GB2151057A - Alarm device - Google Patents

Abstract

An alarm device comprises a loudspeaker 37 energized when the mains supply to a transformer 11 is broken. When the supply is broken, a relay 12 closes a contact 16 so that power from a battery 15 is supplied to a driver oscillator circuit including transistors 21 and 22 and an astable multivibrator 30 energizing a loudspeaker 37. The battery connection can be broken by a switch 17 which is normally operable by a key or similar restricted arrangement. The particular circuit arrangement of the driver oscillator circuit and of the a stable multivibrator enable high power to be achieved from a small battery 15. <IMAGE>

Description

SPECIFICATION Alarm device This invention relates to alarm devices which are arranged to operate when the mains supply is cut off. Such devices contain a battery connected to an alarm system through a contact of a mains-operated relay, the contact being opened when the relay is energised.

When the mains supply is broken the contact closes and the battery energises the alarm.

In burglar alarm systems, a large battery is required to operate the alarm system. When such an alarm device is applied to smaller items than a house, it is important that the alarm system including the battery should also be small. In order that the battery can be small, the alarm system must be efficient. It is the object of the present invention to provide an efficient alarm system. This object is achieved by providing an oscillator driver circuit followed by a bridge oscillator which itself drives a loudspeaker.

An example of the invention will now be described with reference to the accompanying circuit diagram.

A transformer 11 has a primary winding connected to the mains supply and a secondary winding connected to a rectifying diode 1 3 and across a 1000 microfarad smoothing capacitor 1 4 to a relay 1 2 and a stack of nickel cadmium cells 1 5 through a limiting resistor 1 8. The stack of cells 1 5 is connected through a normally opened relay contact 1 6 and an on-off switch 1 7 to the oscillator circuit to be described below.

When the mains supply is functioning, the relay 1 2 is energised so that the contact 1 6 opens and the stack of cells 1 5 is charged.

When the mains supply fails, the relay 1 2 is de-energised, and the contacts 1 6 closes so that the cells 1 5 energise the oscillator, assuming the switch 1 7 is closed.

The right-hand side of the circuit is a conventional astable oscillator, comprising a pair of BC546 transistors 21 and 22, modified to produce a specially-shaped waveform by two 10 microfarad capacitors 23 and 24. The capacitor 23 connects the collector of ransistor 21 to the base of transistor 22. The capacitor 24 de-couples a 100 kilohm resistor 25 forming part of a potential divider for the base of transistor 22 with a 10 kilohm resistor 26. The output of the oscillator driver circuit is taken across a 10 kilohm resistor 27 decoupled by a 100 microfarad capacitor 28 to a bridge oscillator generally referenced 30 comprising a further pair of the cross-coupled BC546 transistors 31 and 32, driving a pair of TIP 31 transistors 33 and 34 and a pair of TIP 32 transistors 35 and 36.An 8 ohm loudspeaker 37 has one terminal connected to the line joining the collectors of transistors 33 and 35, and the other terminal is connected to the line joining the collectors of transistors 34 and 36. The effect is that during one half cycle of oscillation the transistors 33 and 36 conduct to drive the terminals of loudspeaker 37 in opposite directions, whereas in the other half cycle transistors 34 and 35 conduct to drive the terminals of loudspeaker 37 in the other sense. In this way, four times more power of the sound output can be achieved than with a single transistor. This high power output means that the stack of cells 1 5 can be made small and the whole alarm device reduced in size.This makes it suitable for attachment to portable equipment such as an audio or video recorder, the alarm sounding when the recorder is removed from the mains supply. In this case, the switch 1 7 will be operated by a key so that the alarm can only be switched off by an authorised user. If an unauthorised removes the electrical equipment from the mains supply, the alarm will sound continuously, drawing the attention to the fact that the equipment is being removed by an unauthorised person. The sound emitted by the loudspeaker is given a warbling effect by the capacitors 23 and 24 so that the sound varies over a range of frequencies and so becomes more noticeable.

The invention can also be applied to electrical devices where disconnection from the mains may be accidental but can lead to undesired effects, such as a good freezer. The alarm will sound whenever the mains supply is broken, drawing attention to the fact that food in the freezer is liable to perish unless the mains supply is restored.

1. An alarm device comprising an oscillator driver circuit driving a bridge oscillator, and a loudspeaker driven by said bridge oscillator.

2. A device as claimed in claim 1 wherein the bridge oscillator comprises symmetrical halves, each half comprising a transistor whose base is connected through a capacitor to the collector of the transistor in the other half and through a respective resistor to a common input terminal connected to said oscillator driver circuit.

3. An alarm device as claimed in claim 1 or claim 2 wherein the bridge oscillator comprises a pair of symmetrical halves, each half comprising a pair of opposite type transistors connected in series across the power supply to the bridge oscillator, the junction between said two transistors in each half forming a terminal for connection to said loudspeaker.

4. An alarm device as claimed in any one of claims 1 to 3 wherein said oscillator driver circuit comprises a pair of identical transistors whose collectors are connected at least indirectly to the bases of the other transistor.

5. A device as claimed in claim 4 wherein the collector of one transistor is connected

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Alarm device This invention relates to alarm devices which are arranged to operate when the mains supply is cut off. Such devices contain a battery connected to an alarm system through a contact of a mains-operated relay, the contact being opened when the relay is energised. When the mains supply is broken the contact closes and the battery energises the alarm. In burglar alarm systems, a large battery is required to operate the alarm system. When such an alarm device is applied to smaller items than a house, it is important that the alarm system including the battery should also be small. In order that the battery can be small, the alarm system must be efficient. It is the object of the present invention to provide an efficient alarm system. This object is achieved by providing an oscillator driver circuit followed by a bridge oscillator which itself drives a loudspeaker. An example of the invention will now be described with reference to the accompanying circuit diagram. A transformer 11 has a primary winding connected to the mains supply and a secondary winding connected to a rectifying diode 1 3 and across a 1000 microfarad smoothing capacitor 1 4 to a relay 1 2 and a stack of nickel cadmium cells 1 5 through a limiting resistor 1 8. The stack of cells 1 5 is connected through a normally opened relay contact 1 6 and an on-off switch 1 7 to the oscillator circuit to be described below. When the mains supply is functioning, the relay 1 2 is energised so that the contact 1 6 opens and the stack of cells 1 5 is charged. When the mains supply fails, the relay 1 2 is de-energised, and the contacts 1 6 closes so that the cells 1 5 energise the oscillator, assuming the switch 1 7 is closed. The right-hand side of the circuit is a conventional astable oscillator, comprising a pair of BC546 transistors 21 and 22, modified to produce a specially-shaped waveform by two 10 microfarad capacitors 23 and 24. The capacitor 23 connects the collector of ransistor 21 to the base of transistor 22. The capacitor 24 de-couples a 100 kilohm resistor 25 forming part of a potential divider for the base of transistor 22 with a 10 kilohm resistor 26. The output of the oscillator driver circuit is taken across a 10 kilohm resistor 27 decoupled by a 100 microfarad capacitor 28 to a bridge oscillator generally referenced 30 comprising a further pair of the cross-coupled BC546 transistors 31 and 32, driving a pair of TIP 31 transistors 33 and 34 and a pair of TIP 32 transistors 35 and 36.An 8 ohm loudspeaker 37 has one terminal connected to the line joining the collectors of transistors 33 and 35, and the other terminal is connected to the line joining the collectors of transistors 34 and 36. The effect is that during one half cycle of oscillation the transistors 33 and 36 conduct to drive the terminals of loudspeaker 37 in opposite directions, whereas in the other half cycle transistors 34 and 35 conduct to drive the terminals of loudspeaker 37 in the other sense. In this way, four times more power of the sound output can be achieved than with a single transistor. This high power output means that the stack of cells 1 5 can be made small and the whole alarm device reduced in size.This makes it suitable for attachment to portable equipment such as an audio or video recorder, the alarm sounding when the recorder is removed from the mains supply. In this case, the switch 1 7 will be operated by a key so that the alarm can only be switched off by an authorised user. If an unauthorised removes the electrical equipment from the mains supply, the alarm will sound continuously, drawing the attention to the fact that the equipment is being removed by an unauthorised person. The sound emitted by the loudspeaker is given a warbling effect by the capacitors 23 and 24 so that the sound varies over a range of frequencies and so becomes more noticeable. The invention can also be applied to electrical devices where disconnection from the mains may be accidental but can lead to undesired effects, such as a good freezer. The alarm will sound whenever the mains supply is broken, drawing attention to the fact that food in the freezer is liable to perish unless the mains supply is restored. CLAIMS

1. An alarm device comprising an oscillator driver circuit driving a bridge oscillator, and a loudspeaker driven by said bridge oscillator.

2. A device as claimed in claim 1 wherein the bridge oscillator comprises symmetrical halves, each half comprising a transistor whose base is connected through a capacitor to the collector of the transistor in the other half and through a respective resistor to a common input terminal connected to said oscillator driver circuit.

3. An alarm device as claimed in claim 1 or claim 2 wherein the bridge oscillator comprises a pair of symmetrical halves, each half comprising a pair of opposite type transistors connected in series across the power supply to the bridge oscillator, the junction between said two transistors in each half forming a terminal for connection to said loudspeaker.

4. An alarm device as claimed in any one of claims 1 to 3 wherein said oscillator driver circuit comprises a pair of identical transistors whose collectors are connected at least indirectly to the bases of the other transistor.

5. A device as claimed in claim 4 wherein the collector of one transistor is connected through a capacitor to the base of the other transistor, the base of said other transistor being connected to a potential divider.

6. An alarm device as claimed in any one of the preceding claims comprising a battery connected through a relay contact to supply power to the oscillator driver circuit and the bridge oscillator and a relay responsive to the presence of an external mains supply to hold said relay contact open.

7. A device as claimed in claim 6 comprising a switch in series with said relay contact, said switch being operable by a security device such as a key.

8. An alarm device comprising the arrangement of circuit components illustrated in the accompanying drawing.

9. An alarm device substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.