 |
Description  |
|
|
CROSS REFERENCE TO COMMONLY ASSIGNED COPENDING APPLICATION
Reference is made to U.S. Pat. No. 4,791,935 by Baudino et al for an
"Oxygen Sensing Pacemaker" issued Dec. 20, 1988.
BACKGROUND OF THE INVENTION
The present invention relates to cardiac pacing generally and in particular
to a cardiac pacemaker which regulates pacing rate based upon sensed
percentage of oxygen saturation of the blood.
The relationship between oxygen saturation of the blood and pulse rate in a
healthy heart is well known. This relationship has given rise to numerous
proposals for pacemakers which regulate pacing rate in response to sensed
blood oxygen level These pacemakers attempt to restore the natural
relationship between blood oxygen level and pulse rate in order to provide
a pacemaker which paces the heart at a rate appropriate to the
physiological demands of the patient's body.
An early proposal for such a pacemaker is set forth in U.S. Pat. No.
4,202,339, issued to Wirtzfeld et al. This pacemaker takes the form of an
asynchronous pacemaker which does not sense the heart's underlying
electrical activity. U.S. Pat. No. 4,467,807, issued to Bornzin and
incorporated herein by reference in its entirety discloses a pacemaker in
which the oxygen sensor's function is integrated with a sense amplifier
for sensing intrinsic heart activity. This approach is believed superior
to that of Wirtzfeld in that it avoids competing with underlying heart
activity and allows the heart to beat at its own, underlying rhythm if
that underlying rhythm is appropriate.
SUMMARY OF THE INVENTION
The present invention provides a refined version of an oxygen sensing
pacemaker in which the oxygen sensor is mounted on the pacing lead. The
oxygen sensor requires only two conductors for operation, yet includes a
two wavelength reflectance oximeter for increased accuracy. Experience in
the field of cardiac pacing has shown that one of the more vulnerable
portions of the pacing system is the pacing lead, which is exposed to
mechanical stresses. Therefore, it is felt that the simpler the structure
of the pacing lead is, the more reliable and durable it is likely to be.
In addition, reducing the number of conductors to two allows for easier
installation of the pacemaker and lead and allows the use of standard
bipolar pacemaker and lead connector configurations. For these reasons, an
oxygen sensor requiring only two conductors is believed desirable.
The sensor and circuitry are also configured to minimize the current drain
imposed by the sensing regime, while retaining the high degree of accuracy
and linearity provided by a two wavelength reflectance oximeter. The
sensor includes an oscillator which sequentially activates red and
infrared diodes. The activation times of the infrared and red diodes are
determined by the amounts of infrared and red light reflected by the
blood. The sensor draws substantially more current when the red diode is
activated than when the infrared diodes are activated. This allows the
pacemaker to measure the durations of the red and infrared diode
activation periods. This approach provides a signal to the pacemaker which
has high resolution, increased noise immunity, and is less affected by
long term fluid infiltration into the pacing lead. The intermittent
operation provides a desirably low current drain.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram illustrating the functional relationship of the
major elements of the pacemaker.
FIG. 2 is a timing diagram illustrating the basic timing of the pacemaker.
FIG. 3 is a graph illustrating the correlation between the pacing rate
provided by the pacemaker and the percentage of oxygen saturation.
FIG. 4 is a sectional drawing of the assembled oxygen sensor.
FIG. 5 is a plan view of the top of the hybrid circuit within the sensor.
FIG. 6 is a plan view of the bottom of the hybrid circuit within the
sensor.
FIG. 7 is a schematic of the hybrid circuit within the sensor.
FIG. 8 is a timing diagram illustrating the operation of the sensor and
associated circuitry.
FIG. 9 is a timing diagram illustrating the operation of circuitry which
decodes the signals provided by the sensor to provide a signal indicative
of oxygen saturation.
FIG. 10 is a diagram showing how the FIGS. 10A-10D are arranged. FIGS.
10A-10D show schematic of the pacemaker.
FIG. 10A is a schematic illustrating prior art linear and digital pacing
circuitry and its interconnection to the remainder of the pacemaker
circuitry.
FIG. 10B is a schematic of the pacemaker illustrating circuitry associated
with oxygen sensor control and pacemaker timing.
FIG. 10C is a schematic illustrating circuitry associated with powering of
the oxygen sensor, telemetry of sensed oxygen saturation values and timing
of sensor operation.
FIG. 10D is a schematic illustrating circuitry associated with decoding the
signals generated by the oxygen sensor.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the primary components of the pacemaker. Basic
operation of the pacemaker is described in conjunction with the timing
diagram in FIG. 2.
The pacemaker is based upon a VDD pacemaker of the type known to the art.
Such a pacemaker typically includes a ventricular pacing electrode, an
atrial amplifier and a ventricular amplifier, which sense electrical
activity in the atrium and ventricle of the heart, respectively. In the
present embodiment, the atrial amplifier is dispensed with. However, all
timing circuitry associated with the atrial amplifier is retained. In the
present embodiment, the VDD pacing circuitry 10 is taken from the
Enertrax.RTM. pacemaker, manufactured and sold by the assignee of this
application. However, other VDD or DDD pacemakers would provide a suitable
starting point, provided they have at least the inputs and outputs
discussed below. A more detailed description of such pacemakers can be
found in U.S. Pat. No. 3,648,707 issued to Greatbatch and U.S Pat. No.
4,059,116 issued to Adams, incorporated herein by reference in their
entirety.
The basic operation of a VDD pacemaker is well known to those skilled in
the art. Typically, in response to a sensed atrial contraction, the
circuitry initiates timing of an A-V interval. If no ventricular
contraction is sensed during the A-V interval, a ventricular pacing pulse
is generated. After a sensed ventricular contraction or a ventricular
pacing pulse, if no underlying atrial or ventricular activity occurs
within a predetermined V--V interval, a ventricular pacing pulse is
generated. In the present invention, the atrial amplifier is dispensed
with, and the sensor and associated circuitry provide a signal which the
pacing circuitry treats as if it were a sensed atrial contraction. By
varying the timing of this simulated atrial contraction, the underlying
pacing rate of the pacemaker is controlled.
The pacing circuitry is coupled to the heart by means of a pacing lead 12
which includes a tip electrode 14 and the sensor 16. Pacing occurs between
the tip electrode 14 and the can 18 of the pacemaker. Sensing of
ventricular activity also occurs between the tip electrode 14 and the can
18 of the pacemaker. The lead 12 has two conductors 20 and 22, coupled to
the pacing circuitry 10 and to the sensor control/decode circuitry 24,
respectively. Sensor control decode circuitry 24 provides a signal on line
26 indicative of the sensed oxygen saturation percentage.
The operation of the pacemaker can best be understood in conjunction with
the timing diagram in FIG. 2. The ECG trace illustrates the
electrocardiogram of a patient in which a pacemaker according to the
present invention has been implanted. The first QRS complex 50 is a
natural ventricular contraction sensed by the pacing circuitry 10. The
sensed contraction causes generation of a pulse 52 on RTRIG line 372.
Sensing of the ventricular contraction also initiates an atrial blanking
period which prevents the pacer from responding to electrical signals in
the atrium. During the atrial blanking period, a low logic signal 54 is
generated on ABLNK line 378. The atrial blanking signal extends until 120
ms following the sensed ventricular contraction.
The variable timer 28 (FIG. 1) includes an RC timing circuit controlled by
ABLNK line 378 and by the signal indicative of oxygen saturation, provided
by the sensor control decode circuitry 24 on line 26. Variable timer 28
begins timing a variable delay interval on expiration of the atrial
blanking period at 56. Upon time out of the variable delay interval at 58,
timer 28 generates a simulated atrial sense signal 60 on ASENSE line 374.
The simulated atrial sense signal 60 causes the VDD circuitry 10 (FIG. 1)
to initiate the timing of the A-V interval at 62. In addition, the
simulated atrial sense signal 60 causes the pacing circuitry 10 to
initiate the atrial blanking period, driving ABLNK line 378 low at 64. At
the expiration of the A-V interval at 66, a ventricular pacing pulse 67 is
generated, causing a positive signal 68 on RECHARGE line 358. At the
expiration of the atrial blanking interval at 70, timing of the variable
delay is reinitiated at 72. Thus, the interval between a sensed
contraction 50 and the next subsequent ventricular pacing pulse is
determined by the sum of the A-V interval of the plus the variable delay
interval determined by variable timer 28 plus 120 ms. The duration of the
variable delay interval is determined by the signal on line 26 from the
sensor control/decode circuitry 24.
The escape interval following a ventricular paced beat is determined in a
similar fashion Following the pacing pulse 67, the atrial blanking period
remains in effect for a period of 120 ms. Upon expiration of the atrial
blanking period at 70, variable timer 28 reinitiates timing of the
variable delay interval at 72. Time out of the variable delay interval at
74 triggers the generation of simulated atrial sense signal 76 on ASENSE
line 374. This in turn initiates timing of the A-V interval at 78, and
initiates the atrial blanking period at 80, driving ABLNK line 378 low. At
the expiration of the A-V interval at 82, a ventricular pacing pulse 84 is
generated followed by a corresponding positive signal 86 on RECHARGE line
358.
Sensed ventricular contraction 94 follows pacing pulse 84 by an interval
less than the sum of 120 ms plus the A-V delay plus the variable delay.
This terminates timing of the A-V interval at 96. The atrial blanking
period initiated at 98 by time out of the variable delay at 99 will
terminate 120 ms following sensed ventricular contraction 94 At that time,
timing of the variable delay is reinitiated at 102. A positive going
signal on RTRIG line 372 corresponding to sensed beat 94 is illustrated at
104.
Pacing circuitry 10 also includes a V--V timer. As discussed above, this
timer sets a minimum pacing rate. This timer is initiated concurrent with
either sensed ventricular contractions or ventricular pacing pulses. In
the examples of FIG. 2, the V--V timer is initiated at 88, 90 and 92.
Because the total of 120 ms plus the A-V delay plus the variable delay was
less than the underlying V--V interval, the V--V timer does not time out
in FIG. 2. However, in the event that the sum of 120 ms plus the variable
delay plus the A--V delay were greater than the V--V interval, pacing
would occur on the expiration of the V--V interval.
In summary, the pacemaker operates similarly to the pacemaker disclosed in
the Bornzin patent referred to above, in that the escape interval
following either a sensed ventricular contraction or a paced ventricular
contraction is determined by the sensed oxygen saturation percentage. Like
the pacemaker described in the Bornzin patent, sensing of natural
ventricular contractions prior to the expiration of the escape interval
reinitiates timing of the escape interval
The signals on RECHARGE line 358 and RTRIG line 372 are provided to the
sensor control/decode circuitry 24 where they are counted. After every
fourth count, the sensor 16 is activated, and a new value of oxygen
saturation is decoded by circuitry 24. This signal indicative of the
percentage of saturation is provided to variable timer 28 on line 26, and
the variable delay interval is correspondingly changed.
FIG. 3 illustrates the various relationships between oxygen saturation and
pulse rate available in the pacemaker. The pacemaker provides two sets of
curves relating oxygen saturation and pulse rate, which are referred to
hereafter as family A and family B. As discussed above, the pacing rate is
the sum of the A-V interval, the 120 ms atrial blanking period following
ventricular pace or sense, and the variable delay interval determined by
the variable timer 28. In the particular VDD pacing circuitry utilized,
that of the Enertrax.RTM. pacer, there are ten available A-V intervals
ranging from 25 ms to 250 ms in 25 ms increments. These control settings
determine the curves within family A and family B. For example, the curves
illustrated as A10 and B10 employ an A-V interval of 25 ms, while the
curves illustrated as A1 and B1 employ an A-V interval of 250 ms.
The sensor/control decode logic 24, FIG. 1, provides an output signal on
line 26 which is proportional to the sensed oxygen saturation. The
specific relationship between sensed oxygen saturation and the output
signal on line 26 is adjustable between two settings which define the A
and B families of curves illustrated in FIG. 3. At any particular sensed
oxygen saturation percentage, the variable delay interval in the B family
is about one-half of the corresponding variable delay in the A family.
This provides increased flexibility to optimize the pacemaker's operation
for the particular patient in which it is implanted. Within each family of
curves, any of the 10 A-V intervals may be selected to produce a total of
20 possible response curves The specifics of the operation of sensor
control/decode circuitry 24 and variable timer circuitry 28 are discussed
in more detail below in conjunction with the discussion of FIG. 10.
All curves illustrated employ an underlying, V--V interval of 1500 ms,
corresponding to 40 beats per minute. This interval is also programmable
and determines the lowest available pacing rate. As discussed above,
unless the sum of the A-V delay, the variable delay and the 120 ms post
ventricular atrial blanking interval is less than the V--V interval,
pacing will occur on expiration of the V--V interval.
FIG. 4 is a sectional drawing of the assembled sensor ready for
incorporation in a cardiac pacing lead. The sensor includes a machined
sensor body 200 which may be fabricated of titanium. The sensor body 200
serves both as a structural element and as a conductive element. The
distal end 202 of sensor body 200 is provided with a bore 204, in which a
coiled conductor coupled to the tip electrode 14 (FIG. 1) of the pacing
lead 12 (FIG. 1) may be mounted. The proximal end 206 of sensor body 200
is preferably coupled to one of the two elongated conductors 20 and 22
(FIG. 1) in the pacing lead 12. The sensor body 200 thus serves as part of
the conductor coupling the pacemaker to the tip electrode. A wire 208
enters the proximal end of the sensor body 200 via bore 210. Bore 210 is
sealed by means of feed through 212, which may be fabricated of sapphire
and is provided with a metal sheath 213 which is welded to sensor body
200.
Sensor body 200 serves to mount the hybrid circuit 214, which contains the
active circuit elements of the sensor and is provided with a central bore
201 to allow access to the bottom of hybrid circuit 214. The circuitry of
hybrid circuit 214 is coupled to the sensor body at its distal end by
means of strap 216 and is coupled to wire 208 at its proximal end by means
of wire 218. Surrounding sensor body 200 is a transparent sapphire tube
220 which allows the infrared and red light generated by the oximeter
circuitry on hybrid 214 to exit and to reenter the sensor. Sapphire tube
220 is coupled to sensor body 200 by means of welding collars 222 and 224.
Welding collars 222 and 224 are brazed to sapphire tube 220 and welded to
sensor body 200. This construction provides a long term, hermetically
sealed sensor capsule.
Hybrid circuit 214 includes three LED's, and one photo diode. The three
LED's are surrounded by a semicylindrical shield 232 which extends from
the surface of hybrid 214 to the inner surface of sapphire tube 220. The
photo diode is mounted to the surface of hybrid 214 distal to cylindrical
shield 232. This construction minimizes direct propagation of light from
the LED's to the photo diode, and thus ensures that almost all light
impinging on the photo diode has been reflected off of the patient's
blood.
When the sensor is incorporated into a pacing lead, it is expected that a
coaxial configuration will be used employing two coiled conductors. The
outer conductor will be welded or otherwise attached to the proximal end
206 of sensor body 200 and the inner conductor will be welded or otherwise
attached to the proximal end of wire 208. In order to encourage long term
operation of the device, the exterior of the sensor capsule may be covered
by a transparent, polyurethane sheath, which may be continuous with the
outer insulation of the pacing lead and also insulates sensor body 200.
This structure is described in more detail in commonly assigned,
co-pending application by Baudino et al cited above, and incorporated
herein by reference in its entirety.
FIG. 5 shows a top, plan view of hybrid 214. In this view, it can be seen
that the hybrid contains two infrared LED's 226 and 228 and one red LED
230. Diodes 226, 228 and 230 are mounted proximal to shield 232. Photo
diode 234 is located distal to shield 232. In addition, the hybrid 214
contains an oscillator 236. Oscillator 236 alternately energizes the red
diodes 226 and 228 and the red diode 230. The reflected light impinging
upon photo diode 234 determines the relative time periods of energization
of the infrared diodes 226 and 228 and the red diode 230. During the time
when red diode 230 is energized, there is a current flow through the
sensor circuitry of approximately 6 ma. While the IR diodes 226 and 228
are energized, there is a current flow through the sensor circuitry of
only approximately 2 ma. Due to the intermittent operation of the sensor,
the average current drain is only about 10 microamps. The difference in
current drain when the red and IR diodes are activated allows the sensor
control/decode circuitry within the pacemaker to determine the relative
time periods during which the diodes are energized, and thus the ratio of
reflected red and infrared light. This mechanism will be discussed in more
detail in conjunction with FIG. 10, below. Areas 238, 240 and 242 are
metallization areas on the upper portion of the hybrid substrate. Plated
through hole 244 couples area 240 to an additional conductive area on the
back of the hybrid, as does a second plated through hole located under
oscillator 236, coupled to conductive area 242. Wire 218 (FIG. 4) is
coupled to area 238. Strap 216 (FIG. 4) is coupled to area 242.
FIG. 6 illustrates the bottom surface of hybrid 214. Conductive areas 246
and 248 are coupled to conductive areas 240 and 242 on the top of hybrid
214 by means of plated through holes 244 and 250, respectively. Resistor
252 is coupled to conductive areas 246 and 248. Resistor 252 is located
over bore 201 of sensor body 200 (FIG. 4) when assembled. This allows
laser trimming of resistor 252 through the sapphire tube 220. Preferably,
resistor 252 is trimmed so that oscillator 236 provides a 50 percent duty
cycle for activating the red diode 230 and the infrared diodes 226 and 228
in the presence of blood having an 80 percent oxygen saturation.
FIG. 7 is a schematic diagram of the sensor circuitry. In this view, the
interrelation of the various circuit components is more clearly visible.
The connection of sensor body 200, wire 208 and tip electrode 14 is shown
in this drawing. All elements of the drawing correspond to identically
labeled elements in FIGS. 4 and 5. When a power signal is provided across
wire 208 and sensor body 200, the timing function of oscillator 236 is
initiated. Oscillator 236 upon initial turn on activates red diode 230.
The timing periods of the oscillator are determined by the capacitance of
photo diode 234, the fixed resistance of resistor 252, and the variable
resistance of photo diode 234. The amount of light reflected on photo
diode 234 varies its' effective resistance and thus determines the red
diode activation period. At the expiration of the red diode activation
period, oscillator 236 activates diodes 226 and 228. The infrared light
reflected upon photo diode 234 thereafter determines the length of the IR
diode activation period. At the expiration of the IR diode activation
period, the red diode is again activated. This oscillation continues until
the power signal is removed from wire 208 and sensor body 200. There is a
four-fold change in the ratio of infrared to red diode activation
intervals between resting and heavy exercise. The modulation of the power
signal to sensor 16 thus allows oxygen saturation percentage to be
determined with a high degree of resolution.
FIGS. 8 and 9 are timing diagrams which illustrate the operation of the
sensor and associated circuitry. These timing diagrams are discussed in
conjunction with the schematics of the circuitry in FIG. 10, and should be
referred to in conjunction with FIG. 10 in order to understand the
operation of the pacer more fully.
FIGS. 10A, B, C and D illustrate the circuitry associated with the sensor
and its interconnection to a prior art VDD pacemaker circuitry. In
particular, the prior art VDD pacemaker circuitry includes linear and
digital circuitry of the Enertrax.RTM. pacemaker, previously marketed by
Medtronic, Inc., with the exception of the atrial sense amplifier, which
is omitted. While this circuitry is shown as exemplary, the sensor and
associated circuitry are believed to be easily adapted to any modern
programmable VDD or DDD pacemaker circuitry which will provide the
required inputs, outputs and timing periods.
Linear circuitry 300 contains the ventricular sense amplifier, the
ventricular output amplifier, and circuitry associated with telemetry into
and out of the pacemaker. Linear circuitry 300 is coupled to the antenna
via ANT 1 line 308 and ANT 2 line 310. Both the input amplifier and the
output amplifier of linear circuitry 300 are coupled to the tip electrode
14 and to the pacemaker can 18 via lines 312 and 326. Zener diodes 320
provide protection for the sense amplifier in the event of electrocautery
or defibrillation. SENSOR line 324 is coupled to TIP line 326 by means of
back to back zener diodes 322, also protecting the pacing circuitry in the
event of applied defibrillation or electrocautery. TIP line 326 and SENSOR
line 324 in use are coupled to conductors 20 and 22 (FIG. 1) of the pacing
lead 12 (FIG. 1). Timing for the pacemaker is based upon a 32 KHz crystal
oscillator 304 which is coupled to both the linear and to the digital
circuitry.
Digital circuitry 302 takes care of the timing functions of the pacemaker,
including timing of the A-V interval, the V--V interval an the atrial
blanking period discussed in conjunction with FIGS. 1 and 2, above.
Digital circuit 302 also times out a ventricular refractory period, as is
conventional in VDD pacers. In the Enertrax.RTM. pacer circuitry, this is
a programmable parameter. Digital circuit 302 also times out a ventricular
blanking period, as is conventional. Expiration of the ventricular
blanking period is indicated by a signal on VBLNK line 352. On expiration
of the V--V or A-V intervals, digital circuitry 302 generates a signal on
VPACE line 360 and a subsequent recharge signal on RECHARGE line 356. In
response to a sensed ventricular contraction, linear circuitry 300
generates a signal on VSENSE line 354 which in turn triggers generation of
a signal on RTRIG line 372. PIM line 366 carries data received by the
linear circuitry 300 to digital circuitry 302 to select programmable time
intervals, including the A-V interval and the V--V interval and to select
other programmable options such as atrial amplifier sensitivity.
ASENSE line 374 is the input to the digital circuit 302 which would
normally receive the output of the atrial sense amplifier. ASENSEL line
376 is an output from digital circuitry 302, and indicates the atrial
sensitivity level selected via programming. As there is no atrial
amplifier, this line is used to select between the A and B families of
curves and to disable the sensing function. ABLNK line 378 is an output
from digital circuitry 302 and is low during the atrial blanking interval.
In addition, digital circuitry 302 provides outputs controlling the
operation of the telemetry functions of the linear circuitry 300. These
include a telemetry enable signal, generated on TELEN line 370 and a
telemetry data signal generated on TELDATA line 368. The TELEN line 370 is
high when telemetry function is enabled, which occurs only in the presence
of a low signal on REEDSW line 353. TELDATA line 368 is high when the
device is telemetering out digital information from the memory of digital
circuitry 302. When TELEN line 370 is high and TELDATA line 368 is low,
the device is adapted for analog telemetry. Normally, the pacemaker would
telemeter out the electrocardiogram sensed by the tip electrode 14.
Telemetry of this information takes the form of pulse interval modulation,
based upon the current applied to VCO capacitor 332. In the present
invention, the current applied to VCO capacitor 332 is used to telemeter
out the sensed oxygen saturation.
The programming and timing functions of prior art VDD pacemakers are
discussed in more detail in U.S. Pat. No. 4,344,437 issued to Markowitz,
incorporated herein by reference in its entirety. It is believed that one
skilled in the art would be familiar with these basic functions, which
would be present in most modern VDD and DDD pacemakers, and that the basic
sensor related circuit architectures set forth herein would be easily
adaptable to such pacemakers.
The positive terminal of the battery is coupled to B+ line 318, which is
coupled to capacitors 336, 338 and 340 along with resistor 334 to provide
the VCC signal on VCC line 346. This is used as the basic power signal for
the sensor control and timing circuitry. The negative terminal of the
battery is coupled to B- line 328 and to ground. The pacemaker is
preferably powered by an LiMnO.sub.2 cell, which generates a three volt
output and has low internal impedance. The low impedance of the cell
allows it to abruptly increase its current output when required by sensor
activation. Basic timing functions for the sensor circuitry are taken from
the 32 KHz crystal oscillator via XTAL line 348 and from the pacemaker
slow clock via SLOWCLK line 350. SLOWCLK line 350 merely provides a clock
signal at 8 ms intervals. FIG. 10A also illustrates interconnect lines
including the POWERON H line 342, POWERON L line 344, and RATIOCTR H line
382.
FIG. 10B includes the variable timer 28, illustrated in FIG. 1, along with
a portion of the sensor control and decoding circuitry 24. For purposes of
convenience, the sensor control and decode circuitry on FIG. 10B can be
divided into square wave to DC circuit 492 and sensor control circuitry
493.
Sensor control circuitry 493 uses the outputs of digital circuitry 302 to
control sensor timing and operation. Pacing and sensing events are counted
by counter 476, which is incremented by OR gate 466 which is responsive to
either a signal on the RECHARGE line 358 or on RTRIG line 372. As such,
with each sensed ventricular contraction or ventricular pacing pulse,
counter 476 is incremented by 1. When the counter reaches a count of four,
4TH EVENT line 394 goes high. This initiates sensor operation. In
addition, when the fourth event is counted, a negative set signal is
applied to flip-flop 462 via inverter 464. This causes a high signal on
the Q output of flip-flop 462. This in turn serves as a clear signal for
counter 476 through resistor 488. The clear signal sets the output of Q3
of counter 476 low, driving the signal on 4TH EVENT line 394 low and
removing the negative set signal from flip-flop 462. When flip-flop 462 is
set, POWERON H line 342 goes high. The Q output of flip-flop 462
corresondingly sets POWERON L line 344 low.
When POWERON H line 342 goes high, the negative reset is removed from
flip-flop 478. Flip-flop 478 keeps track of the operation of the sensor,
and enables the sensor to operate for one full infrared and one full red
time period. When the signal on RED L line 384 goes high, indicative of
the beginning of the first infrared diode activation interval, the Q
output of flip-flop 478 goes high generating a high signal on SENSORACT H
line 392. On the beginning of the next infrared activation period, the
signal on RED L line 384 will go high again, clocking the Q of flip-flop
478 through, setting SENSORACT H line 392 low. The Q output of flip-flop
478 controls the logic level of SENSORACT L line 391.
FIG. 10B also includes circuitry which disables the sensor in the event
that the programming of the device indicates that sensor operation is not
desired. In the event that sensor operation is not desired, ASENSEL line
376 is set high, which causes FET 434 in the square wave to DC circuit 492
to generate a low signal on the input of OR gate 470. OR gate 470 in turn
generates a negative reset signal to flip-flop 462, preventing the POWERON
H line 342 from going positive to provide power to the sensor.
Counter 474 functions to divide the 32 KHz signal on XTAL line 348 by 16.
This provides a 2 KHz square wave signal on 2 KHz line 386. When the
sensor is disabled, the negative signal from OR gate 478 is inverted by
inverter 472 to provide a positive clear signal to counter 474, preventing
its operation. Inverter 468 inverts the signal on XTAL line 348. This
inverted signal appears on 32 KHz line 388, where it is subsequently used
by the sensor decoding circuitry illustrated in FIG. 10D.
FIG. 10C includes several circuit blocks. Circuit block 490 is the sensor
bias circuitry which powers the sensor 16 (FIG. 1). In response to a low
signal on POWERON L line 344, transistor 488 and associated resistors 484
and 486 couple B+line 318 to TIP line 326, which is coupled to sensor body
200 (FIG. 4). The high logic signal on POWERON H line 342 turns on FET 498
and provides a return path to ground (B-) for SENSOR line 324.
To understand the operation of this circuit, reference is made to the
timing diagram of FIG. 8. RECHARGE line 358 goes high at 626 concurrent
with the negative edge of the slow clock at 620. It remains high until the
expiration of the next subsequent slow clock pulse 622. The negative going
edge 628 of the recharge pulse increments counter 47S (FIG. 10B). Assuming
it is the fourth count, counter 476 drives 4TH EVENT line 394 high at 630.
As discussed above, this sets the Q output of flip-flop 462 (FIG. 10B)
high at 632. This turns on transistors 488 and 498 and provides power to
the sensor circuitry. When transistors 488 and 498 are turned on,
oscillator 236 (FIG. 7) is activated, causing current to flow through red
diode 230 (FIG. 7). However, start up transients make the current level
660 on SENSOR line 324 initially unstable. This first red diode activation
period is therefore not appropriate for use, and is ignored.
When the initial signal on SENSOR line 324 reaches approximately 4 ma due
to red LED 230 activation, the circuitry in circuit block 494 is
activated. Transistors 510 and 520, and associated resistors 508, 516, 518
and 522, along with diodes 512 and 514 act as a toggle flip-flop with a
predetermined power-up state in which transistor 510 is off and transistor
520 is on. Diodes 512 and 514 are collector clamp diodes for transistor
510, and limit the collector voltage swing to 1.2 volts. This enhances the
switching time of transistor 510 and reduces the current drain. When the
current | | |
