Out-of-range personnel monitor and alarm

An out-of-range monitor and alarm system that may be used by a convalescent home or the like to alert an attendant in the home that a supervised person has walked beyond a predetermined prescribed distance. The system includes a base unit that transmits a first signal to a receiver in a mobile unit carried by the supervised person. The receiver includes a threshold circuit that is adjustable to correspond to the prescribed distance and produces a threshold output signal whenever the first signal detected by the receiver drops below the threshold level. In response to the threshold output signal, a transmitter in the mobile unit produces a second signal to a receiver in the base unit, which in turn sounds an alarm indicating that the mobile unit has lost adequate signal strength, as determined by the threshold adjustment, by exceeding the range prescribed for the supervised person. The first and second signals may have equal frequencies if the mobile unit produces an output signal having an identification code. The base unit has decoder for identifying the responsible mobile unit.

TECHNICAL FIELD 
This invention relates generally to alarm systems and particularly to a 
personnel monitor and alarm system that alerts a base unit in a home, 
sanitarium, or the like, when a child or ambulatory patient may be 
straying out of a prescribed range from the base unit. 
BACKGROUND ART 
Many devices have been developed in an attempt to successfully monitor and 
detect the passage of persons from an unconfined area of predetermined 
size. For example, U.S. Pat. No. 3,478,344 to Schwitzgebel et al. 
describes a behavior supervision system including four spaced base 
transceivers communicating with mobile transceivers mounted to the belt of 
each of a plurality of persons to be supervised so that the four base 
units can pinpoint the location of each mobile unit and sound an alarm if 
one leaves a predetermined area. Another alarm system is disclosed in U.S. 
Pat. No. 3,947,832 which describes a miner's helmet containing a radio 
transmitter that will alarm a base unit whenever the miner enters a danger 
zone. 
An object of the invention, therefore, is to provide a means for 
supervisory personnel at a base unit to become alerted when supervised 
personnel carrying a mobile unit leave a prescribed area. 
Another object of the invention is to alert supervised persons carrying a 
mobile unit responsive to a base unit transmitter that they are straying 
from a predetermined prescribed area. 
DISCLOSURE OF INVENTION 
Briefly described, the invention disclosed herein includes a small 
low-powered transmitter and a threshold adjustable receiver in at least 
one small mobile unit, and a low-power transmitter and a receiver in a 
single, fixed base unit. The base unit transmits an omnidirectional first 
signal that is detectable by the receivers in the mobile units. Whenever 
the threshold level in a mobile unit receiver is adjusted to the point at 
which it cannot detect that first signal from the base unit, the mobile 
transmitter becomes activated and emits a second signal that is detected 
by the base unit receiver. The frequency of the second signal may be 
different from the frequency of the first signal. Alternatively, the 
frequency of the second signal may equal the frequency of the first 
signal, provided a unit identification code is transmitted to distinguish 
between different transmitting units. The first and second signals are 
generally frequency independent. The base unit then produces a suitable 
audible alarm or visual display that indicates to the base unit personnel 
that a mobile unit has strayed out of the range predetermined by the 
threshold level adjustment in that mobile unit. If desired, the activated 
mobile transmitter may also initiate an alarm to warn the straying person 
carrying the mobile unit.

BEST MODE FOR CARRYING OUT THE INVENTION 
With reference to FIG. 1, the invention includes a single, fixed base unit 
10 which preferably includes a radio transmitter 12 which transmits from 
its antenna 14 a very low-power omnidirectional first electromagnetic 
signal that may have a maximum range from the base unit to a sensitive 
receiver at a distance of only approximately 800 to 1000 meters. The 
transmitter 12 produces the first signal at a frequency which preferably 
is a very low-power, short wave, VHF or UHF radio signal but which, if 
desired, may be a sonic signal in the ultrasonic range. A frequency of 
approximately 30 MHz is typical. The first signal may be continuous or 
intermittent in nature. The first signal may be modulated with a base unit 
identification code. 
The base unit 10 also includes a receiver 16 that is tuned to receive a 
fixed frequency second electromagnetic signal. The second signal which is 
received is preferably modulated with an identification code indicative of 
a particular transmitting unit. The frequency of the second signal is 
preferably equal to the frequency of the first signal when the transmitted 
signals contain an identification code. Alternatively, the second signal 
frequency may be sufficiently separated from the first signal frequency 
from the transmitter 12 so that the first signal is undetectable by the 
receiver 16. An audible alarm 18 is coupled to the output of the receiver 
16 and is responsive to the second signal detected by the receiver. While 
not essential, it is preferable that both the first and second signals are 
modulated with an audible tone as will be subsequently discussed. The 
alarm 18 may therefore comprise the audible signal modulated on the second 
frequency, or may be triggered by it to sound a bell or other audible 
alarm. The base unit may also produce a visual display instead of or in 
addition to the alarm 18 indicating a particular mobile unit which is 
transmitting the second signal. 
The monitor and alarm system also includes one or more identical mobile 
units 20 or 22 which may be sufficiently small in size to be strapped to 
the wrist of a supervised person or may be worn on the belt together with 
a small power pack. As illustrated in the mobile unit 20, each unit 
includes a fixed frequency receiver 24 tuned to the frequency of the first 
signal transmitted by the base unit 10. The receiver output signal, which 
is preferably an identification code or tone modulated on the first 
carrier signal, is adjustably controlled by a threshold circuit 26 which 
generates an output signal only when the detected receiver output signal 
falls below an amplitude level determined by the adjustment of the 
threshold circuit. Thus, the threshold circuit 26 may be adjusted to 
produce an output signal whenever the mobile unit is moved out of an area 
within which the base unit transmitted first electromagnetic signal may be 
detected. Stated differently, the threshold circuit 26 on any mobile unit 
may be adjusted to produce an output signal corresponding to some 
predetermined prescribed distance from the base unit. 
The threshold circuit 26 in each mobile unit controls the operation of a 
transmitter 28 which transmits an output second electromagnetic signal 
only upon receiving an output signal from the threshold circuit 26. Thus, 
whenever a mobile unit leaves an area prescribed by the setting of the 
threshold circuit 26, the transmitter 28 produces the second 
electromagnetic signal which is detected by the based unit 10 to sound the 
alarm 18 therein. If desired, the mobile unit transmitters may also 
include an audible alarm 30, preferably producing an output signal at the 
modulated tone frequency of the transmitted second signal for alerting the 
mobile unit wearer that the prescribed range has been exceeded. 
FIG. 2 schematically illustrates a method of operation of the monitor and 
alarm system. In FIG. 2 the base unit 10 may be located, for example, in a 
convalescent hospital for ambulatory patients who must take short walks 
from the hospital. The mobile unit 20 may be carried by a first patient 
who may stroll within only a 150 meter radius from the hospital. The 
threshold circuit 26 in that mobile unit 20 is therefore adjusted to a 
value corresponding to a 150 meter distance so that the transmitter 28 
will generate its second electromagnetic output signal at any distance 
exceeding that distance. A second mobile unit 22 may be carried by a 
second patient who is permitted to walk perhaps 500 meters from the base 
unit. Therefore, if it is assumed that both the first and second patients 
walk together, the mobile unit carried by the first patient will sound a 
base unit alarm upon its exceeding the 150 meter range whereas the second 
patient's mobile unit will permit a much further ranging. The visual 
display or alarm at the base unit indicating that a patient has crossed 
the prescribed boundary will then cause an attendant to fetch the errant 
patient. 
In FIG. 3, a base unit has a transmitter 12, connected to an 
omnidirectional antenna 14 and producing a first electromagnetic output 
signal at a first frequency. Transmitter 12 modulates a base unit 
identification code as part of the output signal. The code is stored in a 
reference code memory 32 and is delivered to transmitter 12 on a code bus 
34. Memory 32 is typically a four-pin switch set to one of sixteen 
different settings. A timing circuit 36 connects to transmitter 12 via an 
enable line 38 and produces a transmit enable signal for allowing 
transmitter 12 to produce the first signal and transmit the output signal 
in all directions from antenna 14. As already noted above, transmitter 12 
may be operated in either a continuous or intermittent manner. 
The base unit also has a receiver 16 connected to a receiving antenna 42. 
Receiver 16 receives a signal output, if any, by one of a plurality of 
mobile units at a second frequency. Preferably, the frequency of the 
received second signal is equal to the frequency of the transmitted first 
signal. Timing circuit 36 also connects to receiver 16 via an enable line 
40 and produces a receive enable signal allowing receiver 16 to receive 
signals from mobile units. When operated in an intermittent manner, timing 
circuit 36 alternately allows transmitter 12 and receiver 16 to operate, 
switching back and forth between the transmitter and receiver. When 
operated in a continuous manner, timing circuit 36 enables both 
transmitter 12 and receiver 16 simultaneously. In order to prevent 
possible confusion in identifying received signals when both transmitter 
12 and receiver 16 are simultaneously operated, the base unit 
identification code is preferably set to zero. When transmitter 12 and 
receiver 16 are operated alternately in an intermittent manner, the base 
unit identification code may be set to any of the possible settings. 
Receiver 16 demodulates the received electromagnetic second signal, if any, 
and the resulting serial code signal 44 is differentially amplified with 
respect to a synchronization signal in a bit generator 46 to generate a 
serial data signal. Bit generator 46 is connected to a decoder 54 via a 
data line 48 carrying this serial data signal, a synchronization line 50 
and a clock line 52. Decoder 54 also receives signals directly from 
receiver 16 via two channels 56. Decoder 54 compares these signals on 
channels 56 with the base unit identification code. Exclusive-OR gates in 
decoder 54 connected to both reference code memory 32 via a bus 58 and to 
channels 56 may be used for this comparison. The result of this comparison 
indicates whether the received electromagnetic second signal from antenna 
42 is a "good" signal from a mobile unit or is instead an echo of the 
transmitted first signal from transmitter 12. Decoder 54 also typically 
includes a shift register which converts the serial data signal on line 48 
to a parallel code signal of an equal number of bits containing the mobile 
unit identification code. 
The parallel code signal generated by decoder 54 is stored in a memory 62. 
Decoder 54 connects to memory 62 via an address bus 64 and a buffer 66. 
Memory 62 is typically a static RAM chip and keeps track of the current 
status of the various mobile units, i.e. whether a parallel code signal 
for a mobile unit has been received. 
The base unit has a control panel 68 which may include switches for on or 
off, automatic or manual operation, run or load, clear memory, and enter 
active mobile units. Other switches or buttons for similar functions may 
also be included. Control panel 68 communicates via a control bus 70 with 
a multiplexer 72. Multiplexer 72 likewise communicates with memory 62 via 
a second control bus 74 and a buffer 76 for accessing information stored 
in memory 62. In response to control signals on control bus 70, 
multiplexer 72 may enable the timing circuit 36 via timing enable line 77, 
transfer a mobile unit identification code in buffer 66 to memory 62 or 
drive a display or alarm 18. Control lines, represented by lines 78, 
between memory 62 and multiplexer 72 acknowledge that a code has been 
received, that it is ready for another command and the like. 
Identification code information may be transferred on command from memory 
62 to display 18 via data display buses 82. Display 18 which is typically 
an LED display, indicates in sequence the mobile unit or units, if any, 
which are out of range. 
In FIG. 4, a mobile unit has a receiver 24 connected to a receive antenna 
86 for receiving the coded first electromagnetic signal from the base 
unit. A selector circuit 88 connects to receiver 24 via an enable line 90 
and produces a receive enable signal allowing receiver 24 to receive 
signals from the base unit. Selector 88 also connects to a transmitter 28 
via second enable line 92 for enabling the transmitter 28. Normally, when 
the mobile unit is not out of range, the receiver 24 is continually on and 
the transmitter is off. Whenever the mobile unit moves out of range, 
selector 88 enables the transmitter, and typically also disables the 
receiver. 
Receiver 24 connects to a bit generator 96 via a serial code line 94 and to 
a decoder 104 via channels 106. Bit generator 96 also connects to decoder 
104 via a data line 98, synchronization line 100 and a clock line 102. 
Receiver 24, bit generator 96 and decoder 104 work in the same manner as 
receiver 16, bit generator 46 and decoder 54 of the base unit described 
above. Decoder 104 compares a signal from receiver 24 on channels 106 with 
a base unit identification code stored in a memory 112 to determine 
whether the electromagnetic signal being received is indeed a "good code" 
from the base unit and not from some other mobile unit. Memory 112 
communicates with decoder 104 via a code bus 108. The result of the 
comparison is output from decoder 104 onto a code line 110. 
Receiver 24 also connects to threshold detector 26. Threshold detector 26 
is typically one or more operational amplifiers whose amplifications can 
be varied. Increasing the amplification lowers the threshold of detection 
and increases the distance from base unit over which the mobile unit can 
be still in range. Decreasing the amplification, reduces the range of the 
mobile unit. The resulting electrical signal, if the mobile unit is in 
range, or absence of a signal, if the mobile unit is out of range, is 
transmitted over an in-range line 113 to a pulse detector 114. Code line 
110 also connects to pulse detector 114. 
Pulse detector 114 produces a signal indicative of a good reception by 
receiver 24 when both code line 110 transmits a signal indicative of a 
good code and when in-range line 113 transmits an in-range signal. If 
either code line 110 indicates that the received signal is not from the 
base unit or in-range line 113 indicates that the mobile unit is out of 
range from the base unit, pulse detector 114 produces a signal indicative 
of a poor reception by receiver 24. Pulse detector 114 is typically a 
simple logic gate. Pulse detector 114 connects at its output to selector 
88 via a pulse detection line 116. 
A sync generator 120 receives a signal on a second code line 118 from 
decoder 104. The output of sync generator 120 connects to selector 88 via 
a sinc line 122. Sync generator typically comprises a commercially 
available timer IC generating pulses synchronized with the output of 
decoder 104. Selector 88 enables transmitter 28 whenever sync generator 
120 transmits a pulse to selector 88 and pulse detector 114 transmits a 
signal indicative of poor reception. Otherwise, selector 88 enables 
receiver 24. 
Transmitter 28 transmits an electromagnetic second signal over antenna 126 
having a frequency which is preferably equal to the frequency of the first 
signal transmitted by the base unit. Transmitter 28 communicates via a bus 
124 with memory 112. Memory 112, in addition to storing a base unit 
identification code for comparison in decoder 104 also stores a mobile 
unit identification code particular to the mobile unit. Memory 112 may, 
for example, be an 8-pin switch with four of the switches set to the 
positions for the base unit identification code and the four remaining 
switch pins set to positions for the mobile unit identification code. 
Transmitter 28 modulates the mobile unit identification code on the signal 
transmitted by antenna 126. Thus, whenever the mobile unit is out of range 
from the base unit, the mobile unit transmits its identification code to 
the base unit where it is detected and displayed as described above. 
While the invention has been described for use in a rest home and for 
ambulatory patients, it is clear that it may be used for controlling 
children in a playground or at a picnic. The alarm may also be valuable 
for training hunting dogs or other animals by substituting the optional 
alarm 30 in the mobile unit for a unit that gives the animal a gentle 
electrical shock if it extends its range beyond a prescribed amount.