US3493967A - Amplifier for intercommunication system with fire and burglar alarm features - Google Patents

Description

Feb. 3, 1970 E. o. RESSLER 3,

AMPLIFIER FOR INTERCOMMUNICATION SYSTEM WITH FIRE AND BURGLAR ALARM FEATURES Filed April 3, 1967 l9 LOAD (SPEAKER) Q gOMMON SWITCH POSITIONS a-BURGLAR aFIRE Io-OFF (RESET) c-FIRE ONLY ol-FIRE TEST A A -zzv SUPPLY g gm BURGLAR ALARM SWITCHES R m E L N S E s V E N R 0 D R A H E FIRE ALARM SWITCHES ATTORNEY.

United States Patent 01 3,493,967 Patented Feb. 3, 1970 3 493,967 AMPLIFIER FOR INTERCOMMUNICATION SYSTEM WITH FIRE AND BURGLAR ALARM FEATURES Erhard 0. Ressler, West Chester, Ohio, assignor to Avco Corporation, Cincinnati, Ohio, a corporation of Delare Filed Apr. 3, 1967, Ser. No. 627,751 Int. Cl. G08b 1/08 US. Cl. 340420 Claims ABSTRACT OF THE DISCLOSURE A solid state audio amplifier of the type used in sound intercommunication systems is provided with a feedback network which is adapted either to make the amplifier howl with high frequency oscillations, in the event of a fire, or provide low frequency oscillations of the relaxation type, in the event of a burglary.

An electronic switch in the form of a bistable device controls the potential of a point on the feedback network in order to establish these two conditions of operation. A control switch establishes a response to the sensing of both burglary and fire or fire only. It also has an o position.

BACKGROUND OF THE INVENTION The present invention provides control circuitry which, when used in conjunction with an intercommunication system having a sound channel, provides means for detecting such undesired hazards as burglary or fire, and adds these additional functions to the normal function of an intercommunication system.

The prior art includes many arrangements for providing an audio tone in response to the sensing of a fire and coupling that audio tone into the loud speakers of an intercommunication system. Such a system is shown, for example, in the United States patent to Wooten, No. 2,942,245.

The present invention provides an arrangement which furnishes an alarm in the event of either burglary or fire. In the event of fire it furnishes high frequency oscillations in the form of a howl and in the event of a burglary it furnishes low frequency audible sounds by relaxation oscillations.

The present invention is an all solid state system, simple and reliable in operation, and it includes a rotary switch device operative to provide the functions mentioned in the abstract set forth above.

DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, together with other objects, advantages and capabilities thereof, reference is made to the following description of the accompanying drawings, in which:

FIG. 1 is a circuit schematic of an amplifier and a combined burglar and fire alarm circuit in accordance with the present invention;

FIG. 2 is a circuit schematic showing the switching functions performed with respect to point 43 of the feed back network provided in accordance with the present invention.

DETAIL DESCRIPTION OF THE INVENTION In accordance with the present invention, an audio amplifier in an intercommunication system is made to function as an oscillator and to set up a beep or howl in the loud speaker load of the amplifier, in the event of the presence of a burglar or of fire, which presence is appropriately sensed.

Referring now first to the audio amplifier of FIG. 1, it comprises four stages, the first of which is a pre-amplifier stage incorporating a PNP type transistor 11, the second of which is an intermediate stage incorporating an NPN transistor 12, the third of which is a driver stage incorporating a PNP type transistor 13 and the fourth of which is a push-pull arranged output network comprising PNP type transistor 14 and NPN type transistor 15, connected in complementary symmetry. The junction of the emitters of transistors 14 and 15 is coupled by a series capacitor 16 to an output terminal 17. The other output terminal 18 is grounded. To ground are connected the collector of transistor 15 and the emitters of transistors 11 and 13. A loud speaker load symbolically indicated by the reference numeral 19 is connected across the output terminals 17 and 18.

A voltage feedback connection originates at the junction 20 of the emitters of transistors 14 and 15. For purposes of temperature stabilization the direct current voltage feedback circuit comprises point 20, series resistor 21, series resistor 22 and series resistor 23, the junction 24 of the latter two resistors being connected to the base of transistor 11 and resistors 21-23 forming a voltage divider between point 20 and ground. A portion of the alternating current in the feedback path is bypassed by the series combination of resistor 25 and capacitor 26, in shunt with 2223, some feedback being maintained for purposes of distortion reduction.

The audio input to pre-arnplifier transistor 11 is via the input terminals 27 and 28, terminal 27 being coupled to the base of transistor 11 via capacitor 29.

A direct current reverse-bias voltage of approximately 18 volts is provided for the collector of pre-amplifier transistor 11 via a series connection of resistors 9, 30 and 31, resistor 31 being connected to a 22 volt terminal 32 and resistor 9 being connected to the collector of transistor 11.

The pre-arnplifier transistor 11 is in the common emitter configuration and its collector output is directly connected to the base of transistor 12 and the collector output of transistor 12 is in turn directly connected to the base input of transistor 13, also in the common emitter configuration. Resistor 33 is the collector load of transistor 12. Capacitor 34 is an emitter bypass for transistor 12.

Current in resistor 35, in series with the collector of transistor 13, provides turn-on bias for transistors 14 and 15, resistor 35 being connected between their bases. Resistor 36, being large with respect to resistor 35, is the principal collector load for transistor 13. Resistors 35, 36 and 31 are in series between the collector of transistor 13 and supply terminal 32.

There is a bootstrap coupling from output point 20 of the push-pull output stage (i.e. the emitters) to the junction 37 between resistors 31 and 36, via capacitor 38, and the purpose of this bootstrap connection is to provide a more elevated voltage for the collector of transistor 13 than is available at the source terminal 32.

In accordance with one aspect of the invention, this amplifier is made to oscillate by close-circuiting of a feedback network between the audio output terminal 17 and the collector of transistor 11. This network comprises, in series, a series resistor 41, series capacitor 40, and series diode 39. The diode 39 is so arranged that its cathode has a resistive shunt connection 42 to the 22 volt supply terminal 32 whereby, when the cathode of the diode is not grounded the cathode is forward biased and the diode 39 then establishes a conductive path and a feedback connection between point 17 and the collector of transistor 11, which feedback connection, when closed, causes regenerative reaction and oscillation. The point at which the cathode is connected to resistor 42 is referred to as 43.

When the point 43 is grounded then the feedback con nection is effectively open-circuited and the amplifier circuit functions as an amplifier. On the other hand, when the point 43 is ungrounded then the feedback connection 39, 40, 41 is close-circuited and the amplifier functions as an oscillator.

The broad function of the remaining parts fthe circuit now described is to cause point 43 to be grounded or ungrounded as desired, grounding being the condition desired for normal operation and a negative potential being the condition desired when a warning or fire is to be furnished. For burglary, point 43 is at a less negative potential.

At this point it is emphasized that when point 43 is grounded the amplifier is in the upper part of FIG. -1 functions as such. Reference is now made to FIG. '2. When point 43 is negative enough to make diode 39 conductive, as when the presence of a fire is detected, then the feedback network 41, 40, 39 functions in such a way as to make the amplifier howl at a relatively high but audible frequency. On the other hand, when burglary is detected point 43 is placed at a less negative potential and the feedback network then functions in such a way as to cause the generation of relaxation oscillations which are at a lower frequency and therefore characterize the detection of burglary. In other words, when a fire is detected point 43 is placed at a negative potential. When burglary is detected point 43 is placed at a less negative potential. This switching of point 43 to the desired potential is accomplished by the switching network or bistable device comprising the transistors 50 and 51. Resistors 70 and 67 and the collector-emitter circuit of transistor 51 are in series between high potential supply conductor 72 and conductor 57. The emitter collector circuit of transistor 50, resistor 68 and resistor 71 are also in circuit between conductor 72 and conductor 57. The operation of this switching circuitry is such that when a burglary is detected, removing the ground from 57, then conductor 52 is in effect encircuited with conductor 72 and placed at a minus potential, whereby the diode 61 becomes nonconductive and the potential of point 43 is as determined by the voltage divider which comprises resistor 42 and resistor 65, disposed between conductor 72 and ground, point 43 being effectively the junction of these two resistors.

On the other hand, when a fire is detected the connection between point 43 and ground is simply broken. This will be more readily understood by reference to FIG. 2 in which 61 is a diode and a switch with moving contact Z and fixed contacts X and Y symbolically represents the switch including the transistors 50 and 51 of FIG. 1. It will be seen from FIG. 2 that when one of the burglary sensing switches 58 or 59 is open the moving contact Z moves to X, turning off diode 39 and placing point 43 at a potential determined by the voltage divider 42, 65. On the other hand, when one of the fire alarm switches 54 or 55 is open, point 43 is simply ungrounded. This discussion of FIG. 2 assumes that the switch X, Y, Z operates in such a manner that Z is normally in contact with Y and that when either 58 or 59 is opened that Z will immediately swing over the contact with X.

By analogy, when 58 or 59 in FIG. 1 is open, 57 departs from ground potential and the collector of 51 immediately falls to -22 volts, so that the analog between the bistable switching device of FIG. 1 and the symbolic representation of FIG. 2 is satisfied.

A switch comprising three ganged sections 44A, 44B and 440, the gauging of the rotary contacts being by a suitable mechanical expedient 45, is so arranged as to provide for four distinct conditions of operation. In the extreme counterclockwise position the rotary contacts are encircuited with the respective fixed contacts designated a. In the next position, the rotary contacts are encircuited with the fixed contacts designated b. In the third position, the rotary contacts are cncircuited with the fixed contacts designated 0, and in the fourth or extreme clockwise position with those designated d. The b contact of section 44A is connected to point 43. The a and 0 contacts of switch section 44C are connected to each other and point 43. The I), c and d contacts of section 44B are connected to each other and to the collector of transistor 51. The rotary contacts of switches 44A and 44B are connected to ground.

Let it now be assumed that the switch sections are in the off position. In this condition there is ground connection, from 43 via conductor 62, the b contact of section 44A, conductor 47, and conductor 48 to ground, which open-circuits and disables the feedback connection 39, 40 and 41. This establishes normal amplification of the amplifier.

Parenthetically, for reasons which will appear hereinafter, when the switch sections are in the off position it is desired that the bistable device which comprises transistors 50 and 51 be conductive, i.e., that the collector of transistor 51 be at ground potential and this requirement is accomplished by the connection comprising the collector of 51, conductor 52, contact b of switch section 44B, conductor 48 and ground 60.

Let use now consider the position Bulglary-Fire condition of the switch sections 44A, 44B, and 44C. In this position it is desired that the system be sensitive to the presence either of a burglar or fire. The presence of a fire is indicated by the opening of any one of the serially arranged thermally responsive switches 54, 55, etc., individually staged at the various locations where it is desired that the presence of a fire be sensed. In other words, the presence of a fire is indicated by an open circuit on line 56.

The presence of a burglar is indicated by an open circuit on line 57, by reason of the opening of any one of the switches 58 and 59 which are connected in series and individually located at the various places where security against intrusion is desired.

It will be recalled that the bistable device which comprises the elements 50, 51 is conductive when the switch sections were in the off condition. When the switch sections are placed in the Burglary-Fire position this bistable device remains conductive, effectively short-circuiting conductor 57 to conductor 52 so that a ground circuit is established from conductor 60, switches 59 and 58, conductor 57, the emitter-collector circuit of transistor 51, and conductor 52, diode 61, switches 54 and 55, conductor 56 and the a contact of switch section 44C, conductor 62 and point 43, which point is therefore grounded so that the amplifier functions as an amplifier. Note that the existence of this ground depends on: the closed state of switches 54-55 and 5859 and the conductivity of transistor 51.

Let there now be considered the events which occur upon opening of any one of the fire alarm switches 54 and 55. This action places point 43 completely off ground because there is an opening in the circuit comprising the elements 62, a contact of 44C, 56 and 55, 54. This action causes a single tone oscillation to be produced by regenerative action of the aforementioned feedback components 39-41.

Now the tone which is characteristic of the burglar alarm is different than that which characterizes a fire alarm.

Let there be considered the events which occur when there is a burglar alarm, causing the opening of one of switches 58 and 59 and ungrounding conductor 57. This action causes interruption of current flow through the bistable circuit comprising transistors 50 and 51, whereby the collector of 51 and the conductor 52 fall to approximately 22 volts, the potential of the collector of 51 rendering diode 61 non-conductive so that point 43 is effectively connected to point 64, via 62, the a contact of switch section 44C, conductor 56 and switches 54 and 55. The potential at point 43 is now dependent on the proportions of the resistors 42 and 65 and is such that the feedback elements 39, 40 and 41 are causing blocking type oscillations to occur.

The capacitor 40 charges up in a positive direction, periodically cutting olf diode 39 and this causes the oscillations to be of the blocking type. Once an oscillation stops, capacitor 40 discharges through 43 and 32, again rendering diode 39 conductive.

The bistable is a collector to base coupled complementary pair, the collector to base couplings being via resistors 67 and 68. Capacitor 79 is connected between the emitter and base of transistor 51 for purposes of transient suppression. Appropriate base biases are provided by resistors 70 and 71.

Let use now consider the events which occur when the switch sections employ the fixed contacts c in the Fire Only position. Under this condition contact of switch section 44B short-circuits the collector of transistor 51 to ground. Conductor 52 being at ground potential, switches 58 and 59 are no longer effective. That is, the system does not respond to a burglar alarm when it is set for fire only. In the absence of a fire, 43 remains connected to ground via 62, fixed contact c of 44C, 56, 55, 54, 64, 61, 52, 0 contact of 44B, conductor 48 and ground. In the event of a fire, one of switches 54 or 55 will open, ungrounding point 43.

The d contact-utilization or Fire Test condition of the switch sections simulates the conditions of fire only, on the assumption that one of the fire switches is open. That is, it open-circuits line 56. It also connects the collector of 51 to ground 60.

The invention provides:

In combination:

An audio amplifier,

A resistance capacitance- feedback network 41, 40, 39 in said amplifier which is responsive to a first potential (ground) to cause the amplifier to operate as such, to a second potential (a negative one) to cause the amplifier to oscillate, and to a third potential (a less negative one) to cause the amplifier to relaxation oscillate, said network having a control terminal 43 to which said potentials are applied, and means for applying said potentials to said control terminal comprising:

A source of potential having positive and negative terminals (32 and 18),

A voltage divider comprising a first resistor 42 and normally closed fire sensing means 54, -5 and a second resistor 65 in series between the terminals of said source,

Means comprising a normally enabled diode 61 and normally closed burglary sensing means 58, 59 in series across the second resistor 65 for normally placing said control terminal 43 at the first potential (ground), and

Means (the bistable device including 50, 51) responsive to the opening of the burglary sensing means for disabling said diode to place said control terminal 43 at the third potential (the less negative one),

Said control terminal being connected to the first resistor and the fire sensing means (by 62, a of 44C and 56) so that when the fire sensing means is open the control terminal 43 goes to the second potential (the more negative one) by reason of voltage division.

I claim:

1. In combination:

an audio amplifier,

a resistance capacitance-feedback network in said amplifier which is responsive to a first potential to cause the amplifier to operate as such, to a second potential to cause the amplifier to oscillate, and to a third potential to cause the amplifier to relaxation oscillate, said network having a control terminal to which said potentials are applied, and means for applying said potentials to said control terminal comprising:

a source of potential having positive and negative terminals,

a voltage divider comprising a first resistor and normally closed fire sensing means and a second resistor in series between the terminals of said source,

means comprising a normally enabled diode and normally closed burglary sensing means in series across the second resistor for normally placing said control terminal at the first potential, and

means responsive to the opening of the burglary sensing means for disabling said diode to place said control terminal at the third potential, by reason of voltage division,

said control terminal being connected to the first resistor and the fire sensing means so that when the fire sensing means is open the control terminal goes to the second potential.

2. In combination, an audio amplifier,

a source of potential having positive and negative terminals,

means in said amplifier having a control terminal which is normally at a reference potential but which responds to second and third potentials to cause the amplifier to oscillate in distinct modes,

a normally closed fire sensing means,

a normally closed burglary sensing means,

means responsive to the opening of the fire sensing means for placing said control terminal at the second potential, and

means responsive to the opening of the burglary sensing means for placing said control terminal at the third potential,

the means responsive to the opening of the fire sensing means comprising a first resistor between one of said source terminals and said control terminal,

the means responsive to the opening of the burglary sensing means comprising a second resistor in series with the other source terminal and said fire sensing means and said control terminal,

whereby opening of the fire sensing means places said control terminal at the potential of said one source terminal, and closing of the fire sensing means places said control terminal at the potential of the junction between the fire sensing means and said second resistor.

3. The combination in accordance with claim 1,

means for normally holding the control terminal at the reference potential comprising a diode in series combination with said first resistor and said fire sensing means and said burglary sensing means and said other source terminal, and

means for closing this series combination but responsive to opening of the burglary sensing means to disable the diode and to permit the control terminal to assume the third potential determined by said first and second resistors.

4. The combination in accordance with claim 3 in which the last-mentioned means is a bistable device comprising two transistors of which one has a collector-emitter circuit in series with said diode and said burglary sensing means, whereby the combination of said other source terminal, which is at the reference potential, and the burglary sensing means and said collector-emitter circuit and said diode and said fire sensing means and said first resistor normally cooperate to hold said control terminal at said reference potential.

5. In combination, an audio amplifier,

a source of potential having positive and negative terminals,

means in said amplifier having a control terminal which is normally at a reference potential but which responds to second and third direct-current biasing potentials to cause the amplifier to oscillate in distinct modes,

a normally closed fire sensing means,

a normally closed burglary sensing means,

means responsive to the opening of the fire sensing means for placing said control terminal at the second direct-current potential and means responsive to the opening of the burglary sensing means for placing said control terminal at the third direct-current potential,

the means responsive to the opening of the fire sensing means comprising a first resistor between one of said source terminals and said control terminal,

and the means responsive to the opening of the burglary sensing means comprising a second resistor in series with the other source terminal and said fire sensing means and said control terminal,

8 whereby opening of the fire sensing means places said control terminal at the potential of said one source terminal, and closing of the fire sensing means places said control terminal at the potential of the junction 5 between the fire sensing means and said second re- JOHN W. CALDWELL, Primary Examiner P. PALAN, Assistant Examiner US. Cl. X.R.

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