Apparatus for monitoring child activity

Apparatus for monitoring a child or the like includes a transmitter having a microphone and an oscillator coupled to an antenna within an enclosure for mounting on a child's arm and transmitting a frequency-modulated rf signal from the antenna. A separate receiver, within an enclosure constructed to be hand carried by a parent, includes circuitry for separating the audio and carrier components of the signal received at the receiver, and comparing field strength of the carrier component to a range threshold. When the amplitude of the carrier field strength is less than the threshold value, an alarm is sounded to indicate that the child is out of desired range. The audio component is fed to a speaker so that the parent can monitor the activities of the child.

The present invention is directed to apparatus for monitoring the range of 
a person from a monitoring location, and more particularly to apparatus 
for monitoring both the range to and audio activity around a child. 
BACKGROUND AND OBJECTS OF THE INVENTION 
There have heretofore been proposed in the art a number of schemes and 
techniques for monitoring the range to or location of a person with 
respect to a monitoring station, such as the range to a child or patient. 
In general, systems heretofore proposed have been characterized by a high 
degree of cost and complexity. It is therefore a general object of the 
present invention to provide an apparatus of the described character that 
is of simple and economical construction, that may be worn by a child or 
other person without obstructing daily activity, in which the receiver 
employed by the parent or other monitoring person is transportable in the 
hand or pocket for monitoring the range to and activity around a child 
away from the home during a walk, for example, and that indicates an alarm 
condition in the event that the transmitter unit is removed from the child 
or other person to be monitored. 
SUMMARY OF THE INVENTION 
Apparatus for monitoring range to a person from a monitoring station in 
accordance with the present invention comprises a transmitter having a 
transmitting antenna for radiating a frequency-modulated rf carrier 
signal, and an enclosure with facility for removably fastening the 
enclosure to a person to be monitored. A separate receiver includes 
circuitry coupled to a receiving antenna for monitoring field strength of 
the carrier signal received at the antenna, and for indicating range from 
the receiver to the transmitter as a function of such field strength. In 
the preferred embodiment of the invention, the transmitter includes a 
microphone positioned within the enclosure for generating an audio signal 
as a function of sounds around the transmitter, and the rf carrier signal 
is frequency modulated as a function of such audio signal. The receiver in 
such preferred embodiment of the invention includes corresponding facility 
for separating the audio signal from the frequency-modulated carrier 
signal at the receiving antenna, and an audio speaker for making audible 
at the receiver sounds surrounding the transmitter and detected by the 
microphone. Thus, the parent or other monitoring person may monitor both 
range to the child and activity of the child as reflected by the audio 
sounds detected by the microphone. 
The receiving unit in the preferred embodiment of the invention comprises a 
rectangular enclosure with knurled knobs positioned along opposed 
sidewalls of the enclosure for adjusting both volume of audio radiated by 
the speaker and alarm range between the transmitter and the receiver. The 
transmitter electronics are mounted on a printed circuitboard that forms 
one wall of a transmitter enclosure. The circuitry on the circuitboard 
forms a ground plane positioned internally of the enclosure and with 
respect to straps for fastening the transmitter to a child's arm so that 
the ground plane is capacitively coupled to the skin of the wearer, and 
the wearer's body thus forms part of the transmitting antenna. In this 
way, field strength of the transmitted signal is automatically greatly 
reduced in the event that the transmitter is removed from the wearer, and 
generates an alarm condition at the receiver. 
n accordance with a further aspect of the present invention, the circuitry 
of the transmitter is specifically constructed not only to reduce the 
number of components needed to form the necessary circuit functions, but 
also to reduce power consumption and thereby enhance transmitter battery 
life. This is accomplished by employing an operational amplifier for 
coupling the microphone output to the carrier signal oscillator, and 
connecting the reference or offset voltage input of the amplifier to the 
junction of two series-connected batteries. In this way, circuit 
components for generating separate offset voltages at the amplifier and 
oscillator are automatically eliminated, as is the dc coupling capacitor 
normally positioned between the amplifier output and the oscillator input.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 illustrates apparatus 10 in accordance with a presently preferred 
embodiment of the invention as comprising separate transmitter and 
receiver units 12,14. Transmitter 12 comprises a voice encoder 16 that 
includes a microphone 18 (FIG. 5) and an audio amplifier 20. Voice encoder 
16 is connected to a radio transmitter 22 that includes an rf oscillator 
24 (FIG. 5) for frequency modulating an rf carrier signal and feeding such 
signal to an rf transmitting antenna 26. Receiver 14 includes a receiving 
antenna 28 connected to receiver circuitry 30 for separating the audio and 
rf carrier components of the received signal. Carrier field strength is 
coupled at 32 to a circuit 34 for sampling and holding a signal indicative 
of peak field strength amplitude. Field strength is compared at 36 with a 
threshold 38 selected by the operator. As long as the peak carrier field 
strength is greater than the this threshold, the transmitter is considered 
to be within range and an LED 40 is illuminated. When the peak carrier 
field strength decreases below the range threshold 38, the LED is 
extinguished and an appropriate signal is fed to audio range alarm 
circuitry 42. An audio decoder 44 receives a first signal input from 
receiver 30 indicative of the audio component of the frequency-modulated 
signal at antenna 28, and a second signal input from audio alarm 42. A 
volume control input 46 is also fed to audio decode circuit 44 for 
operator selection of amplitude of the audio voice and alarm signals. The 
output of decoder 44 is fed to a speaker 48. 
FIG. 2 illustrates receiver unit 40 as comprising a rectangular enclosure 
50 of dimensions suitable for holding in an adult's hand, or placement in 
a shirt or a jacket pocket. Range selection and volume control adjustments 
38,46 (FIGS. 1 and 2) include knurled knobs that extend through opposed 
sidewalls of enclosure 50 for operator selection of the corresponding 
control parameters. Antenna 28 extends through an end wall of enclosure 
50, with an on/off slide-type power switch 52 being positioned adjacent 
thereto. An LED 54 adjacent to range adjustment knob 38 indicates received 
signal to the receiver circuitry. The front wall of enclosure 50 includes 
a suitable opening 56 positioned adjacent to speaker 48 for emitting the 
audio sound and alarm signals to the monitoring person. 
FIGS. 3 and 4 illustrate transmitter 12 as comprising a generally 
rectangular enclosure 58. The circuitry of transmitter 12 (FIGS. 1 and 5) 
is mounted on a printed circuitboard 60 that forms one wall of enclosure 
58. Circuitboard 60 is constructed so as to have a ground plane 62 (FIG. 
5) positioned internally of enclosure 58 for capacitive coupling to the 
skin of a wearer when the straps 64 fastened to enclosure 58 are affixed 
to the wearer (as shown in FIG. 4). The wearer's body thus effectively 
becomes part of the transmitting circuitry and the transmitting antenna, 
so that the power of transmission and transmitted field strength are 
greatly reduced in the event that the transmitter is removed from the 
wearer, automatically resulting in an out-of-range audio alarm signal at 
the receiver. Transmitter 58 also includes a slide-type power switch 66 
positioned along one sidewall of enclosure 58, and an LED 68 for 
indicating application of battery power to the transmitter circuitry. 
FIG. 5 illustrates the electronic circuitry of transmitter 12. A pair of 
batteries 70,72 (as well as a third battery 74) are connected in series to 
provide a first dc potential at the junction 76 of batteries 70,72, a 
second dc potential at the positive terminal of battery 70, and a 
reference or ground potential at the negative terminal of battery 74. 
Switch 66 connects the positive terminal of battery 70 to a power bus 78, 
across which LED 68 (FIGS. 3-5) is connected. The negative terminal of 
battery 74 is connected to ground plane 62. Microphone 18 is connected in 
series with a current limiting resistor 80 and an rf choke 82 between bus 
78 and ground plane 62. Amplifier 20 comprises an operational amplifier 
having a reference input 84 connected to junction 76, and a signal input 
86 connected to microphone 18 through a blocking capacitor 88 and a 
resistor 90. The output of amplifier 20 is connected to signal input 86 
through a resistor 92. The power terminals of amplifier 20 are connected 
to voltage bus 78, and to ground 62 through choke 82. 
Rf oscillator 24 includes a PNP transistor 94 having its base connected to 
the output of amplifier 20, its emitter connected through a resistor 96 to 
bus 78, and its collector connected through a coil 98 to ground 62. A 
feedback capacitor 100 is connected between a tap on coil 98 and the base 
of transistor 94. A fixed capacitor 102 and a variable capacitor 104 are 
connected in parallel across coil 98 for setting the base frequency of 
oscillator 24. Antenna 26 is connected to the collector of transistor 94. 
Use of series-connected batteries for powering transmitter 12, and 
connection of amplifier reference input 84 to battery junction 76, provide 
appropriate dc offset for generation of the audio input to the oscillator 
without requiring the usual amplifier input biasing resistors or pull-up 
and pull-down amplifier output resistors. Further, the series-connected 
battery arrangement of FIG. 5 automatically provides biasing for 
oscillator 24, while eliminating the usual biasing resistors for 
transistor 94. Since the audio signal input to the oscillator is carried 
by the dc offset, there is no need for a dc isolation capacitor between 
the output of amplifier 20 and the input of oscillator 24. Choke 82 
isolates negative rf feedback from amplifier 20 and quenches oscillator 
24. The transmitter design of FIG. 5 thus reduces both number of 
components and power consumption as compared with transmitters for a 
similar purpose heretofore proposed.