Receiver for audible alarm

A receiver to detect the audio tone emitted by an activated smoke alarm. The receiver employs an electret condenser microphone to monitor ambient sound. The output of the microphone is applied to an active band-pass filter and then to a PLL tone decoder. The PLL is tunable to provide maximum sensitivity to the frequency of the tone. The microphone is the gain controllable element in an AGC system that acts to keep received signals at a constant level. This insures that all devices operate linearly.

This invention relates to a detector designed to receive the audio signal 
emitted by an activated smoke alarm. 
Many structures are protected by common store-bought smoke alarms. These 
low cost smoke alarms give off an audible tone in the band of frequencies 
most audible to the human ear. It has been noted that this band is from 
approximately 2.5 KHz to 3.5 KHz, and thus most smoke detectors, when 
triggered, give off an audible tone in this range. 
There are many situations in which the audio tone given off by a triggered 
smoke detector can go unnoticed. For example, these smoke detectors have 
limited audio power and so their signal can be masked by ambient sounds. 
Then, too, people can be far enough away from a triggered detector so as 
not to hear the same. Thus it would be desirable to provide a means for 
monitoring or receiving the output of a triggered alarm, and to do so 
reliably. 
Many approaches have been taken to deal with this problem. There are 
patents that describe systems in which the smoke detector is hard wired to 
a central location. However, such wiring usually involves great expense 
and there is the danger that the fire will destroy the interconnecting 
wiring. Then there are patents describing systems in which a plurality of 
smoke detectors are interconnected to a central station by means of a 
wireless RF or ultrasonic link. But such wireless systems are prone to 
interference and require highly sophisticated electronics in their senders 
and receivers. 
Then there are devices designed to receive the audio output of a triggered 
alarm. The device is placed in proximity to a given smoke detector and 
when the latter is triggered, the former responds to the emitted audio 
signal. While such devices appear to have many attributes, they are 
subject to falsing on ambient noise, or have difficulty detecting the 
emitted alarm, or both, and this detracts from their suitability as 
receivers. 
The difficulty encountered by devices heretofore designed to detect an 
audio alarm stems from the type of sound emitted by such alarms and the 
type of ambient noise encountered. Referring to the sound emitted, many of 
these low cost smoke alarms were tested. The emitted audio was of the 
"squeal" type, where squeal designates a high frequency tone in the range 
of 2.1 KHz to 4.1 KHz. The sonic characteristics of many of the devices 
showed that the sound may be continuous or pulsating. However, during 
periods of active sound, the source was essentially sinusoidal (with 
possible distortion). Referring to ambient noise, these noises were either 
random phase noises, long term coherent noises, or short term coherent 
noises. 
The present invention overcomes the above-noted problems associated with 
receivers designed to monitor an audio alarm and provides for a receiver 
which responds only to the sound emitted by an activated smoke alarm. In 
general, the present invention is a self-contained audio or sonic receiver 
which is placed in the vicinity of the smoke detector to be monitored. In 
its preferred form, the receiver comprises an electret condenser 
microphone with automatic gain control (AGC), a sharply defined active 
filter network, and a phase locked loop (PLL) tone decoder. The microphone 
defines the gain controllable element in the AGC system. Ambient audio is 
monitored or picked up by the microphone and passed through the filter 
network. The signal is then sent to a signal amplifier which boosts the 
same to a 2 volt peak-to-peak level. The AGC circuitry controls the 
sensitivity of the microphone so that this 2 volt peak-to-peak level is 
kept relatively constant and not exceeded. The signal is then applied to 
the PLL tone decoder. The PLL is kept insensitive to sub-harmonics and 
higher ordered harmonics of the tone being received so that the PLL will 
discriminate between true smoke detection (i.e., a triggered alarm) and 
ambient noise. The PLL operates in a narrow band mode and will phase lock 
onto the tone of a triggered alarm. Once the PLL locks, the output 
therefrom is used to drive a signaling device such as a telephone dialer 
or a high-powered alarm. 
The PLL decoding of the inventive device, along with sufficient output loop 
filtering, solves the problems inherent in discriminating from the three 
types of ambient noise, noted above. The PLL with lock output (quadrature 
component output, filter, output comparator) in combination with 
sufficient filtering added to the quadrature output allows the lock 
indicator to remain locked during periods of pulsating sound. 
Random phase components will not cause the PLL to lock so they are 
eliminated. Long term coherent sources in the inventive receiver's range 
are eliminated for two reasons. First, the PLL is designed to have a 
narrrow capture range, approximately 15 percent. Hence the device can be 
closely tuned to the frequency of a given alarm. Second, in most of the 
environments measured, most long term sources (usually but not always 
related to the A.C. power line frequency (60 Hz) or harmonics) were never 
close enough in frequency to the tone of the alarm because the tone is 
high pitched. 
Finally, short term noise sources do not cause falsing in the inventive 
device because music and room noise was found to be coherent for only a 
very short time. Sufficient PLL output filtering is used so that harmonics 
of any musical notes, or noise, never exist long enough to lock the PLL. 
The inventive AGC circuitry helps in the reduction of harmonic generation 
by keeping all amplifiers and the input to the PLL in their linear 
regions. The AGC is a special fast attack, slow decay AGC that has 
superior AGC-loop damping characteristics. By reducing the bias voltage on 
the electret microphone, the gain of the transducer is reduced 
substantially. This allows for a very large range of gain reduction at 
minimal cost. The damping characteristics are quick enough to handle the 
rapid pulsations of an activated smoke alarm. This allows for maximum 
receiver sensitivity and a minimum generation of harmonics. 
In all environments tested, there was only one common source of noise, 
which although infrequently a problem, was nevertheless important. Certain 
telephones with mechanical bells, when placed near to the microphone could 
have a ring that might sometimes trigger the PLL. Investigation revealed 
that the bells were of a substantial Q so that, typically, after being 
rung (driven) for about 2 seconds they would continue to oscillate or ring 
for up to 4 seconds more, sometimes at the same frequency of a particular 
alarm. It was found that in these unusual circumstances, an output delay 
circuit with a relatively long period, longer that the ring cycle (the 
ring cycle being typically 6 seconds), and a timer with immmediate reset 
capability would solve the problem. Even with a worst case ring, the PLL 
would, between rings, always unlock momentarily. This momentary unlock 
would reset the timer and thus prevent a false alarm. It should noted that 
some or all of these "anti-ring" techniques, especially the output timer, 
can be implemented in software if the monitoring device is programmable. 
It is therefore an object of the present invention to provide a receiver 
that responds to the audio output of a triggered smoke alarm. 
It is another object of the present invention to provide a receiver that 
responds to the audio alarm of a common store-bought smoke detector and 
that does so without falsing or responding to ambient noise. 
It is a further object of the present invention to provide a detector for 
sonic signals that fall within a defined frequency range wherein the 
detector uses an AGC system to keep signal levels within predetermined 
limits, and that uses a PLL to discriminate between a true signal and 
noise. 
It is a still further object of the present invention to provide a receiver 
for sonic signals wherein the receiver is tunable over a narrow band of 
frequencies thereby allowing the sensitivity of the receiver to be peaked 
to the tone of a given alarm. 
It is another object of the present invention to provide a detector for the 
tone of a smoke alarm wherein the detector can be supplied as a small 
self-contained battery operated unit that can be conveniently placed with 
range of such tone. 
Other objects and features of the present invention will become apparent 
from the following detailed description considered in connection with the 
accompanying drawings. It is to be understood, however, that the drawings 
are designed for purposes of illustration only and not as a definition of 
the limits of the invention for which reference should be made to the 
appending claims.

In detail now and referring now to the drawings, FIG. 1, shows a block 
diagram of the basic detection system according to the inventive design. 
In this simplest system, sound is picked up by an ordinary low cost 
dynamic or piezoelectric microphone 99. The microphone output is applied 
to a signal amplifier 400 which boosts the signal level a predetermined 
amount. The output of amplifier 400 is fed to a PLL detector 800. The 
exact configuration of amplifier 400 and PLL 800 is discussed in detail 
below, with reference to FIGS. 2 and 3. Suffice it to say here, however, 
that the PLL is tunable over a relatively narrow range thus allowing the 
same to be tuned the frequency of the tone from the detector being 
monitored. 
As will be explained below, the PLL lock output is filtered to allow for 
detection of pulsating smoke alarms and rejection of false signals. Tuning 
is accomplished by a single adjustment of the VCO (voltage controlled 
oscillator section of PLL). In all embodiments of the present invention, 
when the PLL locks onto the monitored alarm tone, the output of the PLL 
gives a defined response to such lock by going to a logic low (voltage 
near ground), or if desired, some other predetermined state. In the 
embodiment of FIG. 1, the output of the PLL is fed into a monitoring 
device (for example, an automatic telephone dialer, not shown) triggering 
the same to indicate the presence of an alarm situation. Common such 
monitoring devices are digital dialers which call the fire department 
either directly or through a central monitoring station. Thus the 
monitoring device or dialer responds indirectly, via activation by the 
inventive receiver, to the presence of smoke or fire. 
Referring now to FIGS. 2 and 3 there is shown the enhanced and preferred 
form of the present invention. Sound from a triggered alarm is detected 
and converted to electrical energy by microphone 100. Microphone 100 is an 
inexpensive electret condenser microphone with low voltage operation. The 
microphone should be mounted to the case in which it is held with a sound 
absorbing cushion to prevent vibrations from the mounting from being 
mechanically transmitted to the microphone. This prevents strong 
vibrations from desensitizing the receiver and preventing proper 
operation. Microphone 100 is supplied as an integral, self-contained unit 
and includes condenser plates 102 feeding built-in FET amplifier 104 with 
internally supplied diode 106. FET 104 acts as an impedance converter from 
the high impedance electret capacitor 102 to the relatively low impedance 
of a load resistor 318. An electret microphone is chosen because it allows 
easy gain control by means of an AGC circuit. Note that the DC to the 
microphone is supplied through resistor 318, more being mentioned about 
this later in conjunction with the AGC description. 
A shielded lead 107 applies the output from microphone 100 to an active 
high-pass filter 200 where the signal is amplified and filtered. The 
output of filter 200 is coupled to an active low-pass filter 300 where the 
signal is, again, amplified and filtered. High-pass filter 200 is followed 
by low-pass filter 300 to produce an overall response of a relatively 
wide-band, band-pass filter. This band-pass filter has a low frequency 
pole of 2.1 KHz and a high frequency pole of 4.1 KHz. Hence, the band-pass 
filter is realized by a third order Butterworth high-pass filter, with a 
cut off of 2.1 KHz, followed by a third order low-pass Butterworth filter, 
with a cut off of 4.1 KHz. Each filter has 20 dB of gain. Placing the 
high-pass filter section before the low-pass section improves the 
realization for this application. This is because the primary low 
frequency components, which make up most of the noise environment, are 
removed before amplification, thus improving dynamic range. Then the 
low-pass filter which follows removes high frequency noise to produce a 
clean output, even for high levels of amplification. 
High-pass filter 200 is composed of two parts. A second order stage 
implemented with op-amp 202, and a first order RC network implemented with 
capacitor 206 and resistor 224. This last stage is duplicated for AGC amp 
502, discussed below, with capacitor 208 and resistor 226. Each chain, 
either the signal chain or the AGC chain, sees only one of the real poles, 
and hence each chain sees the same third order filter. The second order 
section of the high-pass filter is a voltage controlled, voltage source 
(VCVS) active filter. Although the VCVS filter offers good accuracy 
without severe restrictions on amplifier gain, 1 percent resistors 
(resistors 214, 216, 218, 220) and 5 percent capacitors (capacitors 210, 
212) are required to insure production stability. Furthermore, the source 
impedance must be kept low to prevent instability and this is accomplished 
by resistor 318. 
Low-pass filter 300 is also composed of two parts. A second order stage 
implemented with op-amp 302, and a first order RC network implemented with 
capacitor 308 and resistor 318. The second order section of the low-pass 
filter is a multiple feedback (MFB) active filter. 5 percent components 
will guarantee stability with the MFB filter. Although MFB filters produce 
severe gain restrictions on the op-amps, the limited gain bandwith product 
of the second order stage makes this type of filter acceptable here. 
Next the signal is split. Part of it is sent to a signal amplifier stage 
400 by means of lead 401, and part of it is sent to an AGC amplifier stage 
500 by means of lead 501. Each amplifier provides either 0 dB or 20 dB 
(1X) or 10X of gain for its respective signal path, depending on whether 
ganged switches 410 and 510 are open or closed. With the switches open, 
the inventive receiver operates in its low gain (40 dB) mode, and with the 
switches closed, the receiver operates in its high gain (60 dB) mode. 
The AGC path consists of AGC amplifier stage 502, peak detection circuitry 
600, and filtering and regulation circuitry 700. Note that the components 
associated with AGC stage 500 (op-amp 502, resistors 506, 508) are 
identical with the components associated with signal amplifier stage 400 
(op-amp 402, resistors 406, 408), thus these amplifiers have identical 
characteristics. Capacitor 606 applies the output from amplifier stage 500 
to voltage doubling diodes 602 and 604 for peak detection. These diodes 
should have low dynamic resistance to provide sharp AGC characteristics. 
The ratio of capacitor 606 divided by capacitor 608 will determine the 
relative attack time of the AGC response while resistor 612 and capacitor 
610 determine the decay time. The decay time is made long enough to allow 
for pulsating smoke alarms. 
When the output of the AGC stage, which mimics the signal amplifier stage, 
is too large, it turns on transistor 706. This transistor discharges 
capacitor 610 very rapidly, reducing the voltage at test point "A." This 
voltage acts as a variable voltage source which supplies the bias to 
electret microphone 100 via load resistor 318. By reducing the voltage on 
the electret condenser microphone, the gain of the transducer is reduced. 
This technique allows the microphone to be the gain controllable stage in 
the AGC system and provides for wide AGC dynamic range. Transistor 706 is 
operating in its reverse mode. This is done deliberately to reduce its 
gain. If the transistor were placed in its normal mode, AGC 
characteristics as to sharpness, attack time, and decay time, would be 
about the same. However, AGC dynamic loop response would be highly 
underdamped. In the inventive technique, AGC action is very nonlinear. 
This gives rise to its very sharp characteristics, and by using transistor 
706 in its low beta reverse mode, damping is improved. It should be noted, 
too, that using transistor 706 in its reverse mode improves its saturation 
characteristics. A sensitivity potentiometer 702 is incorporated in the 
AGC loop. A resistor 704 is added to the center arm of control 702 to give 
the control a nonlinear characteristic. This improves its controllability. 
In use and operation, as will be discussed below, it is recommended that 
major gain reduction be done first with ganged switches 410 and 510, and 
then with potentiometer 702. 
After the signal pases through amplifier stage 400, a lead 411 applies it 
to PLL stage 800 through a voltage divider made up of resistors 412 and 
414. Because of the AGC action noted above, the output of signal amplifier 
402, accessible at test point "B," is kept at a nearly constant 2 volt 
peak-to-peak level. Stage 800 includes a PLL tone decoder 802. This 
configuration of a PLL utilizes a quadrature detector, output filter, and 
output comparator for tone detection. The PLL is decoupled from the power 
supply by resistor 824 and capacitors 810 and 812. The PLL is also placed 
stragetically with on the board or card (not shown) to prevent feedback. 
Since the inventive receiver is a very high gain device whose amplifier 
feeds the PLL directly, care must be taken to insure that the VCO (voltage 
controlled oscillator) is not fed into the amplifier chain. Otherwise a 
positive regeneration will occur making the PLL lock up to itself, 
producing a false alarm. The bandwidth of the PLL is nominally 14 percent, 
which is small enough to reject false signals, yet large enough to listen 
for the smoke alarm tone. Capacitor 814, which makes up the loop filter, 
is chosen to comply with maximum bandwidth operation. Resistors 820, 818, 
and capacitor 816 comprise the RC section of the VCO. Capacitors 806 and 
808 comprise the output filter. Switches 804 and 1004 are ganged and are 
used to align or tune the PLL to the tone of a given alarm, as discussed 
below. Opening this ganged switch disables the PLL output while reducing 
the output time constant dramatically. 
To tune the inventive receiver to a tone of a given alarm, the alarm from 
the smoke detector is momentarily turned on. Switches 804 and 1004 are 
opened. The PLL is tuned to the particular tone with variable resistor 
818. This resistor is preferably of the 20 turn variety. While observing 
LED 828, variable resistor 818 is slowly swept back and forth, noting its 
effect on capture range (not lock range). As resistor 818 is swept, first 
one way, then the other, locking of the PLL is visually observed at two 
distinct sweep points by LED 828 becoming inactive or shutting off. The 
wiper arm of resistor 818 is then centered between these two sweep points 
in the range wherein LED 828 is off. 
Once tuned, switches 804 and 1004 are closed. Such closing not only 
completes the output path from PLL decoder 802, but it connects capacitor 
806 to the output filter network of PLL decoder 802. This large capacitor 
slows the response of the lock and unlock time of the PLL which is 
important for two reasons. First, the slow response eliminates short 
transients from producing an output. Second and more important, the long 
filter constant prevents the PLL output from unlocking during the 
momentary deactive periods of a pulsating smoke alarm. 
A lead 825 applies the output from PLL tone decoder 802 to a delay filter 
stage 900. Often this delay filter is contained in the monitoring device, 
in, for example, the automatic telephone dialer. This would emiminate the 
need of it being in hardware in the inventive receiver. Irrespective of 
where it is placed, delay stage 900 interposes a relatively long time 
period of approximately 10 to 12 seconds, with fast reset, before the 
buffer stage (discussed below) activates the dialer. This 10 or 12 second 
delay is at least as long as the period of one ring cycle and is 
accomplished with capacitor 904 and resistor 918. 
Under an alarm condition, the capacitor will discharge slowly through 
resistors 916 and 918 (with the latter resistor comprising almost the 
entire resistive discharge path) until the voltage at the base of 
transistor 908 rises enough to turn on the same. The emitter of transistor 
908 is applied to a voltage reference, accomplished with resistors 912 and 
914. When transistor 908 turns on, it drives an output driver or buffer 
stage 1000. However, under a no alarm condition and with the delay 
characteristics exhibited by stage 900, this stage acts as anti-ring 
interference for certain sounds caused by the ringing of mechanical 
telephone bells, mentioned earlier. The bell tones may persist long enough 
to lock the PLL VCO for sufficient time for it to produce a locked signal 
output. But since these signals usually disappear before the next ring, 
the PLL will unlock, albeit momentarily. When such unlocking occurs, the 
output of the PLL will momentarily go high turning on transistor 906. 
Transistor 906 will rapidly charge capacitor 904 through resistor 916 thus 
causing an almost instantaneous reset of the timing or delay circuit, 
i.e., capacitor 904 never discharges sufficiently to turn on transistor 
908. It is seen that capacitor 904 is connected to the +5 volt supply and 
not to ground. This is done so that the capacitor will normally have a 
voltage impressed across it, with voltage being removed during an alarm 
situation. If this were not the case, capacitor 904 might degrade with 
age, and not work properly under an alarm condition. If the 
characteristics of stage 900 are implemented in software, then the opening 
of switch 902 will disable the hardware delay of this stage. 
The output of delay stage 900 is taken off of the the collector of 
transistor 908 and directly coupled into the base of output transistor 
1002. This last-mentioned transistor is the output driver or buffer, with 
drive being taken from its collector. Switch 1004, which is in series with 
the lead from the collector of transistor 1002, is used to isolate the 
driver from the device being driven, not shown, while the PLL is being 
tuned, as noted above. 
The power supply for the inventive receiver is seen as section 1100. It 
comprises a low-dropout, three terminal voltage regulator 1102. Regulator 
1102 has a +5 volt output and allows for an unregulated input as low as 
5.8 volts. This permits economical battery operation. Since a single 5 
volt supply is used, op-amp analog ground is derived by voltage divider 
action and filtering with resistors 1120 and 1116, and capacitors 1110 and 
1112. Operation can be either from a 6 volt source or a 12 volt source. 
With a 12 volt source, jumper 1126 is cut placing resistor 1124 in series 
with the unregulated voltage input. Capacitors 1104, 1105, and 1108 are 
for filtering. LED 1114, when lit, indicates that the receiver is "on," 
but is deleted for battery operation. 
Referring now to FIG. 4, there is shown use of the inventive receiver. Such 
use should be readily apparent. The room or building being protected, room 
1300, employs a conventional smoke detector 1301. The inventive receiver, 
diagrammatically seen here as being built into a low profile container 
1302, is then mounted in the room in any convenient location. It does not 
have to be placed in line-of-sight with smoke detector 1301. 
Microphone 100 is set to its maximum sensitivity by placing the wiper arm 
of control 702 at the +5 volt side or terminal. Ganged switches 410 and 
510 are opened. The alarm tone or signal from detector 1301 is temporarily 
turned on and a DC voltmeter placed between test point "A" and ground. 
With this signal, one volt or less should be observed at this test point 
(Under a no signal condition, test point "A" will be approximately three 
volts.). If the voltage is substantially greater than one volt, not enough 
signal is getting to the receiver and switches 410 and 510 should be 
closed increasing signal gain 20 dB or ten times. In practically all 
situations, the added gain will drop the voltage at test point "A" to the 
desired one volt or less. If this does not occur, the receiver should be 
moved to a spot where sonic pickup is improved. With the voltage at test 
point "A" at one volt or less, approximately 6 dB of headroom is provided 
before AGC action is lost or, put another way, before the signal is too 
weak to provide AGC action. Even when AGC action is lost and microphone 
100 is operating at maximum sensitivity (test point "A" would then be at 
approximately three volts), the inventive receiver will respond to even 
weaker signals. But, below the AGC limit level (point at which AGC action 
is lost), the bandwidth of the receiver (PLL) may be reduced. 
When the voltage at test point "A" is within proper limits, and with the 
alarm tone still on, the PLL is then tuned or peaked to the frequency of 
such tone in the manner stated previously. 
An electrical lead 1303 connects the inventive apparatus to, say, an 
automatic telephone dialer 1304. When detector 1301 is triggered by smoke 
or fire, sonic energy 1305 from detector 1301 is picked up by the 
inventive receiver. The receiver locks onto the emitted tone or energy 
whereupon the output stage of the PLL exhibits a defined response, i.e., 
it goes low eventually to turn on buffer transistor 1002. Assuming that 
this buffer transistor is carried in container 1302, a signal from 
transistor 1002 is sent along lead 1303 causing the dialer automatically 
to call indicating an alarm situation. 
For best results it is recommended that the inventive receiver be sheilded 
or enclosed in a metal box to prevent electrical noise from causing 
interference to the high gain amplifier chain. Strong interference may be 
caused by fluorescent lamps and RF transmitters. Proper shielding 
eliminates both sources. Sharp transients at harmonics of the 60 Hz power 
line frequency may cause ringing of the filters if not eliminated by 
shielding techniques. 
In tests outdoors the inventive receiver, when placed in its high gain 
mode, has picked up the sound and phase locked onto the alarm tone of 
typical, store-bought smoke detectors from a range as great as 300 feet. 
But to allow for very poor alarm power from certain detectors, a range of 
25 feet is recommended. 
With regard to the type of band-pass filter chosen and constructed, other 
filters such as the Chebychev filter offer superior skirt characteristics. 
However, the Butterworth third order filter was chosen because it offers 
only minimal frequency peaking in its component sections. Minimal peaking 
means improved dynamic range of the amplifiers, which for low voltage, 
limited gain amplifiers is important. Furthermore, the filter sections 
being of low individual Q, are easier to systhesize and are more stable. 
This results in the use of lower cost, higher tolerance components. 
The AGC action associated with the inventive AGC system acts to reduce 
harmonic generation by keeping all amplifiers and the PLL in their linear 
region. Linear operation improves the sensitivity of the PLL during 
periods of strong out-of-band noise. Also, note that the associated 
outputs from amplifiers 202, 302, 402, and 502, have respective pull-up 
resistors 222, 310, 404 and 504. This greatly reduces distortion and 
allows for a wider dynamic range. 
Electret condenser microphone 100 is manufactured by Panasonic.RTM. 
(division of Matsushita Electric Corporation) and is listed as part number 
WM-034DM. Amplifiers 202, 302, 402, and 502, are supplied in one package. 
This package is an LM 324N quad operational amplifier, and it along with 
the LM 567CN PLL and the LM 330T-5.0 voltage regulator are manufactured by 
the National Semiconductor Corporation. However, equivalent parts are 
supplied by many other companies. 
Temperature variations of the entire system (smoke alarm and inventive 
receiver) should be kept small to reduce drift, especially in the smoke 
alarm. However, since the system is usually installed in a controlled 
environment, drift is not a problem. 
While only a few embodiments of the present invention have been shown and 
described, it is to be understood that many changes and modifications can 
be made hereto without departing from the spirit and scope hereof.