GB2196209A - Supervising alarm circuits at a PBX - Google Patents

Abstract

A micro-computer controlling a PBX provides an alarm function by monitoring loops containing detectors, the micro-computer being switched to and from alarm monitoring mode by codes dialled from an extension. A personal identification may also be required. An alarm condition detected by the micro- computer causes ringing of an alarm bell. One or all of the extensions may also be called. An automatic dialling function may also work its way through a stored list of numbers until a reply is obtained over the exchange line, the nature and location of the alarm being signalled by code or from a voice synthesiser. The alarm can also be recorded on a printer attached to the micro-computer. A door entry phone is also served.

Description

SPECIFICATION Monitoring apparatus The present invention relates to monitoring apparatus, and in particular, to apparatus for monitoring one or more electrical circuits and activating another electrical circuit in the event of a change in state of any of the circuits being monitored.

According to the invention there is provided monitoring apparatus for monitoring a first electrical circuit and for activating a second electrical circuit in the event of a change of state in the first circuit, the apparatus comprising a micro-computer to scan the first circuit, means to store an arming and disarming code to arm and disarm the apparatus, means to compare a received code with the stored code, and means to arm and/or disarm the apparatus on the received code corresponding with the stored code, wherein the apparatus is adapted for connecting to a telephone, the micro-computer being programmable through the telephone, and the arming and disarming code being enterable through the telephone without interferring with the normal operation of the telephone.Preferably, the apparatus includes means for indicating through the telephone, the status of the first circuit on a change of state in the first circuit.

In one embodiment of the invention, the apparatus comprises means for monitoring a plurality of first circuits, and preferably for monitoring four first circuits.

In another embodiment of the invention, the apparatus comprises means for receiving one telephone exchange line. Advantageously the apparatus may be interrogated through a telephone exchange line remote from the apparatus. Conveniently, the apparatus includes means for listening through an "off-hook" receiver of the telephone connected to the apparatus.

One embodiment of the invention includes means to enable the apparatus to be programmed through a telephone exchange line remote from the apparatus.

In another embodiment of the invention, the second circuit comprises an alarm circuit, and preferably, an alarm bell circuit. In another embodiment of the invention, each first circuit comprises circuit loops, comprising any of a number of sensor switches, such as for example, proximity switches, tamper switches, trembler switches, panic buttons, door, window, floor mat switches, smoke detectors and infrared detectors.

Advantageously, the apparatus comprises means to store a number of predetermined telephone numbers, and means to dial the stored numbers in a predetermined sequence on a change of state in any of the first circuits. Preferably, the apparatus comprises means to activate the alarm circuit on a change in state of any of the first circuits. In another embodiment of the invention, means to receive two telephone extension lines are provided, and preferably, means to receive three telephone extension lines are provided. Advantageously, switching means are provided to switch the extension lines. Advantageously, cross-point switching means are provided. In another embodiment of the invention, a telephone bell of one or each of the telephone extensions is activated on a change of state occurring in any of the first circuits.Advantageously, the means to indicate the first circuit which has changed state is provided by delivering an audible signal from the telephone.

Advantageously, the signal comprises a particular number of pulses corresponding to each first circuit. In the case where four first circuits are provided, the first of the first circuits is identified by a single pulse, the second by two pulses, the third by three pulses, and the fourth by four pulses.

Advantageously, the micro-computer is programmable through any one of the extension telephones.

In another embodiment of the invention, the apparatus comprises means to receive dual tone multi-frequency (DTMF) signals from the extension lines. In another embodiment of the invention, means to deliver a decadic and/or DTMF signal on the exchange line is provided in the apparatus.

In a further embodiment of the invention, means to deliver a dial tone is provided in the micro-computer.

In a still further embodiment of the invention, the apparatus is adapted to receive a printer, and preferably, details of the particular first circuit which changed state would be printed by the printer together with the time the change of state took place. In another embodiment of the invention, the apparatus comprises a printer in which the keyboard of a printer is substituted by a cross-point switching means.

In a still further embodiment of the invention, the apparatus comprises a volatile memory. In another embodiment of the invention, the apparatus has means to receive a door phone, and preferably the circuit also comprises means to activate a door lock. Advantageously, the means to activate the door lock is provided by a relay controlled through a relay drive from the micro-computer.

In a still further embodiment of the invention, the apparatus comprises a constant current source for each extension line circuit, the constant current source comprises a transistor, the emitter of which is connected to a voltage supply through a resistor, and the base of which is connected to a reference voltage through a resistor, the reference voltage being retained constant relative to the first voltage supply by a zener diode connected between the first voltage supply and the reference voltage. Further, a second tran sistor is connected with its collector to the first voltage source, its base to the collector of the first transistor, and an output from the constant current source to the extension line being connected to the collector of the first transistor through a resistor, and to the emitter of the second transistor through a second resistor.

In another aspect of the invention, the micro-computer comprises a daughter board connected through a latch for multiplexing the address and data bus between the micro-computer and the daughter board, the daughter board being provided to extend the capacity of the apparatus.

The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of monitoring apparatus according to the invention.

Fig. 2 is a circuit diagram of portion of the apparatus of Fig. 1.

Fig. 3 is a circuit diagram of another portion of the apparatus and Fig. 4 is a circuit diagram of a portion of the apparatus of Fig. 1 which may or may not be fitted to the apparatus.

Referring tithe drawings, and initially to Figs. 1 to 3, there is provided apparatus which in this case is suitable for monitoring four first circuits, namely loops (not shown) of an alarm system (also not shown) of the type used in a house, factory or the like, and for activating a second circuit, namely, an alarm circuit (not shown) in the event of a change of state in any of the loops. The apparatus comprises a micro-computer, namely, IC1 illustrated in Fig. 1. The micro-computer controls the operation of the apparatus, and also carries out the scanning of the loops of the alarm system to be connected to the apparatus, and activates the alarm circuit.The microcomputer IC1 comprises four K bytes of mask programme memory, 256 bytes of RAM, 40 input, output pins and an 8 channel 8 bit analogue to digital converter.

The alarm loops (now shown) are connected into the apparatus through terminals A. It is envisaged that the alarm loops may comprise either one or a number or a combination of any of the following switches: 1. proximity switch, 2. tremble switch, 3. door mat contact, 4. tamper switch, 5. panic button, 6. smoke detector contact, 7. infrared beam contact, 8. infrared and/or ultrasonic detector contact, 9. heat sensors, 10. fluid detectors, 11. pressure detectors.

Indeed, many other types of detectors could be used on their own or in combination with the above. Resistors RG2 and RG3 are provided to facilitate the micro-computer detecting a change of state of any of the alarm loops. A latching relay RL10 activates the alarm circuit (not shown) of the alarm system.

The relay RL10 is driven through a relay drive IC5 by the micro-computer IC1 on detecting a change of state in any of the four alarm loops.

The apparatus also comprises means to receive three extension line telephones which may be connected to extension lines El a, E1b, E2a, E2b and E3a, E3b (see Fig. 2). Provision is also provided for connecting an intercommunicating door phone into the apparatus through the circuit E4a, E4b. Provision for connecting a telephone exchange line into the apparatus is provided by the exchange line circuit La, Lb. These circuits will be described in more detail below. The micro-computer IC1 is programmable through any one of the telephones of the extension lines El to E3. Furthermore, the apparatus is armed and disarmed by entering an arming or disarming code and a personal identification code through any one of the telephones of extension line El to E3.A non-volatile memory IC9 connected to a micro-computer IC 1 stores the arming and disarming code as well as any personal identification numbers and other data as described below. The entered codes and numbers are compared in the microcomputer IC 1 with the stored codes in the non-volatile member IC9.

Before describing the exchange and extension lines circuits and switching means for these circuits, the circuits peripheral to the micro-computer IC1 and the apparatus will be described. The apparatus 1 comprises a power supply circuit indicated by the reference letter D, see Fig. 3. A 220 volt AC mains supply is connected to input terminals LNE of the power supply circuit D. The primary coil Tlp of a transformer TR1 is fed by the live and neutral L and N of the mains supply. A movistor MOV 3 avoids surges in the transformer Tor 1. The secondary coil T1S1 of the transformer TR1 delivers a power supply for ringing the bells on the extension telephone, as will be described below. A secondary coil T1S2 of the transformer TR1 feeds a bridge rectifier BR1, which in turn delivers a +24 volt and a +5 volt supply through regulators RG1 and RG2. Capacitors C18 and C19 are provided to smooth and stablise respectively the voltages.

The micro-computer IC1 and its associated circuits are powered from the 5 volt supply. A 12 MHz crystal XT1 generates the system clock of the mico-computer It 1. A circuit F comprising a resistor R53, capacitor C10 and diode D3 generate the required power on reset signal from IC1 when power is applied.

Resistors R47 and R48 set the external memory mode. The remaining input/output lines on the micro-computer controls the systems electronics in accordance with the E Prom or mask programme.

The switching means which connects the extension lines El, E2 and E3 to the exchange line L comprises relays RL2, RL4 and RL6 controlled through a relay drive IC4 by the microcomputer IC1. Relays RL1, RL3 and RL5 also controlled by the relay drive 1C4 and operated by the micro-computer IC1 deliver a voltage on the extension lines El to E3 from the secondary winding T1S1 of the transformer TR1 to activate the telephone bell of any one of the extension telephones in the event of a change of state in any of the loop circuits.

Internal switching between any of the extension lines El, E2 and E3 and the door phone extension line E4 is carried out by a 4 x 4 x 1 cross-point switch IC2, which is connected to the extension lines at Ex 1, Ex3, Ex4, see Figs. 1 and 2.

Each extension injects special DTMF signals described below into one of four dedicated associated verticals, each extension can be connected via the cross-point switch to any one of four horizontals. Two horizontals are for the two private internal speech paths, while the other two serve to inject dial tone into the extension, or to receive DTMF from the extension for analysis by the DTMF receive circuit. The cross-point switch is under the control of the micro-processor via control lines from port A of the micro-computer.

The extension line circuits El, E2, E3 and the circuit E4 are substantially similar to each other, and it is intended only to describe extension line 1. A constant current source fed from a 24 volt supply comprises a transistor TS1 with associated resistors R1 and R2 connected respectively to the base and emitter of transistor TS1. A reference voltage Vref is connected to the resistor R1 and is derived from a voltage divider circuit comprising a zener diode ZD1 and a transistor R29. Capacitor C5 decouples the reference voltage Vref while resistor R1 buffers the reference when the extension fee circuit is interrupted during dialling or on-hook conditions. In this case, the current through the transistor TS1 is proportional to the voltage Vref and the values of R1 and R2 only.This, thus, presents a high AC impedance at the collector of the transistor TS1. A constant DC current source is provided by the transistor TS2 and associated resistors R5 and R6 in assodiation with transistor TS1.

However, in this case, the circuit presents a fixed low impedance of 600 ohms at the junction of R5 and R6. In this way, DC constant current to save power, and AC finite impedance and telephone impedance matching are realised. Thus, an AC signal injected at the junction of R5 and R6 from the extension phone is presented with very little attenuation to the base of transistor TS2, because of the high impedance of TS1 and can be injected via a decoupling capacitor C1 and DC bias resistors R3 and R4 to the relevant horizontal within the 4 x 4 x 1 cross point switch IC2.

Because the path is a high impedance path, the capacitor C1 can be of low value which has considerable cost, and space saving advantages. Further, a signal injected into the buffer from another extension phone via the cross-point switch IC2 is presented to the base of transistor TS2, which acts as an emitter follower buffer amplifier, and presents a high impedance, low attenuation path to the signal.

The b leg, namely Elb of extension El is connected at Lid1, see Fig. 2 to the input port AN2 of the micro-computer, and a resistor R7 provides a voltage proportional to the current in the extension line El. The micro-computer accepts this information every scan, and processes the received signal to determine the line status, namely off or on-hook, and a decadic digit dialled.

The public exchange line as already discussed is connected into the apparatus through the lines La and Lb. Resistors R34 and R35 and movistors MOV1 and MOV2 provide lightning protection. Relays RL7 and RL8 are controlled by the micro-computer IC1 through the relay drives IC4 and IC5 and provide any one of four terminations to the exchange line, depending on the relative positions of the contacts of the relays RL7 and RL8. When in the position illustrated in Fig. 2, the relays RL7 and RL8 connect the exchange line L onto an extension bus H. Thus, on activation of any of the relays RL2, R14 and RL6, the exchange line can be connected directly to a corresponding extension. When the relays RL7 and RL8 are in the opposite position to that illustrated in Fig. 2, the exchange line is in the idle state.In this case, the line is terminated in a resistor capacitor series termination comprising R36, R37 and C6 to emulate the telephone bell impedance. The exchange line is shorted by activating RL7 and releasing RL8. This condition is used in conjunction with the idle state to generate decadic dialling out to the exchange line. The exchange line can be put on hold by releasing RL7 and activating RL8. This presents a hold circuit indicated by the reference letter Z, which is illustrated in detail in Fig. 1. This circuit serves two purposes. The first being to provide a DC/AC resistance of 600 ohms to the exchange line while an outside call is being transferred from one extension to another.The second purpose is to seize a line and provide an interface for the DTMF transmitter circuit, to enable it to dial DTMF numbers and to provide intercommunication between two similar systems, which will be described below.

A buffer provided by an operational amplifier IC10 connected across the exchange line La Lb to resistors R38 and R39 is connected to one of eight analogue to digital convertor inputs of the micro-computer IC 1 so that the micro-computer can monitor the exchange line status. A voltage proportional to the line voltage presented at the buffer input terminals is provided at the buffer output, and this is presented to the microcomputer IC1. The microcomputer IC1 has a facility for barring certain predetermined calls which may be stored in the non-volatile member IC9, and also it has a facility for repeating the last number dialled, and this is also stored in the non-volatile memory.Accordingly, numbers being dialled are fed to the micro-computer IC 1 through the buffer IC10 and analysed within the microcomputer It 1.

Dial tone is generated by the micro-computer IC 1 and is delivered through the buffer IC10, see Fig. 1 and its associated circuitry resistors R50, R51 and R52 and capacitors C13 and C14 to any one of the extension lines through the cross-point switch IC2. A DTMF receiver provided by integrated circuit IC6 receives DTMF signals from the horizontal of the cross-point switch IC2. Under the control of the micro-computer IC1 an extension is selected and poled to examine the AC content of its signal for DTMF. This is done by enabling the cross-point switch IC2 associated with the relevant extension and the DTMF Rx integrated circuit IC6. This circuit filters out the dual frequency, and checks them for valid frequency and duration and strength of signal.

A resistor R56 and a capacitor C16 determine these parameters. On validating the DTMF signal, the integrated circuit IC6 then informs the micro-computer via the STD output. The micro-computer accepts a binary four bit input via pins Q1 to Q4, which represent the digit selected. The micro-computer then opens the cross-point switch and selects a new extension or waits for more digits to be dialled.

A DTMF transmitter circuit comprises a logic integrated circuit lC7 and a generator integrated circuit IC8. The micro-computer IC1 transmits DTMF tones out to the exchange line first by putting the exchange line onto hold.

The DTMF signal is transmitted through the hold circuit Z.

The micro-computer IC1 also has a facility to communicate with a corresponding microcomputer IC1 of another apparatus at another location over an exchange line. Thus, in the event of one of the alarm loops changing state, the micro-computer can dial any one of a list of telephone numbers stored in the nonvolatile memory IC9 in a predetermined sequence, and on being connected to the dialled number, the data is transmitted on the exchange line in DTMF form from the generator IC8.

Three ports PCO, PC 1 and PC2 of the micro-computer IC1 are provided for connecting to a printer. This would be an optional printer and may or may not be connected.

A 4 x 4 cross-point switch IC3 is provided to operate a calculator printer. In use, the keyboard of the calculator is removed, and replaced by the cross-point switch IC3.

A 50 Hz detector circuit K is provided to calculate the cost of a call in progress. The circuit K receives pulses from the public exchange on the exchange line La and Lb and these are counted in the circuit K.

A relay RL9 controlled by the micro-computer IC1 through relay drive IC5 is provided to activate a door lock, should this be desired.

The micro-computer IC 1 is programmed to scan all the input and output ports every five milliseconds, and any change in inputs is recorded, if required, every five milliseconds.

The scan takes in inputs associated with the alarm loops, the exchange line, the extension lines and the door phone, the DTMF receiver and the 50 HRz detector. These inputs are processed in the micro-computer IC1 and output control signals are delivered to relay drivers, dial tone generator and cross-point switches, DTMF generator and electrical erasible prom memories which in turn control the operation of the alarm circuit, the printers the exchange line and extension lines. The control programme is arranged so as to store in internal RAM the status of the system components and change that state to a new state when a particular sequence of inputs is received during successive scans. The scan timing is generated by an internal timer within IC1.

In use, the apparatus is armed by taking any one of the extension telephones (not shown) off hook. A designated digit is then dialled to access the micro-computer 1. A personal identification number is then dialled on the extension and entered into the micro-computer 1. This is compared with the personal identification number in the non-volatile member IC9, and if the number compares, the apparatus may then be armed by dialling the appropriate digit or digits to arm the apparatus. On the micro-computer IC1 detecting a change of state in any of the alarm loops, the alarm circuit relay RL10 is activated to sound the alarm. The relays RL1, RL3 and RL5 are also made to deliver a voltage from the secondary winding T1S1 of the transformer TR 1 onto the extension lines El to E3 to activate the bell of the extension telephones. If desired, the microcomputer iC1 may be programmed to activate only one of the relays RL1, RL3 or RL5 and thus the bell of only one extension telephone. The micro-computer then dials out the first of the sequence of the predetermined numbers stored in the non-volatile memory IC9, and continues to dial until communication is established with one of the numbers. Date on the alarm loop which has changed state and the time at which it changed state is then transmitted in DTMF to the corresponding apparatus at the other end of the exchange line.When either or both house-holders lift an extension off the hook, data on the alarm loop which has changed state is delivered to the telephone. This is delivered by electrical pulses which give audible signals in the telephone. If the first of the alarm loops has changed state, a single pulse is transmitted. If the second has changed state, two pulses are given, and so on. Where the apparatus is provided with a printer, or a printer is connected to it, then the number of the alarm loop and the time at which it changed state is printed out. If the apparatus is provided with a voice synthesizer then a message may be transmitted through the telephone system.

Where it is desired to talk between extensions, then the appropriate number is dialled for the particular extension, and the microcomputer IC1 activates the cross-point switch IC1 to connect the relevant extensions. Where it is desired to dial an outside line in the normal course of events without affecting the monitoring apparatus, any of the extensions are lifted off hook, and the exchange line is seized. The call is then dialled in the normal way, and provided it is not a barred number, the micro-computer IC1 allows the call to proceed.

Where one wishes to enter barred numbers into the non-volatile memory IC9 this is done by lifting any of the extensions off hook and dialling a predetermined digit. The barred numbers are then entered into the memory IC9.

The telephone numbers and the sequence in which they are to be dialled by the microcomputer IC1 in the event of a change of state of an alarm loop, are similarly entered by lifting any of the extension phones off hook, dialling a predetermined digit, followed by the digits that are to be dialled in the desired sequence.

When it is desired to disarm the apparatus, an extension phone is lifted off hook, a predetermined digit is dialled, followed by the personal identification number of the user, and then followed by a disarming code. Whether the apparatus is in the arm mode or disarm mode, incoming calls can be received, and outgoing calls may be made, as well as internal calls between extension telephones can also be made.

Should it be desired at any stage to change the security code, this is done by entering the old code and following this by the new code.

At any stage whether the apparatus is armed or disarmed, should a caller ring the door bell of the door phone, this is detected by the micro-computer Inc1, and any one or ail of the three extensions El, E2, and E3 are rung.

Communication can then be made between the door phone and the appropriate extension.

Should it be desired to permit the caller to gain entry, the door lock is activated by dialling a predetermined digit, and the micro-computer IC1 through the relay drive IC5 activates the door lock relay RL9.

Referring now to Fig. 4, there is illustrated an alternative arrangement for connecting the micro-computer IC1 into the circuit of Fig. 1 of the apparatus. In this particular embodiment of the invention, additional memory capacity is provided by an additional integrated circuit IC3, which is connected as a daughter to the micro-computer IC1. A latch IC2 is connected between the micro-computer IC1 and the integrated circuit IC3 to multiplex the address and data bus. input and output ports IC5 and IC6 are provided to compensate for the lost ports.

The circuit as is illustrated in Fig. 4 is connected into the circuit of Fig. 1 in place of the micro-computer IC1 through a 64 way QIL connector. While the invention has been described as comprising features such as the 50 Hz detector circuit, the connections for an external printer, or indeed, a facility for connecting a printer operating a calculator printer through the cross-point switch IC3, these features, if desired, could be dispensed with. Furthermore, it will be appreciated that other forms of delivering ringing onto the telephone in the event of an alarm state being detected could be provided and indeed, in certain case, it could be dispensed with. It will also be appreciated that any other suitable power supply could be used. Indeed, in certain cases, it is envisaged that any other suitable power supply could be used.Indeed, in certain cases, it is envisaged that any other suitable power supply could be used. Indeed, in certain cases, it is envisaged that the power supply may be provided by a battery, and in many cases a standby battery may be provided.

Additionally, it will be appreciated that while a particular combination of relays and crosspoint switches have been described as providing switch means between the relays and the extensions and the like, any other suitable arrangement of switch means or combination of switch means could be provided.

While the apparatus has been described as having a facility of being able to comnunicate with a similar apparatus at a remote location through an exchange line, this is not necessary. Furthermore, while the apparatus has been described as having a facility to prevent certain barred calls and repeat dialling of last number, this facility could also be dispensed with without departing from the scope of the invention.

The apparatus includes a listening in feature, whereby it is possible for a user to listen in through an "off-hook" receiver of a telephone extension by dialling the number of the tele phone through the telephone exchange and then dialling an access code. This feature enables the user to check on the status of his home or office through the telephone system remote from his home or office.

Claims (29)

1. Monitoring apparatus for monitoring a first electrical circuit and for activating a second electrical circuit in the event of a change of state in the first circuit, the apparatus comprising a micro-computer to scan the first circuit, means to store an arming and disarming code to arm and disarm the apparatus, means to compare a received code with the stored code, and means to arm and/or disarm the apparatus on the received code corresponding with the stored code, wherein the apparatus is adapted for connecting to a telephone, the micro-computer being programmable through the telephone, and the arming and disarming code being enterable through the telephone, without interfering with the normal operation of the telephone.

2. Monitoring apparatus as claimed in Claim 1 including means of indicating through the telephone the status of the first circuit on a change of state in the first circuit.

3. Monitoring apparatus as claimed in Claim 1, in which the apparatus comprises means for monitoring a plurality of first circuits, and preferably for monitoring four first circuits.

4. Monitoring apparatus as claimed in Claim 1, in which the apparatus comprises means for receiving one telephone exchange line.

5. Monitoring apparatus as claimed in Claim 4 in which the apparatus may be interrogated through a telephone exchange line remote from the apparatus.

6. Monitoring apparatus as claimed in Claim 5 in which the apparatus includes means for listening through an "off-hook" receiver of the telephone connected to the apparatus.

7. Monitoring apparatus as claimed in any one of Claims 4 to 6 including means to enable the apparatus to be programmed through a telephone exchange line remote from the apparatus.

8. Monitoring apparatus as claimed in any one of the preceding claims, in which the second circuit comprises an alarm circuit, and preferably, an alarm bell circuit

9. Monitoring apparatus as claimed in any one of the preceding claims, in which each first circuit comprises circuit loops, comprising any of a number of sensor switches, such as for example, proximity switches, tamper switches, trembler switches, panic buttons, door, window, floor mat switches, smoke detectors and infrared detectors.

10. Monitoring apparatus as claimed in-any one of the preceding claims in which the apparatus comprises means to store a number of predetermined telephone numbers, and means to dial the stored numbers in a predetermined sequence on a change of state in any of the first circuits.

11. Monitoring apparatus as claimed in any one of the preceding claims in which the apparatus comprises means to activate the alarm circuit on a change in state of any of the first circuits.

12. Monitoring apparatus as claimed in any one of the preceding claims in which means to receive two telephone extension lines are provided, and preferably, means to receive three telephone extension lines are provided.

13. Monitoring apparatus as claimed in Claim 12 in which switching means are provided to switch the extension lines.

14. Monitoring apparatus as claimed in Claim 13 in which cross-point switching means are provided.

15. Monitoring apparatus as claimed in any one of Claims 12 to 14, in which a telephone bell of one or each of the telephone extensions is activated on a change of state occuring in any one of the first circuits.

16. Monitoring apparatus as claimed in any one of Claims 2 to 15, in which means to indicate that the first circuit has changed state is provided by delivering an audible signal from the telephone.

17. Monitoring apparatus as claimed in Claim 16 in which the signal comprises a particular number of pulses corresponding to each first circuit.

18. Monitoring apparatus as claimed in Claim 16 to 17 in which the signal is transmitted to a voice synthesizer for producing audible message.

19. Monitoring apparatus as claimed in any one of the preceding claims, in which the micro-computer is programmable through any one of the extension telephones.

20. Monitoring apparatus as claimed in any one of the preceding claims, in which the apparatus comprises means to receive dual tone multi-frequency (DTMF) signals from the extension lines.

21. Monitoring apparatus as claimed in any one of the preceding claims, in which means to deliver a decadic and/or DTMF signal on the exchange line is provided in the apparatus.

22. Monitoring apparatus as claimed in any one of the preceding claims, in which means to deliver a dial tone is provided in the microcomputer.

23. Monitoring apparatus as claimed in any one of the preceding claims, in which the apparatus is adapted to receive a printer, details of the particular first circuit which changes state are printed by the printer together with the time the change of state took place.

24. Monitoring apparatus as claimed in any of the preceding claims, in which the apparatus comprises a volatile memory.

25. Monitoring apparatus as claimed in any of the preceding claims, in which the appara tus has means to receive a door phone, and the circuit also comprises means to activate a door lock.

26. Monitoring apparatus as claimed in Claim 25 in which the means to activate the door lock is provided by a relay controlled through a relay drive from the microcomputer.

27. Monitoring apparatus as claimed in any one of the preceding claims, in which the apparatus comprises a constant current source for each extension line circuit, the constant current source comprises a transistor, the emitter of which is connected to a voltage supply through a resistor, and the base of which is connected to a reference voltage through a resistor, the reference voltage being retained constant relative to the first voltage supply by a zener diode connected between the first voltage supply and the reference voltage, a second transistor is connected with its collector to the first voltage source, its base to the collector of the first transistor, and an output from the constant current source to the extension line being connected to the collector of the first transistor through a resistor, and to the emitter of the second transistor through a second resistor.

28. Monitoring apparatus as claimed in any one of the preceding claims, in which the micro-computer comprises a daughter board connected through a latch for multiplexing the address and data bus between the micro-computer and the daughter board, the daughter board being provided to extend the capacity of the apparatus.

29. Monitoring apparatus substantially as herein described with reference to and as shown in the accompanying drawings.