EP0649551A1

EP0649551A1 - Alarm circuit

Info

Publication number: EP0649551A1
Application number: EP93916043A
Authority: EP
Inventors: Patrick Hugh Carey
Original assignee: CAREY Patrick Hugh; WILES Barrington John
Current assignee: CAREY Patrick Hugh; CONNER, PETER; WILES Barrington John
Priority date: 1992-07-09
Filing date: 1993-07-09
Publication date: 1995-04-26
Also published as: GB9214600D0; AU4575593A; WO1994001843A1

Abstract

An alarm device for an electrical apparatus is provided which provides an alarm signal when the electrical apparatus is moved whilst unplugged from its electrical supply. The alarm device may be supplied with an over-ride means for preventing the initiation or continuation of the alarm signal. The alarm device can be provided inside the protected electrical apparatus, or it can be provided as a separate device which is fixed securely to the protected apparatus.

Description

ALARM CIRCUIT

The present invention relates to an alarm device. More particularly the invention relates to an alarm device for electrical equipment which is supplied with electrical power from an external source such as the mains system.

There are many small and portable mains-connected apparatus such as hi-fi equipment, computers and televisions. These pieces of equipment tend to be valuable and easily disposable and hence are attractive to thieves. Shops, offices and hotels all encounter problems in preventing the theft of these items since they may easily be removed by unplugging the piece of equipment from the mains and carrying it away. Previously various methods have been tried to prevent such theft occurring. For example many shops chain their valuable hi-fi equipment to the display stands to prevent theft. However this is very cumbersome and prevents easy transfer of equipment when required. In addition security is not assured since by mistake the chains may not be secured. Furthermore a determined thief would be able to cut through most available mechanical locking mechanisms with the correct equipment. This invention thus seeks to provide a reliable automatic alarm device for electrical equipment. According to the present invention there is provided an alarm device for an electrical apparatus supplied with electrical power from an external source, having a detector to detect whether the electrical apparatus is connected to the external source; a sensor to sense movement of the electrical apparatus; and alarm means to provide an alarm signal when the detector detects that the electrical apparatus is not connected to the source and the sensing means detects movement of the electrical apparatus. This alarm device therefore prevents the movement of the electrical apparatus once the external electrical supply has been removed.

The alarm device may provide an alarm signal to an external alarm system when the detector detects that the electrical apparatus is not connected to the external source and the sensing means detects movement of the electrical apparatus.

This enables different electrical devices to be monitored from a central location.

Further the alarm device may provide an alarm signal to the external alarm system and to an internal alarm generating means alternately.

The alarm device may further include a timer means which causes the alarm signal to be discontinued when a predetermined time has elapsed since the alarm signal was initiated.

This feature minimises the power drain on the alarm circuitry and minimises annoyance to passers-by while providing an effective alarm deterrent.

In an advantageous embodiment the alarm signal may be re-initiated after the predetermined time if the detector detects that the apparatus is not connected to the source and further movement is detected. In this way continued protection is afforded against the theft of the electrical apparatus after the alarm has been set off and then discontinued.

The alarm device may further include an over-ride means for preventing the initiation or continuation of the alarm signal.

This feature is particularly useful to allow the authorised transportation of the electrical apparatus without the alarm signal being provided.

Preferably the override means can only be operated while the detector detects that the electrical apparatus is connected to the external source. For a better understanding of the present invention, and to show how it may be brought into effect, reference will now be made, by way of example, to the following drawings in which: Figure 1 shows a circuit diagram of a first embodiment of this invention; and

Figure 2 shows a flow diagram of the operation of this invention.

Figure 3 shows a circuit diagram of a second embodiment of this invention.

Figure 4 shows a circuit diagram of a third embodiment of this invention.

A first embodiment of the invention will now be described with reference to Figure 1. This embodiment of the invention would be suitable for detection of the main electrical supply to, for example, a computer.

The alarm circuit of this embodiment is provided with an input control voltage of 5-7 Vdc which is derived from the main electrical supply to the protected apparatus. Whilst this voltage is chosen because a 5V rail is usually provided on electrical apparatus the actual input voltage chosen is not important. Integrated circuit IC1 is a dc-dc converter which converts the input control voltage to an output voltage of approximately 12 Vdc.

The output of IC1 is applied to a voltage regulator IC2, which in this instance is configured as a constant current source. The output of IC2 is supplied to pin 2 of integrated circuit IC3 through a resistor Rl and a diode Dl. The diode Dl prevents any reverse current flowing back into IC2, and in this embodiment Dl is a light emitting diode to give a visual indication of the charge status of the batteries provided for the alarm circuit.

In this embodiment IC3 is a conventional power supply sentinel, such as is used to provide automatic power supply backup. IC3 is supplied with a reference voltage direct from IC1 through a diode D2 on pin 8. The voltage applied to pin 8 of IC3 is limited by a zener diode D3.

IC3 obtains its power through pin 2 from the voltage regulator IC2 when the input control voltage is applied, or from the batteries Bl when the control voltage is not applied. The alarm circuit comprises batteries Bl, a disable switch SI and an integrated circuit IC4 together with its associated circuitry. The alarm circuit further includes a movement switch TGI which may be a vibration, tilt, or similar switch, and also an alarm device BZ1 such as a sounder.

The batteries Bl provide the necessary current to produce the alarm signal. The batteries Bl may be rechargeable so that they may be recharged slowly from the control voltage when the alarm is in its inactive state while the control voltage input is present. If the batteries are run down or have been removed, a sufficient voltage will not be applied to IC3 and the alarm circuitry to permit operation and the alarm device will not operate. In this embodiment IC4 is a timer operating in a monostable mode.

The operation of the alarm circuit will now be explained in more detail.

When the protected apparatus is disconnected from its electrical supply, the control voltage will no longer be applied to IC1 and therefore the reference voltage will no longer be present at pin 8 of IC3. When the absence of the reference voltage at pin 8 of IC3 is detected, pin 3 of IC3 is enabled to supply the earth feed for IC4. Provided that disable switch SI is closed, the electrical circuit around IC4 is now complete and the alarm device is armed. If however, disable switch SI is open the electric circuit is no longer complete and the alarm cannot be sounded. The disable switch may be provided to enable the protected apparatus to be moved when required without setting off the alarm. If provided, the disable switch is advantageously a key switch to ensure security and to minimise the possibility of unauthorised people disabling the device. In order to prevent the false triggering of the alarm circuit when the power is supplied initially, a resistor R3 and capacitor C3 network may be connected across the power rails of IC4. The connection point of the resistor R3 and capacitor C3 is connected to the reset pin of IC4. When the power is initially applied the capacitor C3 charges up through resistor R3. In this way reset pin 4 of IC4 is kept low for a delay period which is dependent upon the time constant of the R C3 network, and thus the condition of the alarm circuit is allowed to become settled before becoming active. Provided sufficient voltage remains applied to IC4 the delay network R3C3 has no further effect on the operation of the circuit.

If the disable switch SI is closed, the circuit will remain in an armed condition until the power is reapplied, or until the protected apparatus is moved and the movement switch TGI closed so setting off the alarm. If the power is reapplied, the reference voltage will reappear on pin 8 of IC3, and pin 3 will be disabled, thus breaking the circuit and disarming the device.

During normal operation pin 2 of IC4 is pulled high through R4. However, when the movement switch is closed, indicating movement of the protected apparatus, a negative spike is applied to pin 2 of IC4 through the capacitor C3 which causes pin 3 of IC4 to go high. The diode D4 is provided to limit the amplitude of any positive spikes.

The first side of a capacitor C4 is connected to pin 2 of IC4 while the second side of C4 is connected to movement switch TGI, and also to the positive power supply through R6.

In the event that unauthorised movement of the protected apparatus is attempted, then the protected apparatus may be abandoned in a tilted position. If, for example, a tilt switch has been used as the movement detector, the tilt switch would subsequently indicate unauthorised movement at all times. This would result in the alarm continuously sounding, falsely indicating an alarm signal and the batteries would become exhausted in a short time. This present arrangement avoid this problem and allows the alarm to be repeatedly triggered when the protected apparatus is moved.

The transistor Tl is preferably configured as a Darlington pair to provide fast switching and high current carrying capacity. Tl is switched "ON" when pin 3 of IC4 goes high, and a current flows through the loudspeaker, thus causing an alarm sound to be emitted. The resistor R5 limits the current supplied to the base of Tl, and hence limits the current through Tl and the sounder to approximately 90 mA.

It may be advantageous to discontinue the alarm signal, once initiated, after a set time in order, for example, to conserve the battery power for longer life. The required timing operation is performed by a resistor R2 and a capacitor Cl. The capacitor Cl is charged via a resistor R2 and the voltage on capacitor Cl is compared to a reference voltage supplied to pin 5 by capacitor C2. After the set time, which is determined by the time constant of the RC network, the alarm circuit is reset and the alarm discontinued. However as long as the reference voltage has not been reapplied to pin 8 of IC3 and the circuit has not been disabled via switch SI, the alarm circuit is still armed and further movement will cause reinitiation of the alarm signal.

The operation of an alarm device according to this invention will now be described with reference to Figure 2.

The alarm device according to this invention is first installed in the electrical apparatus which is to be protected. The alarm device is electrically connected to the protected apparatus in such a way that a control voltage indicative of the external electrical supply to the protected apparatus is supplied to the detection means on the alarm device. The control voltage may be obtained in any way although it may be convenient to supply the detection means of the alarm device from the voltage rails that are present in most pieces of electrical equipment. The alarm device may advantageously be manufactured on a pcb and installed into the protected apparatus. This would be particularly advantageous for apparatus such as computers, since the manufacturer may supply the apparatus as protected apparatus with the alarm device already supplied or, alternatively, the owner or user of a computer will be able to add the appropriate board easily.

Alternatively the alarm device may be supplied as a separate device to be attached to the apparatus to be protected. This may be better for protecting apparatus such as televisions which have already been purchased. In this situation the alarm device may be connected between the power source and the protected apparatus to enable monitoring of the power supply to the protected apparatus. It is preferable that the voltage monitored by the alarm device is of low voltage to make the use of the alarm device safe. The alarm device must be attached to the protected apparatus so as to enable accurate detection of the movement of the protected apparatus. After installation or attachment of the alarm device in stage 1, the alarm device monitors the power supply to the protected apparatus in stage 2 by means of the presence of the control voltage at its detector inputs. When the detector detects, in stage 2, that the control voltage is present, the alarm enters its inactive state (stage 3) . While the alarm is in the inactive state it continually monitors the power supply (stage 2) . When the detector detects in stage 2 that the control voltage is no longer present, and hence that the power has been removed from the protected apparatus, the alarm device enters the alarm armed state (stage 4) .

In the alarm armed state the alarm device is armed and ready to sound. However, the alarm will not sound unless an unauthorised movement of the protected apparatus is detected. If an unauthorised person attempts to remove the protected apparatus by unplugging it and transporting it the alarm will sound. However it is possible that the owner will wish to move the protected apparatus himself. It is observed that unless the power has been switched off the protected apparatus may be moved around freely. Once the power has been switched off the alarm device is in the armed state and the protected apparatus may not be moved. An override switch may therefore be provided which prevents the alarm sounding when the protected apparatus is moved after having been disconnected from the external supply. An effective deterrent to opportunist thieves is therefore provided, while at the same time producing minimal interference to legitimate users. The armed alarm device detects whether the power has been reapplied to the protected apparatus by checking the control voltage input to the detector in stage 5. If the voltage has been reapplied the alarm device returns to the inactive state (stage 3). If the voltage is still absent, the alarm device determines whether authorised movement is intended by checking in stage 6 the status of the override switch. If authorised movement is intended the alarm device monitors whether the override has been released in stage 7. When the override has been released, and normal operation of the alarm device is desired, the alarm device once more ascertains in stage 2 whether the control voltage is present. If the override switch is not set, the alarm device determines whether the protected apparatus is being moved in stage 8. If the protected apparatus is not being moved the alarm device remains in the armed state and rechecks the power, override and movement status again. However, if movement occurs the alarm device enters the alarm trigger state, stage 9, and an alarm signal is sounded.

Once the alarm has been triggered, the alarm device may determine in stage 10 whether the alarm has sounded for a predetermined period. If it has the alarm signal is cancelled and the alarm device is returned to the alarm armed state in stage 4, and any further movement of the apparatus will be detected at stage 8 and retrigger the alarm. The predetermined period may be for example 10 minutes, and may be varied depending on the application. The cancellation of the alarm signal after the predetermined time is provided so as to limit the power requirements of the alarm and therefore to provide an improved life for the alarm device, but may be omitted in certain circumstances. Furthermore in stage 11 the alarm device may check whether the power has been reapplied and, if so, return the alarm device to the inactive state (stage 3) . In addition the override status may be checked in stage 12 and if the override is set the alarm signal is cancelled (stage 13) and the release of the override is monitored in stage 7 as described above.

A second embodiment of the invention will now be described with reference to Figure 3.

This embodiment of the invention would be suitable for an external unit encased in a purpose made box which could be attached securely to the equipment to be protected. In contrast to the first embodiment of the invention, the external unit is powered directly from the power input to the electrical equipment, and has conversion circuitry within the unit to convert the input voltage to the required voltage for the alarm circuitry. The input voltage is most easily taken from the live and neutral wires of the main supply to the protected equipment. In this case, the input voltage is 240 Vrms AC in the United Kingdom. The input voltage is supplied to pins 1 and 8 of voltage converter IC101 through FS101, resistor R101, and resistor R102. Resistor R101 and resistor R102 are provided to limit the peak current which occurs when charging capacitor C102 from a fully discharged state. Resistor R103 is also provided to ensure that the peak current does not exceed the absolute maximum current allowed by the device.

Protection of the voltage conversion circuit is achieved by voltage dependent resistor VDR1 provided to clamp any spikes on the mains input, while resistor R101, resistor R102, and capacitor C101 provide a low pass filter to limit the rate of rise of the voltage across the input of IC101. Capacitor C102 is the preregulator capacitor and is charged once per line cycle. Capacitor C103 prevents the IC101 from turning on during any large input voltage transient. Capacitor C104 is the output filter capacitor. This capacitor can be small in view of the low current operation of the alarm unit: the current is typically equal to or less than 5 mA. Zener diode ZD101 adjusts the output from IC101 to around 12 Vdc.

This circuitry thus provides a safe and reliable dc control signal to the alarm circuitry from the main supply to the unit.

The circuit described above is universal i.e. it operates from an AC voltage input of between 90 and 260 Vrms with a frequency range of 48Hz - 400Hz and so it is not necessary to provide voltage selection via a user operated switch. In effect the unit be connected to virtually all forms of mains power sources currently available around the world without modification. However it may be necessary for some circuit arrangements embodying the invention to provide a selection switch to enable the user to select different supplies in accordance with the electrical supply suitable for the protected equipment. Obviously, it may be necessary to adjust the circuitry described above to accommodate sources of power other than the mains.

The control signal from the power supply is applied to the alarm unit. When present, this signal controls two functions of the unit.

The first function of the control signal is to replenish the internal power of the unit. This is achieved by a simple network of diode Dl and resistor R2 which limits the flow of current to the battery Bl to around 1 mA. This is the recommended continuous charge current for the battery Bl. Diode Dl also prevents any reverse flow of current back into the control signal source. The second function of the control signal is to disarm the alarm via diode D2, resistor R2 and the input of optocoupler IC1.

Optocoupler IC1 functions to arm and disarm the alarm unit in response to the absence or the presence of the control signal derived from the main supply, which control signal indicates whether the unit is switched on or not. When the control signal is present at its input, the output transistor of optocoupler IC1 is biased to the on state. The collector of the output transistor is connected to pins 3 and 4 of monostable IC2. When pins 3 and 4 of monostable IC2 are in the low state (i.e. the control signal is present) monostable IC2 is deactivated. However, when the control signal is not present pins, pins 3 and 4 of monostable IC2 are in the high state and the alarm unit is armed. Monostable IC2 is configured as two independently triggered devices, one triggered on a positive edge signal and the other on a negative edge signal. Once monostable IC2 is activated, either of the monostables can be triggered by mercury loaded vibration detection switch SWl connected between pins 4 and 11 of monostable IC2 and ground. Resistor R6 and capacitor C3 provide a simple method of extending the positive- and negative- going duty cycles of the trigger inputs. Once triggered, each monostable output pulse duration is controlled by their respective RC network, and in both monostables this takes the form of a 10 M resistor and a 100 microfarad capacitor. Diodes D3 and D4 provide a discharge path for capacitors Cl and C2 respectively should the unit suffer a complete absence of power. These component values give an approximately 4 to 5 minutes duration output pulse. In the circuit illustrated, only the positive going output pulse is utilised. Each positive output from the monostable is taken through a diode D5 or D6. The output of these diodes is connected to transistor TRl via resistor R7. As a result once the alarm is triggered by switch SWl, the monostable IC2 generates an output which turns transistor TRl on. Transistor TRl provides a ground return to the alarm output signal generator part of the circuit, comprising IC3, IC4, transistor TR2, and transformer TF1 and their associates components. Thus the triggering of the alarm by SWl causes transistor TRl to switch on which in turn causes the alarm to be sounded by the alarm signal generator.

IC4 is configured as an astable multivibrator operating at around a central frequency of 3 kHz. However, IC3 is also an astable multivibrator, and operates at around 5 Hz. IC3 operates to control the output frequency of IC4 by providing a ramp control voltage at pin 5 of IC4 via resistor R8 and capacitor C7. The output of IC4 feeds transistor TR2 via resistor R13, and TR2 acts to switch pin 3 of transformer TF1 between VCC and ground. As transformer TF1 is configured in the step up auto transformer mode, this results in a varying frequency (warble tone) of around 80 volts peak to peak being applied across PIEZO element device SPK1. The noise level output from this unit is approximately 105 dB.

Once triggered the alarm will continue to sound for a time dependent on the duty cycle of the monostable IC2. Once this duty cycle has been completed the alarm will again assume the stand by alarm active state. However, once triggered, any further movement will restart the alarm time i.e. the time out period of the alarm will be approximately 4 minutes from the latest movement.

The alarm signal can be terminated at any time by simply reconnecting the protected electrical equipment to its power source. This has the effect of forcing the output of the optocoupler IC1 to a low state.

A key switch SW2 is provided for disarming the unit, but this can only be achieved when pins 3 and 13 of monostable IC2 are held low. This means that the operational state of the switch may only be altered when the control signal from the protected electrical equipment is present. Any change of state to this switch when the control signal is absent will trigger an alarm. It will be appreciated by those skilled in the art that many arrangements other than those described above are possible.

Furthermore, it may be advantageous, in certain circumstances, to operate an alarm other than a siren contained within the alarm device when the alarm is triggered. For example, it may be desirable for the alarm unit to interact with a burglar alarm system or another remote alarm system. Further the alarm unit may be configured such that it is able to switch alternately between the remote alarm and the alarm sounding circuitry of the alarm unit itself.

A third embodiment of the invention will now be described with reference to Figure 4.

This embodiment allows a switching interface between the alarm device and an external device i.e. a burglar alarm, an external warning device or an autodial machine which dials police or other security personnel.

Circuitry for supplying the control voltage to the alarm unit (diode Dl and resistor Rl ), circuitry to arm and disarm the alarm (optocoupler ICI and associated circuitry) and circuitry for sounding the alarm (astable multivibrators IC4 and IC5, transistor TR3, transformer TF1, PIEZO element device SPK1 and associated circuitry) are similar in operation to that described above in connection with Figure 3, and a detailed discussion thereof will be omitted here.

The function of monostable IC2 B is similarly unchanged from that described above, and thus functions of the monostable IC2B causing the alarm to trigger when trigger switch SWl detects movement whilst the monstable IC2 is enabled by the optocoupler (which occurs when the power supply to the protected equipment has been removed) will not be described again. The operation of monostable IC2 A has been modified by the waveform shaping circuit, comprising resistor R6, capacitor C5 resistor R7 and diode D4, at the trigger input pin B. This circuity is necessary to maintain the anti-tamper function of keyswitch SW2.

Now, when triggered, monostable IC2 A provides an output to IC3, which is a four input AND gate. The AND gate is configured so that its operation depends on the condition of two pins only, the other two pins being tied to VCC. One of these operating pins is connected to the Q output of monostable IC2 A, and the other is connected to the Q output of monostable IC2B.

When monostable IC2 A pin Q switches to a high state and the output of monostable IC2 B pin Q remains static, i.e. also a high state, AND gate IC3 will provide an output to resistor R9 which, in turn, will turn on transistor TR2. Transistor TR2 activates Relay RA1 for a preset period of time determined by the R.C. network of resistor R3, capacitor Cl, capacitor C2 and Link 1.

The Link component allows two user selectable time constants to be provided: the values shown result in time constants of approximately 4 seconds and 8 seconds, but clearly other values may be chosen if desired.

The outputs of relay RA1 are presented to external devices in the form of a multi pin socket, enabling this device to interface with a multitude of external device Thus, this method provides a safe and completely isolated switching circuit which will neither allow an external voltage condition to affect operation of the alarm device, nor allow an internal voltage condition to be supplied to an external device.

As mentioned before, the output of IC2 A is governed by a user selectable time constant. However, even though a high signal is present at IC2 A pin Q, no signal will be output by AND gate IC3 unless pin Q of IC2 B is also high, which only occurs when the device is in the non triggered state.

Thus operation of relay RA1 is prevented whilst the alarm cycle is in progress since when the alarm signal is triggered the output of IC2 B pin Q is brought low, thereby preventing AND gate IC3 from providing an output signal to transistor TRl via resistor R8.

No conflict of switching is therefore experienced as the operation of monostable IC2 'B' only becomes active on the trailing edge of the output pulse on pin Q of IC2 'A' .

The preceding method has been used in order to minimise the current drain on battery Bl, by ensuring that the alarm sounding circuit and the relay RA1 are not both active at the same time.

As with the original circuit the retrigger function has been retained. However, as the sound circuit timer constant is approximately 4 minutes and the time constant of IC2 A is between 4 and 8 seconds, retriggering will only affect the alarm by extending the duration of the alarm, and will not affect the condition of relay RA1 since the output at Pin Q of IC2 B is low.

Preferably, in any embodiment of the invention the user is able to select the duty cycle of the alarm and is also able to select whether the alarm is connected to the second remote alarm or is connected to the alarm sounding circuitry of the alarm unit itself.

In order to provide the longest working period of the batteries on standby operation or during the alarm period, it is advantageous to use low-current components e.g. CMOS devices in the alarm device.

In addition in some circumstances it may be advantageous to incorporate a warning device which indicates that the alarm has been triggered since the power was last switched off.

Furthermore a confirmatory signal indicating that the alarm device has been armed may also be provided to provide reassurance of protection.

While the preceding description of the invention have been described in terms of voltage detection of the supply voltage it may in some circumstances be advantageous to detect the current supply.

Claims

1. An alarm device for an electrical apparatus supplied with electrical power from an external source, having a detector to detect whether the electrical apparatus is connected to the external source; a sensor to sense movement of the electrical apparatus; and alarm means to provide an alarm signal when the detector detects that the electrical apparatus is not connected to the source and the sensing means detects movement of the electrical apparatus.

2. An alarm device as claimed in claim 1 wherein the alarm device provides the alarm signal to an external alarm system when the detector detects that the electrical apparatus is not connected to the external source and the sensing means detects movement of the electrical apparatus.

3. An alarm device as claimed in claim 2 wherein the alarm device alternately provides the alarm signal to the external alarm system and to an internal alarm generating means.

4. An alarm device as claimed in claim 3 wherein the duration of the alarm signal supplied to the external alarm system and to the internal alarm generating means is selectable by the user.

5. An alarm device as claimed in any preceding claim further including a timer means which causes the alarm signal to be discontinued after a predetermined time.

6. An alarm device as claimed in claim 5 wherein the alarm signal may be re-initiated after the predetermined time if the detector detects that the apparatus is not connected to the source and further movement is detected.

7. An alarm device as claimed in any preceding claim which includes an internal source of power.

8. An alarm device as claimed in claim 7 wherein the internal source of electrical power stores energy supplied from the external source for the electrical apparatus.

9. An alarm circuit as claimed in claim 8 wherein the internal source of electrical power is a rechargeable battery.

10. An alarm device as claimed in any preceding claim further comprising an over-ride means for preventing the initiation or continuation of the alarm signal.

11. An alarm device as claimed in claim 10 in which the over-ride means can only be operated while the detector detects that the electrical apparatus is connected to the external power source.

12. An alarm device as claimed in any preceding claim further comprising indicating means for confirming that the alarm device has been activated.

13. An alarm device as claimed in any preceding claim further comprising warning means for indicating that an alarm signal has been initiated.

14. An alarm device as claimed in any preceding claim wherein the detector detects a voltage which is dependent upon the external electrical supply to the electrical apparatus.

15. An alarm device substantially as herein described with reference to the accompanying drawings.

16. An alarm circuit for an electrical apparatus supplied with power from an external source, which receives a control signal indicating whether the apparatus is connected to the external source, and also receives an indication signal which indicates that an alarm situation has occurred, which generates an output alarm signal when a control signal indicating disconnection of the apparatus from the external supply and an indication signal are received simultaneously.