External Alarm Bell

My house has a 30 year old Securitec alarm system.

Even though the alarm was not enabled, it triggered at 2:30am a few nights ago following a brief power cut.

I could tell there had been a power cut because a mains alarm clock, with no battery backup, had reset itself and the clock display was flashing.

The problem was quickly traced to the sealed 12V lead acid battery in the alarm control panel. It was completely flat and the case was cracked. I have a replacment on order.

When the alarm was triggered, I noticed that the external wall mounted bell was not working properly.

Instead of a loud ring, it gave a pathetic tinkling noise.

This is an old electomechanical device, so it's most likely corroded contacts.

The bell is situated just below the eves with a lean-to roof below it which makes it very difficult to access with a normal ladder.

If I could get access, I'm pretty sure I could repair it simply by cleaning the contacts.

As it's so difficult to get to, I was thinking about leaving the existing bell in place and fitting a new piezo sounder in a different position on the wall which would allow access using a standard extension ladder.

I was hoping I could cut the existing bell cable inside the loft and re-route it to the new sounder.

The problem I have is this:-

Most modern sounders appear to require a permanent 12V supply.

The cable that currently runs to the existing bell has just two pairs of wires - one pair activates the bell, the other pair form part of the anti-tamper loop.

Hence I don't have a spare pair of wires for a separate 12V supply.

There are a pair of terminals inside the control panel that could be used to feed a 12V supply, but that would mean running a new cable with three pairs of wires from the new sounder all the way to the control panel.

That would not be easy.

I'm also not keen on the idea of having to climb a ladder to change the internal backup battery you get in modern piezo sounders.

What I would like is a simple piezo sounder without an internal battery that only requires two pairs of wires - one pair to activate the alarm and one pair for the anti-tamper loop.

So far, I've not been able to find anything suitable. Are such sounders available?

you can wire a bellbox with 4 core but youll loose strobe or tamper

of course, if people maintained their alarms instead of ignoring it until its broke...

Out of interest, do domestic intruder alarms actually make any difference when it comes to deterring criminals?

This 2015 paper concludes they are ineffective:-

https://dspace.lboro.ac.uk/dspace-jspui/bitstream/2134/16638/1/2015%20do%20burglar%20alarms%20increase%20burglary%20risk%20Crime%20prevention%20and%20community%20safety%20published%20version.pdf

In our street, there are a number of alarms that frequently go off at various times of day and night. No one seems to take a blind bit of notice.

Here are some surprising stats:-

https://www.safewise.com/blog/8-surprising-home-burglary-statistics/

brian_mk said:

In our street, there are a number of alarms that frequently go off at various times of day and night. No one seems to take a blind bit of notice.

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This can be a reflection on the neighbours who live in the street, or a system that is a DIY hash-up or systems that have never been maintained. A correctly installed and maintained alarm will not be going off frequently day or night. As Andy said a lot of people do not maintain their alarms, they just ignore it till it stops working. Better designed and installed systems can also have links to alert you directly if they are activated so the relevant people do take notice.

Doc H.

brian_mk said:

In our street, there are a number of alarms that frequently go off at various times of day and night. No one seems to take a blind bit of notice.

Click to expand...

mostly because people dont bother getting them serviced so often false alarms, so they get ignored as its probably yet another false alarm...

I did a bit more investigation & cable tracing yesterday.

It turns out that the cable feeding the existing bell does actually contain three pairs of wires but one pair is not used.

I think I've come up with a possible solution:-

The bell cable along with six cables from the first floor alarm sensors all come together in one single junction box in the loft.

(I call it a junction box but it's actually just a bunch of wires twisted together).

At the junction box, all the first floor alarm sensors are looped together. The associated tamper wires are also looped together.

A single 'artery' cable containing four pairs of wires feeds from the junction box down to the control panel on the ground floor.

The spare pair of wires from the bell cable reach as far as the junction box but are left unconnected - they do not feed down to the control panel.

Artery cable connections:-

black    BELL -

red        BELL +

brown  ALARM2

orange ALARM2

blue      ALARM2 TAMPER

yellow  ALARM2 TAMPER

green   BELL TAMPER

white   BELL TAMPER

The ALARM2 TAMPER and BELL TAMPER wires are looped together in the control panel along with the tamper wires for the ground floor alarm sensors.

While tracing the wires, I discovered that the BELL TAMPER was open circuit. As all the tamper circuits are looped togtether, that makes all the other tamper connections completely useless!

This is almost certainly down to a corroded microswitch in the external bell box and is another reason to replace the old bell unit.

I'm guessing that finding a replacement anti-tamper switch to repair the old bell would not be easy.

The control panel has an auxilliary 12V supply output, the positive side of which is common to the BELL + terminal.

I figure that I could free up one pair of wires in the artery cable that runs up to the loft by looping the BELL TAMPER and ALARM2 TAMPER wires together in the junction box rather than in the control panel.

I could then use the spare wires to feed a permenant 12V supply to a new peizo sounder unit.

The existing control panel has no STROBE output to control the light in a piezo sounder but I think I could live with that.

I don't want to go to the expense of replacing the control panel.

It does mean I would have to put up with the need to use a ladder to change the battery in the piezo unit now and again but that seems unavoidable.

tbh, with wiring like that and at that age, i think youre wasting your time trying to fix it. id be looking at replacing it and either using wireless sensors or rewiring

tbh, with wiring like that and at that age, i think youre wasting your time trying to fix it. id be looking at replacing it and either using wireless sensors or rewiring.

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You may be right, but I'm a retired engineer, so I like trying to fix things (as long as it doesn't involve climbing ladders).

It seems odd to me that the control panel gives no warning that the tamper loop has failed (i.e. is already open circuit when the alarm is initially enabled).

That seems like bad design. I would have expected a warning LED to be illuminated or for the alarm to trigger in this situation.

I'm guessing the tamper alarm must be edge triggered rather than level triggered.

If you disconnect the tamper loop and replace it with a test switch, the alarm does trigger when the switch is opened.

The tamper arrangement appears to operate differently to the alarm circuit.

If the alarm loop is already open circuit when the alarm is first enabled, a LED is illuminated and the alarm sounds.

It sounds as though you may have a tamper or two in parallel with others? They should be in series so any one can activate alarm.