
during extended periods of authorized use. 
The advent of various personal electronic devices, such as so-called 
"personal digital assistants", has also created a need for an inexpensive 
article monitoring system. In particular, it would be desirable to provide 
a cost-effective way to monitor the proximity of a stylus or other user 
input utensil to the electronic base unit. If the monitoring system were 
further configured with some form of alarm, the likelihood of inadvertent 
removal of the stylus from the vicinity of the electronic base unit would 
be substantially diminished. Given that certain personal electronic 
devices are being designed for wireless communication with a host system, 
the potential for radio interference renders RF-type monitoring systems 
even less preferable for such applications. 
Accordingly, a need in the art exists for an article monitoring system 
which allows for flexible periods of authorized use, and which avoids the 
aforementioned shortcomings of RF sensing and detection techniques. 
SUMMARY OF THE INVENTION 
The present invention is directed to alarm system for generating an alarm 
signal upon removal of an article from the vicinity of a base unit. The 
article removal alarm system includes a base unit for generating an 
ultrasonic signal for transmission in the vicinity the base unit. A remote 
unit accompanying the article includes a receiver for receiving the 
ultrasonic signal. A threshold detector provides a threshold signal upon 
failure of the receiver to receive the ultrasonic signal at predetermined 
strength. An alarm unit then generates an alarm signal in response to the 
threshold signal. 
In another aspect, the present invention may be used in conjunction with an 
electronic device having a user interface designed to be responsive to a 
stylus or other input utensil. In this implementation the alarm system is 
configured to generate an alarm signal upon removal of the user input 
utensil from the vicinity of the electronic device. The alarm system 
includes an ultrasonic transmitter, disposed in the electronic device, for 
generating an ultrasonic signal. A remote unit, accompanying the user 
input utensil, includes a receiver for receiving the ultrasonic signal. A 
threshold detector provides a threshold signal upon failure of the 
receiver to receive the ultrasonic signal at a predetermined strength. The 
remote unit further includes an alarm unit for generating an alarm signal 
in response to the threshold signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a preferred embodiment of an article removal alarm 
system 10 of the present invention includes a base unit 14 having an 
ultrasonic transmitter (not shown in FIG. 1) operative to transmit an 
ultrasonic signal through a meshed cover 16. The ultrasonic signal will 
typically be transmitted continuously throughout the area surrounding the 
base unit 14. Mounted upon the base unit 14 may be a small notepad (not 
shown) or the like. 
The article removal system 10 also includes a portable unit 18 having a 
housing 22 adapted to hold a pen 23 or other writing instrument. In this 
regard the housing 22 defines a small circular aperture 24 through which 
protrudes a tip 26 of the pen 23. The housing 22 includes a first 
compartment 30 in which is disposed the pen 23 (shown in phantom), and a 
second compartment 34 designed for enclosure of an ultrasonic receiver 
module 38 and battery 40 (both also shown in phantom). A cylindrical wire 
screen or mesh 44, interposed between first and second sections 30 and 34 
of the housing 22, allows the ultrasonic signal transmitted by the base 
unit 14 to reach the ultrasonic receiver module 38. When the pen 23 is not 
being used, the portable unit 18 may be placed within a receptacle 41 
defined by the base unit 14. Alternately, the ultrasponic signal from the 
base unit 14 may reach the receiver module 38 through an end aperture (not 
shown) defined by housing 22. 
The ultrasonic receiver module 38 includes a threshold circuit (not shown) 
operative to produce a predefined voltage output level when the detected 
ultrasonic signal from the base unit 14 falls below a predetermined level. 
In the exemplary embodiment this results in actuation of an audible alarm 
within the portable unit 18, it being understood that other alarm 
indications (e.g., flashing light) could also be provided. The threshold 
circuit is set to generate the predefined voltage output level, and hence 
trigger the audible alarm, upon movement of the portable unit 18 beyond a 
predetermined distance (e.g., 15 feet) from the base unit 14. 
Because the signal transmitted by the base unit 14 is ultrasonic, movement 
of the portable unit 18 which causes it to become separated from the base 
unit 14 by a wall or door will also generally result in actuation of the 
audible alarm. Hence, movement of the portable unit 18 immediately outside 
of a room within which is disposed the base unit 14 will tend to trigger 
the alarm. Since it will typically be desired that users of the pen or 
other instrument within the portable unit 18 remain within the same room 
as the base unit 14, the use of ultrasonic energy is seen to 
advantageously enable detection of movement of the portable unit out of an 
enclosed area. This contrasts with monitoring systems employing RF energy, 
which are relatively insensitive to movement of a monitored object beyond 
walls and doors. 
FIG. 2 provides a block diagram of the ultrasonic receiver module 38 and an 
audible alarm unit 70. The receiver module 38 includes an ultrasonic 
microphone 54 for generating an electrical signal in response to 
ultrasonic signal energy received from the base unit 14. The electrical 
signal is amplified within an amplifier chain 58, and provided to a 
threshold detector 62. When the magnitude of the amplified electrical 
signal produced by the amplifier chain 58 falls below an adjustable 
threshold, the output of the threshold detector 62 changes to a predefined 
level. A delay filter 66 is designed to prevent spurious changes in the 
output of the threshold detector 62, occurring in response to brief 
interruption of the received ultrasonic signal, from triggering an audible 
alarm unit 70. After passing through the delay filter 66, the filtered 
electrical signal is amplified by a second stage amplifier 68 prior to 
being supplied to the audible alarm unit 70. 
As is indicated by FIG. 2, the audible alarm unit 70 includes an active RC 
oscillator 74 designed to oscillate at a predefined frequency (e.g., 4 
KHz) when the output of the threshold detector 62 changes to the requisite 
predefined level. An audio transducer 78 functions to generate an audible 
signal in response to oscillation of the active RC oscillator 74. 
FIG. 3 provides a detailed schematic representation of the receiver module 
38 and audible alarm unit 70. In the embodiment of FIG. 3 the threshold 
detector 62 comprises an NPN transistor Q4 having a base terminal biased 
at approximately one-half of the available voltage supply. The resistance 
of a threshold adjustment resistor (R.sub.ADJ) may be adjusted in order to 
alter the threshold received signal level at which the alarm unit 70 is 
actuated. The collector of NPN transistor Q4 is operatively coupled to the 
base of a PNP transistor Q5 used to implement the second stage amplifier 
68. As is indicated by FIG. 3, the collector of the PNP transistor Q5 
drives one input of a first 80 of four NOR gates (80, 82, 84, 86) included 
within a standard 74HC02 integrated circuit (I.C.) used to realize the 
active RC oscillator 74. 
Turning now to FIG. 4, in an exemplary implementation the ultrasonic 
transmitter within the base unit 14 includes a crystal-controlled 
oscillator 90 tuned to oscillate at a predefined ultrasonic frequency 
(e.g., 40 KHz). The crystal-controlled oscillator 90 is coupled to a first 
port of a first NAND gate 92 included within a standard 4-input 74HC00 
I.C. 94. A pair of output NAND gates (96, 98) of the 4-input 74HC00 I.C. 
94 are seen to drive an ultrasonic transducer 99 responsible for 
transmitting the ultrasonic energy received by the portable unit 18. 
Referring now to FIG. 5, in an alternately preferred embodiment of an 
article removal alarm system 100 of the present invention the base unit 
ultrasonic transmitter is included within a personal electronic device 
110. The personal electronic device 110 may comprise, for example, a 
personal digital assistant or the like having a user interface responsive 
to a user input utensil such a stylus. Such a user interface could 
comprise, for example, a touch-sensitive or light-sensitive interface 
screen 112. The ultrasonic transmitter may be realized as in FIG. 4, but 
will transmit ultrasonic energy from within a housing 120 of the 
electronic device 110 through a meshed aperture 130. 
The system 100 also includes a portable unit 140 having a housing 142 
adapted to hold a stylus 152 or other user input utensil designed for 
utilization with the interface screen 112. The housing 142 defines a small 
circular aperture 158 through which protrudes a tip 160 of the stylus 152. 
The housing 142 includes a first compartment 170 in which is disposed the 
stylus 152, and a second compartment 174 designed for enclosure of an 
ultrasonic receiver module 178 and battery 180. A cylindrical wire screen 
or mesh 184, interposed between first and second sections 142a and 142b of 
the housing 142, allows the ultrasonic signal transmitted by the 
ultrasonic transmitter within the electronic device 110 to reach the 
ultrasonic receiver module 178. The ultrasonic receiver module 178 is 
designed to operate in a manner substantially identical to that of the 
receiver module 38 (FIGS. 1 and 2). That is, an audible alarm circuit 
within the portable unit 140 will generate an audible alarm when the 
ultrasonic energy received by the receiver module 178 falls below a 
predetermined threshold. 
The previous description of the preferred embodiments is provided to enable 
any person skilled in the art to make or use the present invention. The 
various modifications to these embodiments will be readily apparent to 
those skilled in the art, and the generic principles defined herein may be 
applied to other embodiments without the use of inventive faculty. Thus, 
the present invention is not intended to be limited to the embodiments 
shown herein but is to be accorded the widest scope consistent with the 
principles and novel features disclosed herein.