U.S. patent number 3,828,335 [Application Number 05/341,744] was granted by the patent office on 1974-08-06 for radio-wave detector for discovering the movement of persons or objects in a confined space.
Invention is credited to Gaston Raoul Salmet.
United States Patent |
3,828,335 |
Salmet |
August 6, 1974 |
RADIO-WAVE DETECTOR FOR DISCOVERING THE MOVEMENT OF PERSONS OR
OBJECTS IN A CONFINED SPACE
Abstract
Movements of persons or articles in a monitored space are
detected by a change in the effective capacitance between an
antenna and an associated counterpoise defining that space, the
antenna being energized by an oscillator whose tank circuit is
tuned to a predetermined radio frequency f"e. The oscillator works
into a tuned monitoring circuit resonant at a different frequency
f.sub.o, a capacitive feedback path extending from a tap on the
inductive branch of that circuit to an input of the oscillator for
applying thereto a control voltage which shifts its operating
frequency from f"e to a value fe closer to f.sub.o. This shift in
oscillator voltage is reduced by a lowering of the control voltage
through a further detuning of the monitoring circuit by a movement
to be detected, with resulting change of the operating frequency to
a value f'e between f"e and fe whereby the change in output voltage
due to such detuning is intensified. A load circuit connected to
the monitoring circuit includes a normally de-energized relay whose
energization in response to the aforementioned voltage change
produces a voltage drop across a supply resistor common to the
relay and the oscillator whereby this voltage change is further
stepped up.
Inventors: |
Salmet; Gaston Raoul
(Saint-Maur, Val de Marne, FR) |
Family
ID: |
9095554 |
Appl.
No.: |
05/341,744 |
Filed: |
March 15, 1973 |
Foreign Application Priority Data
|
|
|
|
|
Mar 21, 1972 [FR] |
|
|
72.9800 |
|
Current U.S.
Class: |
340/552; 340/692;
331/65; 361/181 |
Current CPC
Class: |
G01V
3/102 (20130101); G08B 13/26 (20130101) |
Current International
Class: |
G08B
13/26 (20060101); G01V 3/10 (20060101); G08B
13/22 (20060101); G08b 013/26 () |
Field of
Search: |
;340/258,221C ;317/146
;331/65 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Trafton; David L.
Attorney, Agent or Firm: Ross; Karl F. Dubno; Herbert
Claims
I claim:
1. A system for detecting movements in a monitored space,
comprising:
an antenna and a counterpoise therefor jointly defining the space
to be monitored;
an oscillator provided with a tank circuit tuned to a predetermined
radio frequency;
a tuned circuit with a resonance frequency differing from said
predetermined radio frequency coupled to said oscillator for
energization thereby, said tuned circuit being connected between
said antenna and said counterpoise whereby said resonance frequency
is codetermined by the effective capacitance between said antenna
and said counterpoise, said effective capacitance being variable by
a movement to be detected in a sense increasing the difference
between said resonance frequency and said predetermined
frequency;
a feedback path between said tuned circuit and said oscillator for
delivering to an input of said oscillator a control voltage
establishing an operating frequency intermediate said predetermined
radio frequency and said resonance frequency to be radiated by said
antenna, said control voltage varying with changes in said
effective capacitance for shifting said operating frequency toward
said predetermined radio frequency in response to a movement to be
detected; and
a load circuit connected to said tuned circuit, said load circuit
including responder means for indicating a voltage change in said
tuned circuit due to a movement to be detected.
2. A system as defined in claim 1 wherein said load circuit
includes a differentiation network of long time constant in series
with said responder means.
3. A system as defined in claim 2, further comprising diode means
in said differentiation network for bypassing voltage changes in
said tuned circuit of a polarity opposite that due to a movement to
be detected.
4. A system as defined in claim 3 wherein said differentiation
network has a resistive branch and a capacitive branch, said diode
means being connected across said resistive branch.
5. A system as defined in claim 2 wherein said responder means
includes a normally de-energized relay and trigger means for
energizing said relay in response to a significant voltage
reduction in said tuned circuit due to a movement to be detected,
said relay and said oscillator being provided with a common
direct-current supply, further comprising resistance means in
series with said common supply for generating a voltage drop upon
energization of said relay to intensify said significant voltage
reduction.
6. A system as defined in claim 5 wherein said differentiation
network includes a capacitor and a resistor, said load circuit
further comprising a rectifying connection and an integrating
network inserted between said tuned circuit and said capacitor for
charging the latter upon occurrence of said significant voltage
reduction, said resistor enabling delayed discharging of said
capacitor upon restoration of said effective capacitance to normal
whereby said relay remains energized beyond said restoration.
7. A system as defined in claim 6, further comprising a source of
temporary holding voltage for said trigger means and activating
means controlled by said relay for making said source operative
over a limited period.
8. A system as defined in claim 7 wherein said source comprises a
recording medium carrying a message to be announced and circuitry
for deriving said holding voltage from message signals in the
output of said recording means.
9. A system as defined in claim 1 wherein said feedback path is
connected to said tuned circuit at a point whose alternating
voltage decreases upon variation of said effective capacitance by a
movement to be detected, thereby diminishing said control voltage
in response to such movement.
10. A system as defined in claim 9 wherein said oscillator
comprises a transistor with a base lead connected to said tank
circuit for receiving a regenerative feedback voltage therefrom,
said feedback path including a reactance connected to said base
lead for superimposing said control voltage upon said regenerative
feedback voltage.
11. A system as defined in claim 7 wherein said relay is provided
with contact means for delivering to said trigger means a holding
voltage maintaining said relay energized.
Description
This invention relates to a radio-wave detector for discovering the
movement of peoples or articles in a confined space.
It has already been proposed that the movement of people or objects
in a confined space be detected with the aid of radio waves in
response to the alteration produced in the field reaching a
receiver from a transmitter when a person or article enters or
leaves the space in which the radio transmission between the
transmitter and the receiver takes place. A system of this kind is
of course fairly expensive, needing as it does two complete radio
frequency transducers, namely a transmitter and a receiver.
The object of my invention is to provide a detector of this nature
using just a single such transducer, i.e. a transmitter. In
accordance with my present invention, the space to be monitored is
defined by an antenna and an associated counterpoise connected
across a tuned circuit which is coupled to an oscillator whose tank
circuit is tuned to a predetermined radio frequency, designated f"e
hereinafter, differing from the resonance frequency f.sub.o of that
tuned circuit. Owing to the connection of the latter circuit to the
antenna and its counterpoise, resonance frequency f.sub.o is
codetermined by the effective antenna capacitance which in turn is
variable by a movement to be detected in the monitored space, the
sense of capacitance variation due to such movement being so chosen
as to increase the difference between the two frequencies f"e and
f.sub.o. A feedback path between the tuned monitoring circuit,
serving to energize the antenna, and the oscillator delivers to an
input of the latter, such as a transistor base, a control voltage
establishing an operating frequency fe intermediate and frequencies
f.sub.o and f"e, this control voltage varying with changes in the
effective antenna capacitance for shifting the operating frequency
fe toward frequency f"e, i.e., to a new value f'e more remote from
resonance frequency f.sub.o, in response to a movement to be
detected; the resulting voltage change in the tuned monitoring
circuit actuates a responder in a load circuit, connected to the
monitoring circuit, for indicating that change.
Advantageously, pursuant to a further feature of my invention, the
load circuit includes a differentiation network of long time
constant (e.g. 10 seconds); a diode in this network may serve to
bypass voltage changes in the monitoring circuit whose polarity is
opposite that caused by a movement to be detected.
In a system of this kind, when the effective capacitance between
the antenna and its counterpoise is constant or slowly varying, no
signal reaches the responder which preferably includes a relay
triggerable by an amplifier; when, however, that capacitance varies
at a critical rate, a signal is applied to that amplifier to
energize the relay.
Preferably, the amplifier comprises an integrator so that the
device does not respond to interference picked up via the antenna.
The integrator must have a time constant longer than the average
very brief duration of domestic, industrial and atmospheric
interference.
In cases in which the device is to detect the presence of living
beings, more particularly people, whose approach acts to increase
the circuit capacitance and also to damp the tuning circuit, the
device is so adjusted that the transmitter operating point is
positioned on the descending part of the resonance curve of the
tuning circuit --, i.e., at a frequency a little beyond the exact
resonance frequency. The reason for this is that since the
capacitance increase in the tuned circuit is the result of the
presence of a living being in the transmitter field, so that the
resonance frequency of this circuit decreases, and since a living
body has a small dielectric constant and therefore increases
damping, the difference in transmitted energy due to the
displacement of the resonance curve toward the origin and to the
flattening thereof is at a maximum. This appreciable reduction in
the transmitted field is detected by the differentiation circuit
which actuates the responder.
The deactivation of the responder can be delayed, preferably via
the differentiation network, by using negative detection and by
energizing the complete system via a common resistance such as the
internal resistance of the power supply or an auxiliary resistance
in series therewith.
When, as a result of detecting a movement, an integrating network
in series with the differentiation circuit registers a voltage drop
(negative detection) and triggers the responder, the resulting
power consumption causes an even greater voltage drop at the output
of the integrating network; of the detector, owing to this large
voltage drop and the long time constant of the differentiator
cascaded with the integrator, upon termination of the triggering
event (movement of a person) the differentiating capacitor will
take much longer than that time constant to return to its former
state of charge and thus stop the operation of the responder. Once
the system has ceased to operate, it can be restored to its normal
supervisory or monitoring state with removal of the charge
associated with normalization of the voltage.
The system according to the invention is of use for protecting the
monitored premises or other spaces from intruders and for
transmitting recorded messages for advertising, tourist and museum
purposes. In the latter case delayed operation of the responder can
be produced and maintained by detection, with a short time
constant, of message signals going to a loudspeaker, so that the
delay ceases when the speech or sound transmission ceases and the
device returns to its supervisory or stand-by state.
My invention will now be described in detail with reference to the
accompanying drawing in which:
FIG. 1 is a diagrammatic view of a system according to the
invention;
FIGS. 2a and 2b are diagrams explaining the operation of the system
of FIG. 1;
FIG. 3 is an equivalent circuit diagram of the transmission circuit
of the system;
FIG. 4 is a partial block diagram of the complete system;
FIG. 5 is a diagram of its power supply;
FIG. 6 is a diagram showing how deactivation can be delayed when
the system is used with a loudspeaker;
FIG. 7 is a simplified detail of the diagram shown in FIG. 4.
In FIG. 1 I have shown a resonant monitoring circuit comprising an
inductance L and a capacitor C. One side of the circuit is
connected to an antenna A and the other to a counterpoise therefor
(here ground). The actual capacitance of the tuned circuit is
therefore not C but C + Ca, Ca, denoting the
antenna-to-counterpoise capacitance. The resonant frequency is
therefore:
f.sub.o = 1/(2.pi..sqroot.L (C + Ca)) (1)
Let us assume now that a person P approaches the antenna; since the
human body is a conductor, it will increase the
antenna-to-counterpoise capacitance so that the resonant frequency
of the tuned circuit becomes:
f = 1/(2.pi..sqroot.L (C + Ca + Cx)) (2)
Cx denotes the extra capacitance due to the presence of person
P.
If the circuit L-C is energized by a constantamplitude but
variable-frequency alternating current I, the AC voltages at the
circuit terminals in dependence upon frequency are represented by a
curve K in FIGS. 2a and 2b. If the capacitance increases, the curve
which represents the frequency F is the dotted-line curve K.sub.1
of FIG. 2a. If, in this case, the transmission frequency f is set
at a value fe slightly greater than the resonant frequency f.sub.o,
the capacitance increase and therefore the frequency variation will
of course produce a variation .DELTA.V.sub.1 of the AC voltage
across the circuit. Owing to the shape of the resonance curve,
maximum sensitivity is obtained for a value of the frequency fe
such that the normal operating voltage V.sub.1 developed in circuit
L-C is approximately 80 percent of the resonant voltage
V.sub.o.
Also, for a given change in capacitance the voltage variation
.DELTA.V.sub.1 is proportional to the Q factor of the circuit and
inversely proportional to the tuning capacitance C. A very-high-Q
inductance and a low capacitance should therefore be used. To give
some idea about system sensitivity, if C = 500 Pf and Q = 200, a
0.02-Pf variation of C gives a relative variation .DELTA.V.sub.1
/V.sub.o of approximately 1 percent.
Since the human body has a high ohmic resistance, this choice of
position for the frequency fe is advantageous in the case of a
human being entering the field.
As the equivalent circuit diagram of FIG. 3 shows, the extra
capacitance Cx corresponding to the presence of person P and
applied across the circuit is in series with a considerable series
resistive component Rx, which helps to damp the circuit and
therefore to change the curve K.sub.1 into the dotted-line curve
K.sub.2 -- i.e., to reduce the voltage across the circuit. This
leads to an extra voltage reduction .DELTA.V.sub.2, so that the
total variation due to a person entering the field becomes:
.DELTA.V = .DELTA.V.sub.1 + .DELTA.V.sub.2 (3)
The damping effect is therefore cumulative with the effect of the
extra capacitance if the frequency fe is above the frequency
f.sub.o.
In some cases, however, the movement to be detected causes a
decrease in capacitance, as for instance in the case of a
surreptitious opening of an armored door, metal shutter, grid or
closure lattice; in this case, as shown in FIG. 2b, the oscillator
frequency fe is advantageously smaller than the resonant frequency
f.sub.o of the transmitting circuit L-C. When the capacitance
across the inductance L decreases, the resonance curve shifts to
the right (curve K.sub.3) and as in the previous case there occurs
a voltage reduction .DELTA.V.sub.1 ; in this instance, however,
there is no damping variation.
In both cases (FIGS. 2a and 2b) the value of .DELTA.V.sub.1 can be
increased to .DELTA.V.sub.M as will be shown hereinafter.
Conversely, to reduce the system sensitivity to prevent accidental
operation, the offset between the frequencies fe and f.sub.o can be
increased so that operation is shifted to the skirts of the curve
K. The choice of the offset or difference between the frequencies
fe and f.sub.o determines therefore the sensitivity of the
system.
These considerations are used in the design of a detector Det for
which a circuit diagram is shown in FIG. 4. A fixed-frequency
oscillator O works through an adjustable resistance R.sub.1 into an
amplifier A.sub.1 which has a high internal output impedance so as
not to damp the oscillatory output circuit consisting of an
autotransformer L, preferably of the ferrite-core kind, and a
capacitor C.sub.1. The lower terminal end of the oscillatory
circuit is connected to the counterpoise -- i.e., to ground in the
present case -- whereas its upper terminal is tied to the antenna
A. The resistance R.sub.1 serves to control the drive of the
amplifier A.sub.1 so that at resonance of the circuit L-C.sub.1 the
AC voltage between a point a in the amplifier output and ground is
near the maximum which the amplifier A.sub.1 can provide without
being saturated.
The amplifier A.sub.1 therefore behaves like a constant-current
generator. The tap a is so chosen that the total supply direct
current under these conditions does not exceed a given low value,
e.g. 1 mA, when the apparatus is on standby, so that the system can
run for a long time (several months) if battery-energized.
FIG. 7 is a very simplified circuit diagram of the oscillator O and
the amplifier A. The oscillator O is a Hartley circuit comprising a
transistor Tr.sub.1, a ferrite-core inductance L.sub.1 and a
capacitor C.sub.5, the elements L.sub.1 and C.sub.5 forming a tuned
circuit. Depending upon the number of stages in it, the amplifier A
either inverts or does not invert the phase of the signal which it
transmits; in this particular case the amplifier A, which comprises
a single transistor Tr.sub.2, inverts the phase. A feedback
coupling between the tap a and a point g (i.e., the base lead of
the transistor Tr.sub.1 of the oscillator O) is provided by a
capacitor C.sub.6 which therefore feeds back a control voltage that
is always in phase quadrature with the normal voltage on lead
g.
I shall first describe the case of FIG. 2a and of an amplifier
which inverts the phase of the output voltage relatively to the
input voltage. In the absence of feedback coupling the oscillator
frequency fe would be the same as the frequency of the tuned
circuit L.sub.1 -C.sub.5 constituting the tank circuit of
oscillator O.
If the circuit L-C.sub.1 is tuned exactly to the oscillator
frequency fe (f.sub.o = fe), the 90.degree.-out-of-phase control
voltage fed back via capacitor C.sub.6 to point g lags with
reference to the regenerative feedback voltage from tank circuit
L.sub.1 -C.sub.5 present at point g in the absence of such
capacitive feedback, and so the oscillator frequency is altered. It
can be shown that the operating frequency decreases in this case.
Conversely, if the oscillatory detector circuit L-C.sub.1, is
detuned, the lagging control voltage decreases and the frequency fe
increases, tending toward the natural frequency f"e of the tuned
circuit L.sub.1 -C.sub.5.
Consequently, the movement of curve K toward K.sub.2 when
capacitance Cx is connected in parallel with capacitances C and Ca
(FIG. 3) is enhanced by the effect of frequency fe shifting to a
higher value f'e (FIG. 2a), closer to the natural frequency f"e of
tank circuit L.sub.1 -C.sub.5, and so the detectable voltage
variation becomes .DELTA.V.sub.M instead of .DELTA.V.sub.1.
Thanks to the high Q of the circuit L-C.sub.1, which gives a very
sharply peaked curve K, the variation .DELTA.V.sub.1 and therefore
the sensitivity of the system can be increased by a factor of
approximately 3 to 5.
In the case of FIG. 2b (detuning of circuit L-C.sub.1 by decrease
of capacitance), the circuit arrangement shown in FIG. 7 again
increases the frequency fe, but in this case such increase
counteracts the effect of curve K shifting toward K.sub.3. To
obtain a similar effect -- i.e., a variation of oscillator
frequency fe in the sense opposite to the variation of the
detector-circuit frequency -- the feed-back circuit should be
connected to a point g' which is symmetrical with reference to the
point g relatively to the neutral point h of the oscillator tuned
circuit. Of course, the connections to points g and g' must be
reversed if the amplifier does not invert the phase of the output
voltage.
Because of the autotransformer effect of the inductance L of the
detector circuit, the voltage between the point a and ground is
normally several tens of volts. A proportion of this voltage is
taken off at a top b (FIG. 4) and fed via a rectifying connection,
constituted by a diode D.sub.1, to an integrating circuit
comprising a resistance R.sub.2 in parallel with a capacitor
C.sub.2. As will be apparent, the alternating voltage of radio
frequency fe (or f'e) developed in the tuned circuit L-C.sub.1
builds up a negative potential, of a magnitude proportional to the
radio-frequency voltage, on the ungrounded terminal of integrating
capacitor C.sub.2.
Autotransformer tap b is so chosen that the voltage thus detected
is large but its detection does not cause appreciable damping of
the circuit L-C.sub.1. As an example, the DC voltage across the
resistance R.sub.2 can be something like 50V, whereas the AC
voltage between the tap a and ground may be only about 5V r.m.s. Of
course, and as explained with reference to FIG. 2a, in most cases
the inductance L is adjusted to above the resonance frequency (or
if such adjustment cannot be provided, the capacitance C is so
adjusted) to give a voltage across the circuit L-C.sub.1 of about
80 percent of the voltage at resonance.
The frequency chosen is approximately 30 KHz, which is low enough
for the transmitted voltage and its harmonics not to interfere with
radio broadcasting, yet high enough to be able to use high-Q
inductances of reduced size.
The negative voltage detected at the ungrounded terminal c of
network R.sub.2 C.sub.2 is transmitted to a DC amplifier A.sub.2
via a differentiation network C.sub.3 -R.sub.3. The function of
this network, whose time constant is on the order of 10 seconds, is
to transmit at a point d only relatively rapid variations of the
voltage at point c signaling the approach of a person, and not to
transmit very slow variations, due for example to variations of the
ambient temperature or of the supply voltage (upon exhaustion of
the cells).
Since the voltage at c is negative and the approach of a person
reduces this voltage, such approach produces a positive voltage at
d.
The amplifier A.sub.2, energizing a relay R.sub.1, is so designed
that for zero or negative voltage at d the voltage across the relay
R1 is zero, whereas for even a very small positive voltage (e.g. on
the order of 0.1V) at the point d the amplifier A.sub.2 energizes
the relay R1 sufficiently for the same to become operative.
Preferably, the input impedance of the amplifier A.sub.2 is very
high -- several megohms -- so that a very high detected voltage can
be produced at the point c with low power consumption. The
magnitude of the resistance R.sub.2 should therefore itself also be
very high.
Advantageously, the amplifier A.sub.2 is a conventional NPN
transistor connected as a cathode follower, so as to have a high
input impedance, followed by another NPN transistor arranged as a
voltage amplifier, in turn followed by a voltage-amplifying PNP
transistor driving the relay R.sub.1. One of the stages of
amplifier A.sub.2 includes an integrating network Int, having a
time constant of 0.2 to 0.5 sec, for general interference
suppression.
Relay R1 has two contacts r.sub.1, r.sub.2, the former actuating a
responder, e.g. triggering an alarm AL, whereas the latter
preserves the response by the application of an appropriate voltage
+v to one of the transistors of amplifier A.sub.2. This obviates
the need for a direct holding contact on the relay; the holding
effect of voltage +v can be controlled by any parameter, e.g. as
described below with reference to FIG. 6.
Since the circuit arrangement responds to the appearance of a
positive voltage at the point d, a diode D.sub.2 can provide very
rapid absorption of negative potential variations at that point so
that when such variations occur, for instance, at switch-on, the
apparatus is immediately ready for operation -- i.e., there is zero
voltage at the point d.
Detection of increasing rather than decreasing voltage swings at
the input terminal b is possible if the amplifier A.sub.2 is
sensitive to negative voltages (PNP stage), in which case the shunt
diode D.sub.2 would be inverted.
If the detector is self-restoring to the standby or monitoring
state, it is preferable for many uses of the invention to have a
signal of limited duration rather than a steady signal. Various
auxiliary means are known for providing a delay giving a signal
lasting for a few tens of seconds; in the present case, however,
there is a very simple way of achieving this result with virtually
no addition of extra items.
The delay procedure can be clearly understood from the following
example.
If the system consumes, say, 1 mA on standby, its consumption is
e.g. 30 mA when relay R1 operates, because of the energy used up by
this relay. If a resistance Rs (FIG. 5) is connected in series with
the associated power supply S and is of such magnitude that the
supply voltage energizing the detector part Det of the system drops
by e.g. 10 percent when the relay is thus energized, all the AC and
DC voltages will decrease in substantially the same proportion.
More particularly, the voltage integrated at the point c, which was
-50 V, becomes -45 V, and the initial voltage change which
triggered the alarm and which was just a few tenths of a volt is
converted into a much greater swing as a result of the voltage drop
developed across resistor Rs. A positive voltage above 5 V
therefore appears at the point d. To dissipate this voltage by way
of the differentiation network, capacitor C.sub.3 must first
discharge through resistor R.sub.3 sufficiently for the voltage at
d to decrease to e.g. 0.1 V or less. When this voltage has been
reached, the relay R1 returns to normal and stops the
responder.
Consequently, if the value of capacitance C.sub.3 is chosen
appropriately, a signal lasting for approximately 1 minute can be
produced without any other delay means being used.
Conversely, however, when the relay R1 releases, a very large
negative voltage variation occurs at the point d; as previously
described, this variation is, however, absorbed very rapidly by the
diode D.sub.2 and the detector is ready almost immediately for
further operation.
Since the detector operates basically on the principle of varying
the state of tuning of a tuned transmitting circuit, the main
variation being capacitive and the secondary variation being in the
damping, there is no need to use a vertical antenna in association
with a counterpoise forming a horizontal mass plane. The antenna
its counterpoise can be e.g. two metal strips or even wires
connected to the two ends of the oscillatory circuit and extending
parallel to each other. The strips can be placed on the ground, if
the same is not conductive (floor), or positioned vertically on
either side of an entrance which it is required to protect. The
sensing element formed by the antenna and its counterpoise can be
devised differently to suit individual cases. Inter alia, in a room
or the like the antenna can be in the ceiling and the counterpoise
can be below it on the floor.
Thanks to its high sensitivity, the system is highly versatile. For
instance, with a vertical antenna 1.50 meters long and a ground
plane which is either inherently conductive or made so, e.g. by
latticework, a person can be detected at up to about 8 meters from
the antenna -- i.e., assuming that the antenna is accessible from
all directions, the operative area of the system is on the order of
200 m.sup.2.
An obvious use of the system is for protection against unwanted
intrusions. It can also be used to detect movement, e.g. for
automatic door opening, lighting of passageways (timers), or
counting people.
Also, its delay feature makes it very suitable for advertising
purposes as, for instance, to trigger a tape recorder which
broadcasts an advertising announcement or a commentary on an
article on show in a museum.
FIG. 6 shows one such adaptation of the invention. An endless
magnetic tape m contains a text which may be repeated a number of
times, the spacing between repetitions being such that the end of
the text and the start of its repetition are separated by an
interval of a few seconds. For instance, in its commercial
application a tape recorder MAG containing the tape m is under the
control of the movement detector hereinbefore described. The two
systems are located near the place where possible clients may pass
by. When a client approaches, the relay of the detector Det starts
the tape recorder MAG. For a brief period the tape recorder
transmits no signal to loudspeaker H, but the relay R1 remains held
by the delay means hereinbefore described; in this case the delay
is fairly short, e.g. 5 seconds. At a predetermined time the tape
recorder MAG starts to read out the text through a loudspeaker H.
After some time the internal delay of the detector Det terminates,
but the detector relay R1 locks as a result of detection of the
transmitted modulation; the voltage across the loudspeaker H is
detected by a network D.sub.4 -R.sub.4 -C.sub.4 and applied through
holding contact r.sub.2 of relay R1 of FIG. 4 to hold the relay
while the low-frequency modulation -- i.e., the transmitted message
-- continues. Upon cessation of the message the holding voltage
disappears, the relay R1 releases and the tape recorder MAG stops.
The procedure can restart when someone else passes nearby.
Clearly, the time constant of the detector network R4 C4 must be
long enough for the hold not to be likely to disappear between
individual words of the message, and short enough for terminating
the hold at the end of the message, e.g. after 2 seconds of
silence, so that the system is restored to standby for someone else
to pass by.
Of course, recording using a continuous tape can be replaced by any
other kind of sound recording, such as one using a disk with
automatic return of the pickup arm.
* * * * *