IES60093B2 - Interface circuit for an alarm control panel - Google Patents

Description

This invention relates to an interface circuit for connection between an alarm control panel and a bell or other audible or visual alarm.

An alarm control panel is a circuit which receives input(s) from one or more sensors (for example IR or vibration detectors) and, according to certain built-in criteria, determines when an alarm is to be raised based upon the received input(s).

One example of such a control panel is that known as the Phonewatch control panel marketed by Telecom Eireann which monitors the condition of sensors located around a building and provides an alarm to a central station through the public phone lines. However, this panel is not capable of powering a local internal or external bell.

It is therefore an object of the present invention to provide an interface circuit for connection between an alarm control panel and a bell or other audible or visual alarm.

Accordingly, the present invention provides an interface circuit for connection between an alarm control panel and a bell or other audible or visual alarm device, wherein the interface circuit includes means responsive to an alarm signal from the control panel to activate the alarm device, the interface circuit having its own power supply which also powers the alarm device when so activated.

While the invention is clearly useful in the case of an alarm control panel which is insufficiently powerful to ring a local bell or other alarm, the invention is nonetheless applicable to control panels which can raise 5 a local alarm, as a substitute for or in addition to the capabilities of the panel.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block circuit diagram of an interface circuit, and Figures 2 to 6 are detailed circuit drawings of the various blocks indicated in Figure 1.

The interface circuit shown in Figure 1 of the drawings, which is contained within a suitable housing 20 20, is designed for connection between, on the one hand, a Phonewatch control panel and, on the other hand, an internal bell, an external bell and a strobe. The connections to the control panel are shown at the left of Figure 1, and consist of a pair of terminals 10 for 25 connection to and monitoring of the 6 volt d.c. power supply of the control panel, a bell terminal 11 for receiving a 2Hz a.c. alarm signal from the control panel when the latter signals an alarm condition, and a tamper terminal 12 for supplying a tamper signal to 30 the control panel in the case of tampering with the , internal or external bell.

I I The connections to the alarm devices are shown at the right of Figure 1, and consist of a set of four terminals 13 for connection to an external bell, a set of four terminals 14 for connection to an internal bell, and a set of two terminals 15 for connection to a strobe. The external bell terminals 13 consist of + and · - power supply terminals, a trigger terminal TRIG and a tamper terminal TAMP. The internal bell terminals 14 £ consist of + and - power supply terminals and a pair of tamper terminals TAMPER. The strobe terminals 15 consist of + and - power supply terminals only. The actual internal and external bells and the strobe are, of course, not shown in the drawing. However, each of the two bells would be connected to its respective set of terminals by a respective 4-core cable, and the strobe would be connected to its set of terminals by a 2-core cable. All three sets of terminals 13, 14 and 15 are contained within the housing which contains the interface circuit, so that only the emerging cables would be visible from outside.

The interface circuit itself, the internal and external bells and the strobe are all powered by a 12 volt d.c. supply derived from the mains by a regulated power supply unit 16. The output of the power supply unit also trickle charges a backup battery connected to the pair of terminals 17, which will take over in the event of mains failure. The 6 volt power supply input from the control panel at the terminals 11 is primarily for the purpose of enabling the interface circuit to monitor such power supply and provide an alarm if it is removed, as will be described.

The strobe is connected to the positive side of the regulated 12 volt supply (+12 volts) via a fuse Fl and · to circuit ground (earth) through the emitter-collector path of a power transistor TS3. However, the transistor t TS3 is normally off (that is, it is off in the absence of an alarm signal from the control panel at the terminal 11) so it does not conduct and the strobe is not activated.

Similarly, the internal bell is connected to the positive side of the regulated 12 volt supply via a fuse 5 F2 and to earth through the emitter-collector path of the same power transistor TS3. Again, since the transistor TS3 is normally off the internal bell does not ring.

When a 2Hz a.c. alarm signal is placed on the bell terminal 11 by the control panel, this signal is rectified by the diode D7 and charges up the capacitor C4. This switches on the transistor TS9, which in turn switches on the power transistor TS3. When the power transistor TS3 turns on the 12 volt power supply circuit for both the strobe and the internal bell is completed, so that both are activated.

The external bell operates somewhat differently. The 20 - power supply terminal is connected directly to earth and the + terminal is connected through a fuse F4 to +12 volts through the emitter-collector path of a power transistor TS5. The base of the transistor TS5 is connected to earth through the emitter-connector path of 25 a further transistor TS6. The transistor TS6 is normally switched on by the +6 volt supply from the control panel, and in turn switches on the transistor TS5. Thus in normal circumstances the external bell is powered by +12 volts through the transistor TS5.

However, despite the completion of a power supply circuit from +12 volts through TS5 and F4 the external bell does not ring. This is because it also requires a signal at the trigger terminal TRIG in order to ring.

This trigger signal is provided by the power transistor TS3 turning on in response to an alarm signal placed on the bell terminal 11, as previously described, and connecting the TRIG terminal of the external bell to earth.

€ The reason for this arrangement is to monitor the 6 volt power supply of the control panel. The external bell is assumed to be a so-called self activating bell which has its own internal battery. A characteristic of such bells is that they will ring automatically using their internal battery if external power is removed. Therefore, provided the 6 volt power supply is present from the control panel the power transistor TS5 will be turned on as described above and the external bell will be connected to the +12 volts at the collector of TS5. The external bell will therefore not ring, except in response to a signal at its trigger terminal TRIG. However, should the control panel be disconnected from the interface circuit, or should its power supply fail, the transistor TS5 will turn off. This will cause the +12 volts to removed from the external bell, which will then ring on its own internal battery regardless of any trigger signal.

The interface circuit has a number of light emitting diodes (LEDs) to indicate various conditions of the interface circuit.

If the fuse Fl for the strobe blows, current will flow through the red LED D12 and resistors R19 and R16 to earth. This will cause the LED D12 to light. Further, the voltage at the junction of the resistors , R19 and R16 will drop to about 6 volts, which will cause the diode D5 to conduct. This will turn on the >· transistor TS2 so that current will flow through the right hand (red) side of the trible LED D23.

Similarly, if the fuse F2 for the internal bell blows, current will flow through the red LED D24 and resistors R38 and R18 to earth, causing the LED D24 to light. Also, the voltage at the junction of the resistors R38 and R18 will drop to about 6 volts, which will cause the diode D4 to conduct. This will turn on the transistor TS2 so that current will flow through the right hand side of the trible LED D23.

If the fuse F4 for the external bell blows, current will flow through the red LED D25 and resistors R37 and R35 to earth and cause the LED D25 to light. Further, the voltage at the junction of the resistors R37 and R35 will drop to about 6 volts, which will cause the diode D8 to conduct. This will turn on the transistor TS2 so that current will flow through the right hand side of the trible LED D23.

If the battery fuse F3 blows, then current will be drawn through resistors R4, R8 and R17 to turn on the transistor TS2, so that current flows through the right hand side of the trible LED. Fuse F3 will normally blow when excess current flows from the +12 volt power supply where the battery terminals have been short circuited or where the battery has been reverse polarised.

Thus if any fuse Fl, F2, F3 or F4 blows, the right hand side of the trible LED D23 will light, and (except in the case of the battery fuse) the particular fuse which has blown will be indicated by the lighting of the « respective LED DI2, D24 or D25.

To monitor for mains failure, the output of the mains transformer Tl, which is normally 12 volts RMS, is applied to the cathode of a diode D3. The latter will conduct on negative half cycles to charge a capacitor C5. The time constant of the capacitor C5 and a resistor R12, which are connected in parallel to the +12 * volt power supply, is sufficiently high that the voltage at the anode of the diode D3 remains below the break-over voltage of a Zener diode DIO. However, should the mains fail, the capacitor C5 will discharge through R12 bringing the voltage at cathode of the Zener diode DIO above the break-over voltage. At this point the Zener diode will conduct so that a yellow LED D2 in series therewith will light. Thus mains failure will be indicated by the LED D2 lighting.

The foregoing assumes that the battery remains fully charged, since if the mains fails, the +12 volts needed to supply the current through the LED D2 is derived from the battery. Therefore, it is important to know if the battery is fully charged. This is indicated by the green LED D6, which is normally lit but goes out if the battery voltage falls below 10 volts.

The interface circuit further detects and indicates tampering with the internal or external bell, or with the circuit housing.

The lower pair of the set of internal bell terminals are connected together at the internal bell. Further, within the interface circuit the lowest terminal is connected to earth and the terminal above it is connected to the base of a transistor TS4. Thus the base of the transistor TS4 is at or near earth potential, and the transistor TS4 is therefore normally turned off. r However, if the cable connecting the internal bell to the set of terminals 14 is cut, the potential at the V base of the transistor TS4 rises, and the transistor TS4 turns on. This both lights the yellow LED Dll and, via a diode D21, turns on a normally off transistor TS1. This permits a current to flow to earth from the +12 volts at the emitter of the transistor TS1 through the resistor R24 and the left hand (green) side of the trible LED D23, causing the latter to light. The turning on of the 5 transistor TS4 also turns off a normally on transistor TS10, which provides a tamper signal to the alarm control circuit at the terminal 12.

The external bell is wired for both short and disconnect tamper, by monitoring the voltage across a 470 ohm resistor connected to the tamper terminal TAMP. If the 470 ohm resistor is shorted, a capacitor CIO connected to the base of a transistor TS7 will discharge and turn off the transistor. Current will then flow through a resistor R21, diode D17 and resistor R22 to turn on a normally off transistor TS8. On the other hand, if the 470 ohm resistor is disconnected, the capacitor CIO will charge and turn on the transistor TS8 directly. In either case the yellow LED D18 will light to indicate the tamper, and the diode D22 will conduct to turn on the transistor TS1. As described above, this will cause the left hand side of the trible LED D23 to light, and send a tamper signal to the alarm control panel via the terminal 12.

In order to detect unauthorised removal of the housing of the interface circuit, a switch SI is located within the housing at a position where it is held normally closed by the presence of the housing. However, 30 if the housing is removed or its position relative to the internal components is disturbed, this is detected by the switch SI opening. This turns on the normally off transistor TS1, causing the left hand side of the trible LED D23 to light and sending a tamper signal to 35 the alarm control panel via the terminal 12.

Thus, in summary, if there is a tamper the left hand (green) side of the trible LED lights, and the nature of f the tamper (if it is associated with a bell) is indicated by the lighting of the particular LED Dll or L· D18. Also, the tamper is signalled to the alarm control panel at the terminal 12.

Naturally, if the trible LED 23 shows yellow, this indicates that there is both a fuse fault and a tamper.

Claims (5)

CLAIMS:

1. An interface circuit for connection between an alarm J control panel and a bell or other audible or visual 5 alarm device, wherein the interface circuit includes means responsive to an alarm signal from the control panel to activate the alarm device, the interface circuit having its own power supply which also powers the alarm device when so activated.

2. An interface circuit as claimed in claim 1, wherein the interface circuit monitors the power supply of the alarm control panel and activates the alarm device if such power supply is removed.

3. An interface circuit as claimed in claim 2, wherein the interface circuit has means for detecting the cutting of conductors connecting the alarm device to the interface circuit, and means for signalling the alarm 20 control panel upon such detection.

4. An interface circuit as claimed in any preceding claim, further including a plurality of light emitting devices which indicate different conditions of the 25 interface circuit.

5. An interface circuit substantially as described with reference to the accompanying drawings.