U.S. patent number RE28,754 [Application Number 05/578,206] was granted by the patent office on 1976-03-30 for bed egress alarm circuit.
This patent grant is currently assigned to William Beaumont Hospital. Invention is credited to Kenneth J. Cook, Norman H. Horwitz.
United States Patent |
RE28,754 |
Cook , et al. |
March 30, 1976 |
Bed egress alarm circuit
Abstract
A condition responsive circuit for generating an output signal
upon the opening of a switch. An FET draws nominal current when the
switch is closed. When the switch is opened the FET conducts to
generate an output. The output may include an audio alarm, an
oscillator and a transmitter to generate an alarm at a distant
location, or a visual indicator.
Inventors: |
Cook; Kenneth J. (Oak Park,
MI), Horwitz; Norman H. (West Bloomfield, MI) |
Assignee: |
William Beaumont Hospital
(Royal Oak, MI)
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Family
ID: |
26990960 |
Appl.
No.: |
05/578,206 |
Filed: |
May 16, 1975 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
337955 |
Mar 5, 1973 |
03852736 |
Dec 3, 1974 |
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Current U.S.
Class: |
340/515; 340/666;
340/530; 340/602; 340/573.1; 600/595; 327/208; 200/85R;
327/516 |
Current CPC
Class: |
A61B
5/1115 (20130101) |
Current International
Class: |
A61G
12/00 (20060101); G08B 021/00 () |
Field of
Search: |
;340/279,421,278,240
;307/252R,279,304,296 ;128/2S ;200/85R |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1,145,514 |
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Mar 1969 |
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UK |
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1,563,013 |
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Apr 1970 |
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DT |
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Primary Examiner: Caldwell; John W.
Assistant Examiner: Wannisky; William M.
Attorney, Agent or Firm: Cullen, Settle, Sloman &
Cantor
Claims
What is claimed is:
1. A condition responsive circuit for signalling the departure of a
patient from a bed or the like by generating an output signal upon
the opening of a switch, comprising:
a switch being normally closed by the weight of a patient in a
bed;
an FET having source, drain and gate electrodes, said gate
electrode connected to said switch for normally drawing only a
nominal current and for continuously biasing said gate electrode to
render said FET non-conductive;
a normally off output means coupled to said at least one of
.Iadd.said .Iaddend.source and drain electrodes for generating an
output signal when said FET is rendered conductive;
the opening of said switch to remove the continuous bias from said
gate electrode;
delay means coupled to said switch and said gate electrode for
maintaining a bias at said gate electrode to prevent said FET from
being rendered conductive for a predetermined time interval after
the opening of said switch; and
said output means including an SCR, an SCR switch, and a signal
means, all connected in series;
said SCR being rendered conductive by said FET being rendered
conductive to thereby generate an output signal from said signal
means, and said SCR being thereafter rendered non-conductive only
by operating said SCR switch.
2. A condition responsive circuit for signalling the departure of a
patient from a bed or the like by generating an output signal upon
the opening of a switch, comprising:
a switch being normally closed by the weight of a patient in a
bed;
an FET having source, drain and gate electrodes, said gate
electrode connected to said switch for normally drawing only a
nominal current and for continuously biasing said gate electrode to
render said FET non-conductive;
a normally off output means coupled to at least one of said source
and drain electrodes for generating an output signal when said FET
is rendered conductive;
the opening of said switch to remove the continuous bias from said
gate electrode; and
delay means coupled to said switch and said gate electrode for
maintaining a bias at said gate electrode to prevent said FET from
being rendered conductive for a predetermined time interval after
the opening of said switch.
3. The circuit of claim .[.1.]. .Iadd.2 .Iaddend.and further
including:
second switch means connected in series with said normally closed
switch for testing the operation of said circuit by removing said
continuous bias from the FET gate.
4. The circuit of claim .[.1.]. .Iadd.2 .Iaddend.wherein said
output means includes a normally non-conductive electronic switch
rendered conductive when said FET is rendered conductive.
5. The circuit of claim 4 wherein said output means further
includes an oscillator coupled in series to said electronic switch,
said oscillator being energized when said FET and said electronic
switch are rendered conductive.
6. The circuit of claim 4 wherein said output means further
includes a normally off transmitter which is turned on by said FET
and said electronic switch being rendered conductive, the frequency
of the transmitter being modulated by said oscillator.
7. The circuit of claim 4 wherein said electronic switch is an
SCR.
8. The circuit of claim 7 and including third switch means in
series with said SCR for rendering said SCR non-conductive after
said SCR is rendered conductive by said FET being rendered
conductive.
9. The circuit of claim .[.1.]. .Iadd.2 .Iaddend.wherein said
switch is pressure sensitive.
10. The circuit of claim .[.1.]. .Iadd.2 .Iaddend.wherein said
output means includes an audio alarm.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to condition responsive circuits
and, more particularly, to a condition responsive circuit which may
be utilized in environments such as hospitals and the like for
signalling the departure of a patient from a hospital bed.
One of the common hospital accidents occurs when a patient climbs
off a hospital bed and injures himself such as by falling or
stumbling. This will frequently occur when a patient who is too
weak to walk unaided deliberately attempts to climb off the bed. It
may also occur when the patient is not totally aware of his
physical acts for one reason or another.
The departure of a patient from a hospital bed results in the
removal of pressure (weight) from the bed. Therefore, the present
invention is responsive to a change in conditions, namely, changes
in pressure.
While various systems have been developed for detecting the pesence
or absence of pressure, it must be appreciated that in a hospital
environment it is undesirable to have a hospital bed connected to a
source of electric power. Not only is there a psychological problem
when the bed itself is connected to a source of power, but there is
always a danger of shock if liquids are spilled on the bed.
Therefore, it any current is to be supplied to the bed it must be
nominal. However, if a self-contained battery system is utilized,
the useful life of this system is quite short if the battery is
always in an on condition.
It is therefore a principal object of the present invention to
provide a condition responsive circuit which normally draws only a
nominal current.
It is a further object of the present invention to provide a
hospital bed alarm system which signals the departure of a patient
from a bed.
It is yet another object of the present invention to provide a
self-contained signal circuit responsive to a change in a physical
condition, such as the presence or absence of pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing objects of the present invention together with other
objects and advantages which may be attained by its use, will
become more apparent upon reading the following detailed
description, taken in conjunction with the drawings.
In the drawings, where like numeral identify corresponding
parts:
FIG. 1 is a schematic circuit diagram of one embodiment of a
condition responsive circuit;
FIG. 2 is a partial schematic illustration of a second embodiment
of the present invention;
FIG. 3 is a schematic illustration of an alarm signalling circuit
which may be utilized with the condition responsive circuit of the
present invention; and
FIG. 4 is a schematic diagram of a circuit for remote transmission
of an alarm signal.
DETAILED DESCRIPTION OF THE INVENTION
Referring first to FIG. 1, there is shown a field effect transistor
or FET 10 having source, drain and gate electrodes. A normally
closed switch 12 having a plunger 14 is connected to the gate
electrode of the FET 10. The junction of the switch 12 and the gate
electrode are coupled through a resistance 16 to ground and the
source electrode is coupled through a resistance 18 to ground.
A transistor 20 has its base electrode connected through a resistor
22 to the source electrode of the FET 10. The emitter of the
transistor 20 is coupled through a resistor 24 to ground. Signal
means 25 is coupled between the collector of transistor 20 and the
drain electrode of the FET 10.
A source of voltage, such as -9 volts available from a small dry
cell, is connected to one side of a normally closed switch 28. The
other side of switch 28 is coupled to one pole of a double pole
single throw switch 30 which serves as an on/off switch. The other
side of the on/off switch is connected to switch 12.
The other pole of switch 30 connects a source of +9 volts, also
from a dry cell, to the drain electrode of FET 10. Two separate dry
cells are preferred for reasons to be explained.
OPERATION
In the use of the circuit of FIG. 1 in the environment of a
hospital bed, it is important that only a nominal current be
present to avoid a drain on the two batteries. Since it is desired
to determined when a patient departs from the bed, the switch 12
having a plunger 14 may be a switch such as that manufactured by
Packard Electric of Ohio, bearing Part. No. 82A5A-1216876 which is
a pressure sensitive switch. Thus, the pressure or weight on the
switch is equivalent functionally to pressing and releasing on the
plunger 14 shown schematically in FIG. 1.
With the on/off switch or double pole single throw switch 30
closed, there is a path from the -9 volt battery through the switch
12 to the gate of the FET 10 and through the resistor 16 to ground.
Switch 12 is the type which is normally closed, that is, the
plunger 14 is normally in. If the pressure sensitive switch is
utilized, this represents the presence of a patient in a hospital
bed. Under this condition, virtually no current flows through that
portion of the circuit including the switch 12. The high voltage
drop across resistor 16 which appears at the gate of the FET 10,
biases the source to drain junction of the FET in an off
condition.
Upon the departure of the patient from the bed, equivalent to the
plunger 14 of the switch 12 being released, the circuit
therethrough is opened removing the voltage source from the gate
electrode of the FET 10. At this time, current will flow from the
+9 volt source through one pole of the switch 30 and from the drain
to the source electrode of the FET 10. This current flow turns on
the transistor 20 permitting current to flow from the nine volt
source through the signal means 26 to generate an output
signal.
When the patient gets back into bed and his weight depresses the
plunger 14 of the pressure sensitive switch, the signalling portion
of the circuit is turned off.
An optional capacitor 32 coupled across resistor 16 provides a time
delay. Thus, once the patient departs from the bed, the voltage at
the gate of the FET 10 drops as the capacitor discharges. Once the
capacitor has discharged, then the FET conducts to permit the alarm
or signal to be given. The purpose of this optional capacitor 32,
shown connected with dotted leads, is to prevent an immediate
signalling if the patient momentarily moves clear of the switch
while rolling over in bed.
The test or push button switch 28 is a normally closed switch which
may be deliberately opened to test the operation of the system.
Thus, opening the switch 28 is equivalent to opening the switch
12.
It must be appreciated that the portion of the circuit from the +9
volt battery across the FET and oscillator to ground normally draws
no current. That is, no current flows to that portion of the
circuit because the FET is biased off.
The current which flows through the portion of the circuit from the
-9 volt battery and through the switch 12 is nominal because of the
high resistance of resistor 16. However, since there is some
nominal current, there is always the possibility that the -9 volt
battery will lose power. However, the particular circuit of this
invention has the additional feature that upon the loss of power of
the -9 volt battery, the effect is the same as the opening of the
switch 12. That is, the voltage at the gate of the FET 10 is
removed and a signal is sounded.
As an alternate embodiment for the present invention, reference is
made to FIG. 2 wherein there is illustrated a silicon controlled
rectifier or SCR 34 having its cathode grounded and its anode
connected to one side of a normally closed pushbutton switch 36.
The embodiment of FIG. 2 replaces the portion of FIG. 1 shown in a
dotted block 37 including the transistor 10 and the resistor 24.
Thus, the gate of the SCR is connected to the resistor 22 and the
side of the switch 36 not connected to the SCR is, of course,
connected to the signal means 26.
The purpose of this embodiment is based upon the property of an SCR
that once it fires or conducts it is necessary to actually remove
the current from the anode to the cathode rather than taking away
the signal from the gate. This provides the increased benefit of
requiring a deliberate turn-off of the alarm by resetting switch
36.
With reference to FIG. 3, the first embodiment of an alarm is
illustrated. This circuit may be utilized as the signal means 26 to
generate an alarm at the hospital bed.
The audio alarm of FIG. 3 which is coupled between the drain
electrode and the +9 volt source on one side and either the switch
36 of FIG. 2 or the transistor 20 of FIG. 1 on the other side
includes an .Iadd.UJT .Iaddend..[.FET.]. 38 which is normally
biased off and which has its .[.source.]. .Iadd.base.sub.2
.Iaddend. electrode connected to a voltage divider comprising two
resistors 40 and 42. The junction of resistors 40 and 42 is coupled
to the base of a transistor 44. The emitter of the transistor 44 is
coupled back to the main circuit of FIGS. 1 or 2 and the collector
of transistor 44 coupled to one side of a speaker 46. The other
side of the speaker 46 is coupled back to the .[.drain.].
.Iadd.base.sub.1 .Iaddend. electrode of the .Iadd.UJT
.Iaddend..[.FET.]. 38.
Also included in the alarm means of FIG. 3 is a resistor 48 .[.to
provide a bias to the gate of the FET 38, a capacitor 50 coupling
the gate of the FET back to the circuit of FIGS. 1 or 2 and a.].
.Iadd.and a capacitor 50 which form an RC timing network. The
emitter of UJT 38 is connected to the common point between resistor
48 and capacitor 50. A .Iaddend.second capacitor 52 across the bias
resistor 48 and the capacitor 50 .Iadd.serves as a by-pass
capacitor. .Iaddend.
In operation, when the patient leaves the bed thereby removing the
bias from the FET 10, the conduction of the transistor 20 (or the
SCR 34) as previously explained permits current to flow through the
alarm or signal means of FIG. 3. This current flow .[.cuts off the
bias to the FET 38 normally provided by the voltage drop across the
resistor 48, and.]. .Iadd.biases on the UJT 38. The voltage across
the resistor 48 and capacitor 50 is time varying and when the UJT
conducts, a current pulse is fed to .Iaddend.the voltage divider of
resistors 40 and 42 .Iadd.which .Iaddend.turns on transistor 44 to
conduct the current through the speaker 46. The .Iadd.UJT 38,
.Iaddend.capacitor.[.s.]. .Iadd.50 .Iaddend.and resistor.[.s.].
.Iadd.48 .Iaddend.provide a .[.resonant.]. .Iadd.time varying
signal .Iaddend.circuit which provides the alarm to be driven
through the speaker 46.
Since the alarm going off at the patient's bedside might startle
the patients or otherwise create a disturbance, especially in the
evening, another embodiment of the present invention contemplates
the transmission of an alarm system to a remote area such as a
nursing station. FIG. 4 discloses one such circuit which may be
utilized as the signal means 26.
FIG. 4 includes a conventional unijunction oscillator to frequency
modulate a radio frequency transmitter. The conventional
unijunction oscillator includes an .[.FET.]. .Iadd.UJT .Iaddend.54
having its .[.source.]. .Iadd.base.sub.2 .Iaddend. electrode
coupled through a resistor 56 back to the circuit of FIG. 1 or 2
and being .[.normally biased.]. in an off condition .[.by a voltage
developed across a resistor 58 between the +9 volt source and the
gate electrode..]..Iadd.. .Iaddend.When the transistor 20 of FIG. 1
or the SCR 34 of FIG. 2 conducts, current flows .[.through the
resistor 58.]. and turns on the .[.FET.]. .Iadd.UJT .Iaddend.54.
This functions as a .[.Colpits oscillator.]. .Iadd.unijunction
oscillator .Iaddend.to FM modulate a .Iadd.Colpits oscillator
.Iaddend.transmitter which includes a voltage divider network of
resistors 60 and 62 having their junction coupled to the base of a
transistor 64. The emitter of the transistor 64 is coupled by a
parallel combination of a resistor 66 and a capacitor 68 back to
the circuit of FIG. 1 and having the collector coupled to a
parallel LC circuit comprising inductor 70 and variable capacitor
72. The transmitter of FIG. 4 operates in a conventional
fashion.
Thus, there has been described a circuit for generating an output
signal upon a change in condition, namely, a change in pressure.
The output signal may generate an alarm at the location of the
switch or by a radio frequency transmitter generating a signal at a
remote station. The circuit is battery operated and draws
essentially no current when the patient is in bed, thereby
providing greater useful life for the batteries.
Various modifications may be made to the present invention. For
example, the signal means may include a light or other signalling
devices as are well known in the art. Furthermore, the pressure
responsive switch 12 may be replaced by other condition responsive
switches such as those which respond to the presence or absence of
moisture. In that case, the circuit may be utilized to detect if a
patient spills liquids in his bed or may be further utilized to
detect the presence or absence of fluid flowing in a tube being
utilized for intravenous feedings.
In the preferred embodiment of FIG. 1, FET 10 is a type 2N5653 and
transistor 20 is a NPN-2N4400. Resistor 16 is 15M ohms, resistors
18 and 22 are 10K and 4.7K ohms, respectively, and capacitor 32
would be 0.22 uf for a two-second delay. In FIG. 2, the SCR 34 is a
2N5060.
The component values for the signal means of FIG. 3 are: .[.FET.].
.Iadd.UJT .Iaddend.38, 2N4871; transistor 44, 2N4400; resistors 40,
42 and 48, 470 ohms, 4700 ohms and 6.8K ohms, respectively, and
capacitors 50 and 51 0.1 uf and 100 uf.
In FIG. 4, .[.FET.]. .Iadd.UJT .Iaddend.57-MU4894, transistor 64,
2N3294, resistors 56, 58, 60, 62 and 66; 47K, 500K, 33K, 10K and
1K, respectively, capacitor 68; 10pf and variable capacitor 72,
1.5-7p would provide suitable operation.
Therefore, the foregoing is a description of the preferred
embodiment of circuitry only and should not be interpreted in a
restrictive sense but only as exemplifying the underlying concepts
of the present invention. The invention may be further developed
within the scope of the following claims:
* * * * *