Swimming pool intrusion alarm system

The present apparatus is designed to determine the presence of an intruder in a swimming pool by detecting a predetermined change in water level caused by displacement. The surface of the pool water forms the lower boundary of an air filled acoustic cavity, and cavity resonance is excited therein by a sonic transducer coupled to the cavity and also to a signal amplifier, causing the cavity to determine the frequency of oscillation of the feedback amplifier. A rise in water level caused by intruder displacement decreases the vertical dimension of the cavity so that the cavity has a different resonant frequency than before intrusion, and the amplifier therefore oscillates at a different frequency. The predetermined change in amplifier signal output frequency is sensed by an electrical network and converted to an alarm device activating signal to indicate an intrusion.

BACKGROUND OF THE INVENTION 
It is the purpose of the present continuation in part application to 
describe a simplified and improved embodiment over that disclosed in my 
co-pending application, Ser. No. 738,523, filed Nov. 3, 1976 now 
abandoned. This invention permits the detection of the presence of a child 
in the protected swimming pool in time to prevent drowning. The apparatus 
of the present invention can detect the presence of a child in the pool 
whether he falls in or enters the water gently. The present apparatus can 
be installed at any location on the pool or even at a remote location and 
connected to pool water by pipe or syphon without range or sensitivity 
problems. Unlike the prior art, the apparatus of the present invention 
does not respond to waves, ambient sound, water pressure or transitory 
water disturbance, nor does the present invention utilize floats or moving 
mechanical parts which have troubled the prior art. 
SUMMARY OF THE INVENTION 
In the apparatus of the present invention propagated wave resonant cavity 
apparatus is utilized to sense relative water level. Hydraulic integrating 
apparatus utilizing the lower portion of said hollow body as a water 
capacity chamber analogous to an electrical capacitor, and a duct or 
syphon tube of predetermined resistance to water flow admitting water to 
said hollow body, analogous to electrical resistance, to comprise a 
hydraulic integrator analogous to an electrical RC integrator. The output 
signal from said sensing and integrating apparatus is a frequency 
proportionate to relative average water level. In the absence of an 
intruder, said frequency has a steady value which is herein termed the 
reference value signal. When an intrusion occurs, water level rises to 
cause an increase in frequency which is sensed by an electrical network 
responsive to phase or frequency change to produce an alarm device 
actuating signal. 
Although the preferred embodiment utilizes a reciprocal sound transducer, 
this disclosure teaches that a microwave radio frequency transducer with 
said hollow body conductive, and a microwave frequency amplifier and 
suitable frequency networks could be utilized. Further, it should be noted 
that although a cylindrical body is described, other forms can be 
utilized. For example, a solid resonatable body can be utilized. Also, 
while a preferred non-inverting amplifier is shown, others known to the 
art are suitable. While a preferred phase and frequency responsive network 
is shown, other known to the art are suitable. While a preferred 
triggerable, latching alarm network is shown, others known to the art are 
suitable. 
OBJECTS OF THIS INVENTION 
To provide an improved swimming pool intrusion signal system including an 
output signal and an alarm unresponsive to waves, ambient noise, 
transitory water disturbance, water pressure, rain or evaporation. 
To provide a swimming pool intrusion system without floats or moving 
mechanical parts. 
To provide a swimming pool intrusion system alarm which does not require 
readjustment for change in water level due to rain or evaporation. 
To provide a swimming pool intrusion alarm apparatus having equal detection 
sensitivity at any location on the pool.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1 which shows the detecting apparatus, The drawing is 
a sectional view through the axis, viewed in the plane of the paper, of 
hollow cylindrical body 102 mounted vertically by bracket 101 to pool side 
100, partially immersed, vented to atmosphere by vent hole 105, and with 
pool water admitted to the interior of said body by duct 103, said duct 
offering predetermined resistance to water flow. Initial or reference 
water level 106, variable to water level 107, and air space over said 
contained water within said body having a principal dimension 108 variable 
to dimension 109. Reciprocal transducer 104 is mounted to cover hole 110 
at the upper end of body 102, so that said transducer is operatively 
coupled to air space within body 102. Transducer 104 has connection 
terminals A and B. 
FIG. 2 shows a non-inverting amplifier network comprised of coupling 
capacitors C1, C2, transistors Q1, Q2, bias resistors R1, R3, collector 
load resistors R2, R4, input terminal A, output terminal B, positive power 
terminal P, negative power return and signal return terminal G. 
FIG. 3 shows the circuit network of a first embodiment of the present 
invention, comprised of the apparatus of FIG. 1 with terminals A and B 
connected to the respective terminals of the amplifier of FIG. 2, and 
output terminal B of said amplifier coupled through capacitor C1 to input 
terminal 2 of phase locked loop integrated circuit 302 comprising 
integrated circuit NSC 565, made by the National Semiconductor Corp. Said 
integrated circuit is utilized with bias resistors R1, R2, R3, R4, 
programming resistors R5, R6, programming capacitor C3, bypass capacitor 
C2, a low pass network comprised of resistor R7 and capacitor C4. The 
signal output of said phase locked loop network from terminal 7 connected 
through said low pass network through coupling capacitor C5 to the input 
of a PNP inverting amplifier comprised of transistor Q1, bias resistors 
R8, R9, and load resistor R10. The output of said inverting amplifier is 
direct connected to the input of a triggerable regenerative pulse forming 
network comprised of NPN transistor Q2, PNP transistor Q3, load resistors 
R11, R13, feedback resistor R12, feedback capacitor C6, blocking diode D1. 
The OUtput of said pulse forming network from the cathode of diode D1 is 
direct connected to a pulse summing network comprised of resistors R14, 
R15, and capacitor C7. The output of said pulse summing network is 
connected to the input of a triggerable, latching, alarm switching network 
comprised of silicon controlled rectifier SCR-1, load resistor R16, and 
bypass capacitor C8. Alarm bell 303 is connected across load resistor R16. 
The apparatus is powered by battery B1 through switch S1. 
Referring to the system shown in FIG. 3 and its operation, the apparatus of 
FIG. 1 is shown mounted at pool side with body 102 half immersed to 
initial water level 106. When no intruder has entered the pool, the water 
level within the chamber of wave resonatable body 102 has an average and 
steady reference level, stabilized by the integrating action of said 
chamber water capacity together with said flow resistive duct. Resonatable 
herein denotes "capable of being made to resonate". The air space above 
water within body 103 has a vertical dimension 108 comprising the largest 
internal dimension of said chamber which determines the lowest frequency 
of cavity resonance. Transducer 104 is connected as a signal feedback 
element to amplifier 301 at terminals A and B respectively, to cause said 
amplifier to oscillate, and the sound output of said transducer to excite 
cavity resonance within the hollow body 102 at a frequency related to 
dimension 108. Said cavity resonant signal in turn determines the 
frequency of oscillation of amplifier 301. As long as there is no intruder 
caused change in water level, said frequency is unvarying, and coupled to 
the input of phase locked loop 302, does not vary the D.C. level of output 
from 302 to cause an alarm actuating signal. However, when an intruder 
causes a rise in water level due to displacement, say to water level 107, 
then internal major dimension air space 108 decreases to dimension 109 
with a consequent increase in cavity resonant and amplifier frequency. 
Said frequency change signal causes phase locked loop network 302 to have 
a negative going change in signal output which is filtered free of 
oscillation frequency by low pass filter R7, C4 and coupled through 
capacitor C5 to the inverting amplifier network including transistor Q1, 
and the output of said inverting amplifier connected to the base of 
transistor Q2, the input of said triggerable pulse forming network, to 
cause said network to emit a train of positive pulses of a time duration 
depending upon the magnitude of the change in water level. Said train of 
pulses is integrated by the integrator network R14, R15, C7, with R14 
adjustable to control sensitivity, and the integrated positive voltage 
imposed upon the gate of SCR-1 to cause SCR-1 to latch in the conducting 
state and energize said alarm bell. Load resistor R16 sustains the current 
through the silicon controlled rectifier during the times the bell 
contacts are open, to prevent the SCR from turning off. Alarm persists 
until switch S1 is opened momentarily, to reset the apparatus. 
Slow change in water level due to rain or evaporation is not sufficient to 
cause alarm and the apparatus stabilizes at the new water level. 
Slow changes in water level due to rain or evaporation do not produce 
enough delta h in a short enough time interval to actuate the alarm. 
It should be noted that Section AN46 of the publication "Linear 
Applications-Vol. 1," by National Semiconductor Corp. 1973 provides a 
detailed discussion of the Phase Locked Loop Integrated Circuit. 
Referring now to FIG. 4, the circuit network of a second embodiment of the 
present invention, comprising the apparatus of both FIGS. 1 and 2 together 
with the circuit network to be described. The apparatus of FIG. 1 with 
terminals A and B connected to the respective terminals of the amplifier 
of FIG. 2, and output terminal B of said amplifier coupled through 
capacitor C1 to input terminal 2 of phase locked loop integrated circuit 
402 comprising integrated circuit NSC 565, made by the National 
Semiconductor Corp. Said integrated circuit is utilized with bias 
resistors R1, R2, programming resistors R3, R4, programming capacitor C2, 
bypass capacitor C3, a low pass network comprised of resistor R5 and 
capacitor C4. The signal output from terminal 7 of said phase locked loop 
network is connected through said low pass network and current limiting 
resistor R7 to the inverting input terminal 2 of operational amplifier 
403, comprised of integrated circuit NSC-741 made by the National 
Semiconductor Corp. The signal output from terminal 7 of said phase locked 
loop is also connected to the input of an integrating network comprised of 
resistor R6 and capacitor C5, and the output of said integrating network 
is connected through current limiting resistor R8 to the non-inverting 
input terminal 3 of said operational amplifier 403. R9 null resistor is 
connected from terminal 3 to ground. Feedback resistor R10 is connected 
from output terminal 6 of amplifier 403 to input terminal 2 and functions 
as a gain control. The output of amplifier 403 from terminal 6 is 
connected across load resistor R12 and to the input of a triggerable, 
latching alarm switching network comprised of silicon controlled rectifier 
SCR-1, load resistor R11, and bypass capacitor C6. Alarm bell 404 is 
connected across load resistor R11. The apparatus is powered by batteries 
B1 and B2 connected through dual switch S1. 
A third embodiment not shown is comprised of the apparatus of FIG. 4, with 
the exception that duct 103 of the apparatus of FIG. 1 is predetermined to 
offer minimum resistance to water flow, that is fully open so that 
hydraulic integration does not occur, and resistor R5 and capacitor C4 are 
now proportioned to comprise an integrating network instead of the former 
low pass network. The circuit configuration of this embodiment looks the 
same as that of the second embodiment, however, the change in proportion 
results in a different mode of operation. 
Referring now to the second embodiment, in operation, the apparatus of FIG. 
1 is shown mounted at pool side with body 102 half immersed to initial 
water level 106. When no intruder has entered the pool, the water level 
within the chamber of body 102 has an average and steady reference level, 
stabilized by the integrating action of said chamber water capacity 
together with said flow resistive duct. The air space above water within 
body 102 has a vertical dimension 108 comprising the largest internal 
dimension of said chamber which determines the lowest frequency of cavity 
resonance. Transducer 104 is connected as a signal feedback element to 
amplifier 401 at terminals A and B respectively, to cause said amplifier 
to oscillate, and the output sound of said transducer to excite cavity 
resonance within said hollow body 102 at a frequency related to dimension 
108. Said cavity resonant signal in turn determines the frequency of 
oscillation of amplifier 401. As long as there is no intruder caused 
change in water level, said frequency is unvarying, and coupled to the 
input of phase locked loop 402, does not vary the D.C. level of output 
from 402 to cause an alarm actuating signal. However, when an intruder 
causes a rise in water level due to displacement, say to water level 107, 
then major internal dimension air space 108 decreases to dimension 109 
with a consequent increase in cavity resonant and amplifier frequency. 
Said frequency change signal causes phase locked loop network 402 to have 
a negative going change in signal output. Prior to intrusion during the 
steady D.C. output from terminal 7 of the phase locked loop, capacitors C4 
and C5 have been charged to that steady D.C. level and said equal levels 
imposed upon the differential inputs of the operational amplifier 403 so 
that the amplifier has no output signal, R9 having been adjusted to give 
zero output, and R3 adjusted to cause the internal oscillator in 402 to 
have approximately the frequency of cavity resonance. The negative going 
signal output of phase locked loop 402 at time of intrusion is filtered of 
cavity resonant frequency by low pass filter R5, C4 and impressed through 
current limiting resistor R7 upon terminal 2 inverting input of 
operational amplifier 403. The same phase locked loop output is impressed 
upon the input of integrator network R6, C5 and the output of said 
integrator connected to the non-inverting terminal 3 input of amplifier 
403. Due to the time constant of said integrator, there is a time interval 
during which inverting input 2 of said amplifier has a negative input 
which triggers SCR-1 to the conducting state and energizes the alarm bell 
404. Load resistor R11 sustains the current through SCR-1 during the time 
the bell contacts are open, to prevent the SCR from turning off. Alarm 
persists until switch S1 is opened momentarily to reset the apparatus. 
Slow change in water level due to rain or evaporation is not sufficient to 
cause alarm and the apparatus stabilizes at the new water level. 
Referring now to the third embodiment, in operation, the circuit network of 
a third embodiment looks identical to that of the second embodiment, 
however, the values of three parameters are herein changed to give a 
different mode of operation. First: water admitting duct 103 of FIG. 1 is 
now predetermined to offer minimum resistance to water flow so that there 
is no hydraulic integration in this embodiment, and water level within the 
chamber of body 102 now varies according to water wave action. The output 
signal of phase locked loop network 402 from terminal 7 now is a signal 
responsive to water waves but having an average level response to average 
water level. Second: the values of resistor R5 and capacitor C4 are 
increased so that the former low pass network becomes an integrator 
network with longer time constant than that of the prevailing water waves, 
but of shorter time constant than that of integrator network R6, C5. In 
the absence of an intruder capacitors C4 and C5 charge to the average D.C. 
potential output from terminal 7 of the phase locked loop 402 and the 
differential inputs of amplifier 403 see equal input signals, so that 
amplifier 403 has no signal output. 
When an intruder in the pool causes a rise in water level due to 
displacement, although there is water wave rise and fall of level within 
the chamber of body 102, it is now at a higher average level and the 
frequency varying signal imposed upon the input of phase locked loop 402 
has a higher average value, and therefore the output of said phase locked 
loop has a lower average D.C. level. The integrated D.C. change in level 
in the negative going direction reaches input 2 of the amplifier 403 
before it reaches input 3 of said amplifier because of the longer time 
constant of the integrator in the input to terminal 3 than that in the 
input to terminal 2, so there is a time interval during which terminal 2 
becomes more negative than terminal 3 and said amplifier has a positive 
output signal which triggers SCR-1 to the conducting state, and energizes 
alarm bell 404. 
It should be noted that disclosure herein sets forth an intrusion detection 
apparatus for swimming pool or body of water which is activated by small 
change in average hydraulic head occurring because a person or object has 
caused a change in water displacement by changing the degree of immersion. 
A non-alarm state reference average hydraulic head is first determined by 
the present apparatus. Means are provided to detect a predetermined degree 
of departure, occurring during a predetermined time level, from said 
reference average hydraulic head, to produce an alarm activating signal 
upon said departure. Alarm indicating means, operable by said alarm 
activating signal, are provided to indicate that intrusion has occurred. 
Further means are provided to accommodate relatively large, long term 
changes in said non-alarm state reference hydraulic head, without 
requiring any adjustment of said apparatus. In the present apparatus, said 
reference hydraulic head is averaged over a time interval longer than the 
period of any waves or swells prevailing in the protected body of water. 
In the instant invention, transducer sensed hydraulic head signals are 
integrated over a time interval which is long compared to the period of 
prevailing waves but short compared to the time for significant change due 
to rain, evaporation, or change in reference displacement. Detection of 
small average change in hydraulic head, several orders of magnitude 
smaller than prevailing wave amplitude, is accomplished by integrating 
transducer sensed signals over a predetermined time interval and comparing 
with value sensed and integrated over a different predetermined time 
interval. 
Hydraulic integrating means are herein illustrated and described. Other 
embodiments are possible, taught by the present disclosure. Electrical 
network integrators, or combined electrical and hydraulic integrators are 
feasible in the present invention. 
Referring now to FIG. 5, another embodiment is illustrated, the cylindrical 
hollow housings 502 and 509, shown in vertical section, are each of 
predetermined internal volume and vertical internal dimensions 524 and 525 
respectively. They are securely mounted with axis vertical by mounting 
bracket 523 from the side of the pool, and partially submerged in pool 
water so that mark 501' upon the outside of each housing, coincides with 
the greatest expected depth of water surface 501 in the pool. This insures 
that the pool water can drop as far as level 522 without causing internal 
dimensions 524 and 525 to exceed design range values, since dimensions 524 
and 525 determine the frequency of resonance of airspace 502' and 509' 
respectively. Airspace 502' within housing 502 is vented to the atmosphere 
by a small hole 503, and airspace 509' within housing 509 is vented to the 
atmosphere by small hole 500, in order to equalize internal and external 
air pressures. The space within housing 502 is herein considered the 
primary chamber, and the space within housing 509 the secondary chamber. 
Water 520 within the primary chamber is admitted from the pool through 
primary duct 504 which offers a predetermined resistance to water flow. 
Water 521 within the secondary chamber is admitted from the pool through 
secondary duct 511 which offers predetermined resistance to water flow. 
Hole 526 is located in the sidewall of housing 502 at one half the average 
of dimension 524 above the surface of water 520, and hole 527 is similarly 
located at one half the average of dimension 525 above the surface of 
water 521 in the sidewall of housing 509. Reciprocal transducer 505 is 
mounted to cover hole 526, and reciprocal transducer 512 is mounted to 
cover hole 527. Transducer 505 is thus coupled to air column 502', and 
transducer 512 is coupled to air column 509'. Transducer 505 is connected 
as a feedback element into amplifier 508 network by wires 506 and 507, and 
transducer 512 is connected as a feedback element into amplifier network 
515 by wires 513 and 514, causing amplifiers 508 and 515 to oscillate at 
the resonant frequency of each air column respectively. The signal outputs 
of amplifiers 508 and 515 are connected to the dual inputs 507 and 514 of 
network 516. Electrical network 516 comprises a mixer-detector or other 
network capable of giving an output signal only when the frequencies of 
its two input signals are not equal. The output of mixer-detector 516 is 
connected to the input of signal amplifier 518 by connection 517. The 
output of signal amplifier 518 is connected to the input of triggerable 
alarm circuit network 520 by connection 519. Alarm bell 521 operable by 
the output of network 520, when network 520 is triggered to the alarm 
state by a signal of predetermined amplitude from signal amplifier 518, is 
connected to the output of network 520. 
Referring now to the apparatus of FIG. 5 in operation, when no intruder has 
entered the swimming pool water, the average water levels within the 
chambers of housings 502 and 509 and the water level of the pool are 
substantially equal. Said water level may be anywhere between 501 and 522 
in FIG. 5, and for the purpose of the present invention this is the 
reference level, and only its state of equality is pertinent. Under the 
reference condition air columns 524 and 525 have been preselected equal. 
Coupled air column 524 and transducer 505 resonate to cause amplifier 508 
to oscillate at the frequency for which dimension 524 is one half wave 
length. Since air columns of dimensions 524 and 525 are equal when there 
is no intrusion, then air column 525 coupled with transducer 512 resonate 
to cause amplifier 515 to oscillate at the same frequency as amplifier 
508. Under this condition of equal frequency signal inputs mixer-detector 
516 can have no output and there is no alarm actuation. When an intruder 
enters the swimming pool water, displacement causes the water level to 
rise slightly, and water flows into the primary fluid chamber 502' through 
primary duct 504, and into secondary fluid chamber 509' through secondary 
duct 511. Since the product R.sub.A V.sub.C for the secondary fluid 
chamber has been preselected smaller than the R.sub.A V.sub.C for the 
primary fluid chamber, referring to the curves and equation of FIG. 6, 
then at preselected time t the level of water in the secondary fluid 
chamber is delta h higher than the water in the primary fluid chamber, 
causing the air column of dimension 525 to be smaller than that of 524, so 
that column 525 with transducer 505 resonate at a higher frequency than 
column 524 with transducer 512. Now the two signal inputs to 
mixer-detector 516 are two unequal frequencies and mixer-detector 516 has 
a signal output to signal amplifier 518, whose output triggers the alarm 
circuit network 520 to the alarm condition, and sounds bell 521 to 
indicate an intrusion. If the pool water level changes from level 501 
toward level 522 due to evaporation, or back again due to rain, the change 
is too slow to produce enough delta h (see FIG. 6) to cause an alarm. In 
this case the primary and secondary fluid levels remain substantially 
equal as they change and stabilize at a new reference level. 
Signal amplifiers 508 and 515, shown in functional block diagram form in 
FIG. 5, each comprises known to the art operational amplifier LF13741 made 
by National Semiconductor Corp. Santa Clara, CA 95051, and herein utilized 
in the non-inverting A.C. amplifier circuit network shown on page 43 and 
FIG. 2.52 of the publication linear and Interface Circuit Applications, 
published by Texas Instrument Corp. in 1974. Included are necessary 
coupling, bias and power supply elements and connections. The signal 
output of amplifier 508 is connected to a first input of mixer-detector 
516 by wires 507', and the signal output of amplifier 515 is connected to 
a second input of mixer-detector 516 by wires 514'. Mixer-detector 516, 
shown in functional block diagram form, comprises the known to the art 
electrical signal mixer-detector network shown and described in Electronic 
Circuits Design Casebook, pages 80 and 81, published in 1971 by 
McGraw-Hill Publishing Company, and includes necessary coupling, bias and 
power supply elements and connections. Mixer-detector 516 can have an 
output signal only when its two input A.C. signals are unequal in 
frequency, and said output is a D.C. voltage. The output of mixer-detector 
516 is connected by wires 517 to the input of signal amplifier 518. Signal 
amplifier 518, shown in functional block diagram form, comprises a known 
to the art electrical signal amplifier network, operational amplifier SN 
72709, made by Texas Instrument Corp. Dallas, Texas 75222, connected in 
the D.C. amplifier, non-inverting circuit network shown on page 41, FIG. 
2.50 of the publication Linear and Interface Circuit Applications, 
published in 1974 by Texas Instruments Corp. and includes necessary 
coupling, bias and power supply elements and connections. The output of 
signal amplifier 518 is connected to the input of alarm circuit network 
520 by wires 519. Alarm circuit network 520, shown in functional block 
diagram form, comprises the known to the art triggerable, latching alarm 
circuit network shown on page 425, upper diagram of FIG. 16.42, utilizing 
the described alternate input, triggerable by a positive signal voltage, 
in the publication Transistor Manual, Seventh Edition, 1964, published by 
General Eelctric Corp. and includes necessary coupling, bias and power 
supply elements, and connections, as well as a sensitivity control and a 
reset switch pushbutton. The output of alarm circuit network 520, 
comprising D.C. power supply voltage applied by relay contact closure in 
alarm circuit network 520, is connected to the input of alarm bell 521 by 
wires 528. Alarm bell 521, shown in functional block diagram form, 
comprises a known to the art bell, of voltage rating compatible with the 
apparatus power supply, exemplified by catalog number 275-498, sold by The 
Radio Shack Div. of Tandy Corp. 
FIG. 6 is based on the equation as shown and described in the drawings. 
The natural phenomena of cavity wave resonance utilized in the apparatus of 
the present invention are described in the following publications: SOUND 
by Alexander Efron, published by John Rider Publishing Co., 1957, page 58; 
REFERENCE DATA FOR ENGINEERS, 4th edition published 1956 by International 
Telephone and Telegraph Corp., see page 635. 
The instant invention has been shown and described herein in what is 
considered to be the most practical and preferred embodiment. It is 
recognized, however, that departures may be made therefrom within the 
scope of the invention and that obvious modifications will occur to a 
person skilled in the art.