U.S. patent number 3,713,126 [Application Number 05/116,331] was granted by the patent office on 1973-01-23 for burglar deterrent timing switch.
This patent grant is currently assigned to Novar Electronics Corporation. Invention is credited to Joseph C. Stettner.
United States Patent |
3,713,126 |
Stettner |
January 23, 1973 |
BURGLAR DETERRENT TIMING SWITCH
Abstract
A burglar deterrent having a timing switch connected to the
lights of a room or building for actuating these lights for a
selected period of time in response to a sound made by an intruder.
The timing switch has an SCR connected to the switch terminals. The
gate of the SCR is connected to a timing circuit which comprises a
series connected capacitance and diode connected from the gate to
the anode of the SCR. A second SCR has its main terminals connected
across the capacitance for, at times, discharging the capacitance
in response to a trigger signal at the gate of the second SCR. A
zener diode is connected for limiting the charging voltage applied
to the capacitance. A variable resistance is connected from the
gate of the first SCR to its cathode to permit selection and
control of the delay time.
Inventors: |
Stettner; Joseph C. (Akron,
OH) |
Assignee: |
Novar Electronics Corporation
(Barberton, OH)
|
Family
ID: |
22366547 |
Appl.
No.: |
05/116,331 |
Filed: |
February 18, 1971 |
Current U.S.
Class: |
340/527; 340/566;
327/460; 327/397; 340/815.74; 340/384.71 |
Current CPC
Class: |
G08B
13/1672 (20130101); G08B 15/002 (20130101) |
Current International
Class: |
G08B
15/00 (20060101); G08B 13/16 (20060101); G08b
013/00 () |
Field of
Search: |
;340/258R,258D,276,261,377,322 ;307/293 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Caldwell; John W.
Assistant Examiner: Slobasky; Michael
Claims
I claim
1. An intruder deterrent for actuating an illumination means for a
selected period of time in response to a sound, said deterrent
comprising:
a. a sound transducer and amplifier means for picking up said sound
and converting it to an audio trigger signal; and
b. a timing switch means connected in series to said illumination
means and to a source of AC power therefor, said timing switch
means having a trigger input terminal connected to the output of
said transducer and amplifier means, for actuating said
illumination means for a selected period of time in response to an
occurrence of said audio trigger signal and for then automatically
deactuating said illumination means said timing switch means
further comprising:
1. an electronic switch having its main terminals connected at the
switch means terminals and having a control input terminal;
2. a timing circuit comprising a series capacitance and rectifier
connected between said control terminal and a source of AC control
input power at one of the main terminals of said switch means, the
rectifier being polarized to permit control input current flow for
charging said capacitor by time spaced current pulses through said
control input; and
3. capacitance discharging means including an electronic switching
device connected across said capacitance for at times discharging
the capacitance in response to said triggering signal.
2. An intruder deterrent according to claim 1 wherein
a manual switch is connected parallel to said timing switch for
manual illumination of said illumination means and for
automatically making said timing means operable when said manual
switch is opened.
3. An intruder deterrent for actuating an illumination means for a
selected period of time in response to a sound, said deterrent
comprising:
a. a sound transducer and amplifier means for picking up said sound
and converting it to an audio trigger signal;
b. a timing switch means connected to said illumination means and
to a source of power therefor, said timing switch means having a
trigger input terminal connected to the output of said transducer
and amplifier means, for actuating said illumination means for a
selected period of time in response to an occurrence of said audio
trigger signal and for then automatically deactuating said
illumination means, said timing switch means comprising:
1. an electronic switch having its main terminal connected at the
switch terminals and having a control input terminal wherein said
electronic switch is a first thyristor with a gate as said control
input terminal;
2. a timing circuit comprising a series capacitance and rectifier
connected at one end to said control terminal and conductively
connected at the opposite end to a source of control input power,
the rectifier being polarized to permit control input current flow;
and
3. capacitance discharging means connected across said capacitance
for at times discharging the capacitance in response to said
triggering signal wherein said capacitance discharging means is a
second thyristor having its main terminals connected across said
capacitance and having its gate connected to receive said
triggering signal.
4. An intruder deterrent according to claim 3 wherein
said first thyristor and said second thyristor are silicon
controlled rectifiers and said timing circuit is connected to the
anode terminal of said first thyristor.
5. An intruder deterrent according to claim 4 wherein
a variable resistance is connected between the gate terminal and
the cathode of said first thyristor for control of the time delay
of said timing circuit.
6. An intruder deterrent according to claim 4 wherein
a zener diode is connected between a node of said timing circuit
and the cathode terminal of said first thyristor for limiting the
voltage applied to the capacitance.
7. A timing switch according to claim 6 wherein
a variable resistance is connected between the gate terminal and
the cathode terminal of said first thyristor for control of the
time delay of the time delay of said timing circuit.
8. A timing switch according to claim 7 wherein
the zener voltage of said zener diode is relatively small compared
to the maximum instantaneous voltage at the main terminals of said
first thyristor.
9. An intruder deterrent according to claim 3 wherein
a. a triac has its principal terminals serially connected to said
load and to said source of power and has a control gate
terminal;
b. a bridge rectifier circuit is connected at its bi-directional
nodes to said gate terminal and a principal terminal of said
triac;
c. said timing switch is connected to the uni-directional nodes of
said bridge rectifier circuit; and
d. a power supply filtering circuit has its input connected to the
uni-directional nodes of said bridge rectifier and its output
connected to a power supply terminal of said transducer and
amplifier means;
whereby said timing switch controls the firing of said triac and
said amplifier is disabled when said timing switch is on.
10. An intruder deterrent according to claim 3 wherein
a manual switch is connected parallel to said timing switch for
manual illumination of said illumination means and for
automatically making said timing means operable when said manual
switch is opened.
Description
BACKGROUND OF THE INVENTION
This invention relates to a timing switch, and more particularly
relates to a timing switch for a burglar deterrent wherein the
timing switch can actuate lighting in response to an input trigger
signal and hold the lighting on for a long period of time.
As explained more completely in my copending application Ser. No.
145,134, I have found that burglaries may be deterred if the sound
that an intruder makes is used to illuminate the lights in the room
in which he is located. A circuit is needed to receive the sound
and to cause the sound to turn on the lights, to hold the lights on
for a selected period of time and then to extinguish the lights.
This is desirable so that if the lighting is falsely actuated the
circuit will reset itself. Furthermore, the intruder, upon seeing
the lights illuminated, will initially fear that he has been
detected. After the lights go off, he will known that his sound is
causing the lights to go on and off. He will operate in fear that
his next move will cause the lights to illuminate, and consequently
he will most likely leave the premises before he has completed his
crime.
For this purpose, a timing circuit is needed which is capable of
turning the lights on in response to the sound and then holding the
lights on for an unusually long period of time. Many types of
timing circuits are well known in the art. Most depend on
resistance-capacitance charging or discharging. However, the
problem with conventional resistance-capacitance discharging
circuits is that if extremely long time delays are needed, for
example on the order of five minutes, a very large value of
capacitance and a very large value of resistance are required.
Unfortunately, however, such conventional timing circuits require
expensive, high stability, capacitors. In addition, output
circuitry, for reading the capacitor voltage causes a significant
change in the circuit's time constant. The slow charge or decay of
such a circuit makes the obtaining of a gate trigger signal
difficult. Inexact triggering may result.
What is needed, therefore, is a timing circuit using conventionally
sized resistances and capacitances, and yet which is capable of
providing a time delay on the order of five minutes while at the
same time providing an exact triggering signal for a thyristor.
Many unijunction transistor timing circuits have been designed
which provide excellent timing circuits for some applications.
However, such UJT timing circuits require a continuously applied
power supply voltage in order to function properly. A continuously
applied power supply is impractical with a burglar deterrent timing
switch. The reason is that, in order to provide a timing switch
which is very quickly and easily installed in conventional lighting
circuitry without any significant disassembly or rewiring of the
lighting circuitry, the timing switch should be connected in series
with the lighting load and the AC power generator. With such a
series connection, the timing switch may be simply installed in the
ordinary switch box used for manually switching the lights. The
timing switch together with a parallel connected manual switch if
desired, is simply connected to the pair of wires conventionally
connected to the manual switch. At this pair of wires a continuous
power supply is not available because these wires are shunted by
the switch each time the switch turns on.
Furthermore, a trigger pulse is needed for each half cycle of
operation. A UJT timing circuit would be unable to provide any
timing pulses after the switch is turned on.
What is needed therefore is a timing circuit which can provide a
long time delay and provide pulses for each half cycle during the
timing switch on time and still be capable of being connected in
simple series connection with the lights and the AC generator.
SUMMARY OF THE INVENTION
The invention is a timing switch for connection to a load and to a
source of power for permitting a load energizing current flow
through the load and for continuing such current flow for a
selected period of time. The timing switch comprises an electronic
switch means having its main terminals connected at the timing
switch terminals and having a control input terminal. The timing
switch also has a timing circuit which comprises a series
capacitance and rectifier connected at one end to said control
terminal and conductively connected at the opposite end to a source
of control input power. The rectifier is polarized to permit
control input current flow. A capacitance discharging means is
connected across the capacitance for at times discharging the
capacitance.
For burglar deterrent use, a sound transducer and amplifier means,
for picking up the sounds of an intruder and converting it to an
audio trigger signal is connected at its output to the input of a
second electronic switch connected as a capacitance discharge
means.
It is therefore an object of the invention to provide an improved
timing circuit using relatively moderate or small circuit component
values while providing for long time delays.
Another object of the invention is to provide a timing circuit for
operating a thyristor which provides a trigger signal for each half
cycle while the switch is on and the load is energized.
Another object of the invention is to provide a timing capacitance
of low maximum voltage rating and therefore of small physical
size.
Another object of the invention is to provide a timing switch which
turns the lights off after a selected period of time without
gradual light dimming.
A further object of the invention is to provide an intruder
deterrent with a timing switch which turns lights on for a selected
period of time in response to the intruders sounds and then
extinguishes the lights.
Further objects and features of the invention will be apparent from
the following specification and claims when considered in
connection with the accompanying drawings illustrating embodiments
of the invention.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a simplified version of the
invention for use in explaining the operation of the invention.
FIG. 2 is a schematic diagram of the preferred embodiment of the
invention.
FIG. 3 is an oscillogram illustrating the charging of the timing
capacitance with the triggering angle enlarged out of scale for
purposes of illustration.
FIG. 4 is a schematic diagram of a complete intruder deterrent
circuit constructed according to the invention.
In describing the preferred embodiments of the invention
illustrated in the drawings, specific terminology will be resorted
to for the sake of clarity. However, it is not intended to be
limited to the specific terms so selected, and it is to be
understood that each specific term includes all technical
equivalents which operate in a similar manner to accomplish a
similar purpose. For example, the term "connected" is not limited
to "directly connected" but rather includes connection through
other components, such as a resistor, where the function or
operation is similar to direct connection and is functionally
equivalent.
DETAILED DESCRIPTION
FIG. 1 shows a simplified version of the timing switch which has
timing switch terminals 10 and 12. This timing switch is connected
to a load 14, such as an illumination means, and to a source of
power 16. A manual switch 18 is provided to manually energize the
load 14 when desired. A silicon controlled rectifier 20 functioning
as a first electronic switch has its main terminals connected at
the switch terminals 10 and 12 and has a control input terminal at
its gate terminal 22. A series connected capacitance 24, and
rectifier 28 are connected as a timing circuit. One end of the
timing circuit is connected to the gate terminal 22 of the SCR 20.
The other end is connected to a source of control input power at
the anode of the SCR 20. The rectifier 28 is directed in a polarity
to permit gate current flow through the gate of the SCR 20.
A second silicon controlled rectifier 30 is connected with its main
terminals across the capacitance 24 to operate as a capacitance
discharging means for at times discharging the capacitance 24. The
silicon controlled rectifier 30 is actuated for discharging the
capacitance 24 by a triggering signal occurring at its gate 32.
The simplified circuit of FIG. 1, can be operated manually by the
manual switch 18. Closing of the manual switch 18 would illuminate
the illumination means 14 in the conventional manner so long as the
switch 18 remains closed. Opening of the switch 18 automatically
initiates the operation of the timing switch. Operation of the
timing switch would begin with it in a de-energized condition prior
to the instant the manual switch 18 is opened. When the manual
switch 18 is opened, current may flow through the diode 28 and the
gate 22 of the SCR 20 whenever the AC voltage at the main terminals
of the SCR 20 is in the positive, forward biasing polarity. With no
voltage stored on the capacitance 24, the SCR 20 will fire within
the first few degrees of the forward half cycle. It will fire as
soon as the gate current reaches the gate firing current. This
means of course that the illumination means 14 will be illuminated
during each forward half cycle. During each reverse half cycle, the
diode 28 prevents discharge of the capacitance 24 and, of course,
the SCR 20 because of its characteristic can not conduct.
Assuming that no triggering signal is present at the gate 32 of the
SCR 30, the SCR 20 will continue firing on each positive half cycle
as the capacitance 24 continues to accumulate charge from the
periodic and short gate current pulses in the SCR 20. It should be
noted, and is of great importance, that after the SCR 20 fires,
which initially will be early in the half cycle, its main terminal
voltage will go nearly to zero and consequently charging voltage
will no longer be applied to the capacitance 24. Thus, the
capacitance 24 charges only until the SCR 20 fires. It is charging
for only a small portion, of the forward half cycle and for none of
the reverse half cycle. This, as will be seen, is what accomplishes
the long time delay.
Eventually, after enough gate current pulses, the capacitance 24
will become fully charged. Because of the short duration and small
magnitude of the gate current pulses, this will take a long time.
Current then cannot flow through the capacitance 24 and therefore
the SCR 20 will no longer be able to fire. A long time delay has
been obtained because the capacitance charges during only a small
fraction of the positive half cycle. After the selected long period
of time the capacitance will finally be fully charged, the SCR 20
will no longer fire and consequently the illumination means 14 will
be turned off. The system will now await the occurrence of a
trigger signal at the gate 32 of the SCR 30.
When a trigger signal is applied to the gate 32 of the SCR 30, the
SCR 30 will fire and, regardless of what then occurs at the gate
32, will continue to conduct and discharge the capacitance 24.
Immediately upon the firing and therefore short circuiting of the
capacitance 24, gate current can again flow through the gate
terminal 22 of the SCR 20, and fire the SCR 20. Thus, the
occurrence of the trigger signal immediately initiates illumination
of the illumination means 14 and the voltage on the capacitance 24
is returned close to zero. When the capacitance has discharged, so
that it can no longer maintain a minimum holding current through
the SCR 130 the discharging SCR 30 will go to its off state,
assuming that no trigger signal is present at the gate 32. The
capacitance will then begin recharging while the illumination means
is illuminated. Again, after a long period of time the capacitance
24 will be recharged and the illumination means 14 will be turned
off.
It should be pointed out that although I prefer a capacitance
discharge means which is an electronic switch operated by a trigger
signal, for other applications the timing circuit can use merely a
bleeder resistor for discharging the capacitance. For example, the
basic circuit of FIG. 1 may be modified and used to provide a delay
turn off switch. A bleeder resistor can replace the SCR 30. Its
resistance must be large enough so that it would drain less charge
from the capacitance 24 during each cycle than is put on the
capacitance 24 by the gate current pulses, in order to permit
charge to accumulate and eventually turn the switch off.
With such a circuit, opening of the manual switch 18 will initiate
periodic charging of the capacitance and the light will continue on
for a period of time. After sufficient charge has accumulated on
the capacitance 24, the lights will turn off because, as before, no
gate current can flow through the SCR 20. So long as the manual
switch remained open, the capacitance 24 will be maintained fully
charged with the lights off. When the manual switch is closed, the
lights come on, no gate current pulses can be applied and the
bleeder discharges the capacitance 24. The bleeder resistance must
be small enough to discharge the capacitance within a suitable
time.
As another alternative, a similar shunt bleeder resistor could be
used in combination with the SCR 30. Its value of resistance must
be large enough so that it would remove less charge from the
capacitance each cycle than is added. The result would be an
increased time delay. The smaller the value of the bleeder resistor
in ohms, the longer the time delay obtained.
In FIG. 2, a preferred embodiment of the invention utilizes many of
the same components which are illustrated in FIG. 1 with the same
reference numerals used as in FIG. 1, but with the numeral 1, added
in the hundreds place. Thus, the timing circuit of FIG. 2 comprises
in series a current limiting resistance 126, the diode 128 and the
capacitance 124. A zener diode 140 has been connected between a
node of the timing circuit and the cathode of the SCR 120. The
zener diode 140 limits the voltage which is applied to the
capacitance during the operation of the circuit. The resistance 126
is large enough to limit the zener diode 140 current to a current
which is insufficient to significantly energize the load 114. It
should be small enough to not significantly affect the timing of
the circuit. For example, I have used a 22 K ohms 1 watt resistor
as the resistor 140.
In functional effect, the zener diode 140 is connected across the
capacitance 124 for limiting the voltage applied to the
capacitance. This can be stated because, during charging, the diode
128 and the gate-cathode junction of the SCR 120 will exhibit a
very low impedance. Obviously, at other times during circuit
operation, because of the effect of a reversed semi-conductor
junction, the zener diode 140 is electrically disconnected from the
capacitance 124. However, when we refer to the zener diode as
connected across the capacitance, we mean during the charging of
the capacitance when this is of consequence in limiting the
capacitance voltage. The relative positions of the series
components could be changed or interchanged and other equivalent
connections of the zener diode could be possible.
A variable resistance 142 is connected between the gate terminal
122 of the SCR 120 and the cathode terminal of the SCR 20. The
variable resistance 142 controls and varys the time delay of the
timing circuit so that a desired charging time may be selected. A
sound transducer and amplifier means 150, for picking up the sound
of an intruder and converting it to audio trigger signal, is
connected to the input gate terminal 132 of the SCR 130. It
comprises a microphone 152 connected to the input of an audio
amplifier 154.
For reasons which will become clear, it is desirable to select a
zener diode, having a zener voltage which is relatively small when
compared to the maximum instantaneous voltage at the main terminals
of the SCR 20. For example, I prefer to use a zener diode 140
having a zener voltage of approximately 12 volts. We can then use
an electrolytic capacitor as the capacitance 124, having a
capacitance of 20 microfarads and a maximum voltage rating slightly
above 12 volts, thus permitting use of a capacitor having a small
physical size.
Operation of the circuit illustrated in FIG. 2 similarly begins
with opening of the switch 118 and with the timing switch
components initially in a de-energized condition. FIG. 3
illustrates the voltage V.sub.s of the source of power 116, the
charging current i.sub.c of the capacitance 124 and the voltage
V.sub.scr across the SCR 20. Beginning at time t.sub.o and
continuing into the positive half cycle, the voltage on the SCR
will initially be the voltage of the power source 116. Initially,
right after the SCR voltage passes through zero, a positive voltage
will be applied to the gate 122. Within the first few degrees of
the positive forward half cycle, sufficient current will flow
through the discharged capacitance 124 to fire the SCR 120. When
the SCR fires, the potential across the SCR will go nearly to zero
with the result that charging voltage is no longer applied to the
capacitance 124. Thus, immediately after time t.sub.o a small spike
of charging current will put a relatively small charge on the
capacitance 124. For the major remainder of the forward half cycle
and for all of the reverse half cycle no charging current will flow
through the capacitance 124. The zener diode 140 will not yet have
conducted because the firing of the SCR 20 shorts it out before its
zener voltage is reached.
During the subsequent half cycles, as the capacitance 124
accumulates charge, the spikes of charging current become
progressively a few degrees later in the positive half cycle.
However, in all cases minimum gate firing current must be reached
and the SCR 120 must fire before the voltage applied to the
capacitance reaches the zener voltage of the diode 130 or the SCR
120 cannot fire at all. This is true because after the capacitance
charges to the zener voltage of, for example, 12 volts, the voltage
applied to the capacitance can go no higher because the zener diode
140 will conduct. If the minimum firing gate current is not
exceeded when 12 volts is applied to the series capacitance and
gate, it will not be exceeded in any part of the cycle. Whenever
the capacitance become charged to the zener voltage, sufficient
gate firing current cannot flow in the SCR 120. If a conventional
house supply voltage is used having 117 volts RMS and therefore a
peak instantaneous voltage of 165 volts, then triggering must
always occur in the first few degrees of the forward half cycle
before the supply voltage reaches approximately 12 volts or it will
not occur at all. Therefore, as with the circuit in FIG. 1, the
illumination means 114 is illuminated until the capacitance 124 is
fully charged. The difference however is that the capacitance will
be fully charged when it reaches the zener voltage of the zener
diode 140. Furthermore, because firing of the SCR 120 occurs in the
first few degrees of the forward half cycle, it always occurs where
the voltage has its maximum slope. This always must result in a
high-sloped gate current for accurate firing.
It should also be noted that during each reverse half cycle, the
zener diode will be forward biased. This prevents application of a
large reverse voltage on the diode 128 and therefore eliminates
reverse leakage current in the diode 128 which would otherwise tend
to discharge the capacitance 129.
Variation of the variable resistance 142 permits variation of the
total series resistance of the timing circuit. This permits control
of the length of time required to charge the capacitance 124 and
therefore of the length of time that the illumination means 114 is
energized. The smaller the resistance 142 is made, the larger the
current pulses through the capacitance 124, the shorter the
capacitor charge time and the shorter the time delay. With the
smaller gate to cathode resistance and greater current pulses, a
smaller proportion of the total current flows through the gate leg
of the current divider comprising the gate and the resistance 142.
Because the zener diode forces the circuit to either trigger in the
first few degrees of the positive half cycle or prohibits it from
triggering at all, the lights will not gradually dim at the end of
the timing cycle as they would if triggering occurred progressively
later and later in the forward half cycle until it did not occur at
all.
It should be noted that if the ratio of the value of the resistance
126, R.sub.126, to the selected gate to cathode resistance
R.sub.142 is extremely large, some dimming would occur. Therefore I
normally avoid such operation. Dimming would occur because nearly
all of the source 116 voltage V.sub.s would be dropped across the
resistance 126. In this condition, the minimum gate firing current
would be reached later in the half cycle. For example, with
R.sub.126 / R.sub.142 very large, 12 volts might be reached at the
zener diode 140 when the source voltage V.sub.s is at its
90.degree. peak of 165 volts. Thus, setting of the resistance 142
to a value near zero for nearly zero time delay might produce some
dimming.
After the capacitance 124 has charged and the lights are
extinguished, any sound at the microphone 152, will be amplified by
the amplifier 154 and will be applied to the gate of the SCR 130 to
cause discharge of the capacitance 124. Discharge will occur as
with the circuit of FIG. 1. The lights will be illuminated at the
beginning of discharge, as soon as the SCR 130 begins conducting.
When the capacitance is discharged, operation will then continue
and repeat that which is described above.
FIG. 4 illustrates a complete intruder deterrent system. This
system utilizes a triac 260 for direct control of the lights 214.
As before, components similar to those of FIGS. 1-3 have the same
reference numerals but with a 2 in the hundreds place.
Sound is received and amplified by a transducer and amplifier means
illustrated generally as 250. Its internal circuitry is illustrated
but not described because the internal circuitry forms no part of
the present invention. It receives sound at the microphone 264 and
provides a trigger signal at its output terminal 266. The timing
circuit, illustrated generally as 268. is like that illustrated in
FIG. 2 (although the ground connection is different to avoid firing
of the triac 260 with amplifier biasing current), the similarity in
reference numerals again indicating corresponding components.
However, in this circuit, the timing switch terminal 210 and 212,
instead of being directly connected to the load 214 and the
generator 216 as in FIG. 2, are instead connected to control the
gate of a triac 260. The timing switch is connected to the
uni-directional nodes 270 and 272 of a bridge rectifier. The
bi-directional nodes 274 and 276 of the bridge rectifier are
connected to the gate 278 and a principal terminal 280 of the triac
260. The principal terminals 280 and 282 of the triac are connected
directly to the load 214 and the generator 216.
The power supply for the amplifier is taken from the same
uni-directional nodes 270 and 272 where, whenever the timing switch
268 is not conducting, a full wave rectified sine wave is
available. This is then filtered by a power supply filter 286 which
has a zener diode 288 for regulation. The output of the power
supply filter 286 is applied to the power supply terminal 290 of
the transducer and amplifier means 250.
The operation of the circuit of FIG. 4 is similar to that already
described above. A sound at the microphone 264 provides a trigger
signal at the gate of the SCR 230. The SCR 230 then fires to turn
the timing switch 268 on and begin conduction through the SCR 220.
This immediately initiates conduction of the triac 260 and
illuminates the lights 214. The capacitance 224 begins its periodic
charging as before and when it is sufficiently charged the SCR will
turn off and then so will the triac 260.
An important result is that, as soon as the timing switch 268 turns
on and the SCR 220 begins conduction, the input of the filter 286
will be shunted by the timing switch itself. This will deprive the
transducer and amplifier means 250 of its source of power. It is
thereby diasabled from providing further trigger signals. This fact
insures that, after the selected time period has elapsed, the
lights will go off. Also, if an audible alarm is connected
paralleled to the light 214, the sound it produces will not
continuously create trigger signals which would hold the lights on
continuously. Once the circuit is actuated, it will turn itself
off. If it was actuated by mistake by a nonrecurrent sound, it will
stay off. If the sound which actuated it later recurs, it will
again come on.
The timing switch 268 is operated during both half cycles; that is
in full wave operation. This, of course, doubles the number of
current pulses each cycle which charge the capacitance 224. The
time delay will accordingly be shortened if the same component
values and adjustments are used.
It is to be understood that while detailed drawings and specific
examples given describe a preferred embodiment of the invention
they are for the purpose of illustration only, that the apparatus
of the invention is not limited to the precise details and
conditions disclosed and that various changes may be made therein
without departing from the spirit of the invention which is defined
by the following claims.
* * * * *