Shared Knowledge
"AT&T Home Security System - Replace or Add a Siren"
25 February 2012
Summary: if you need to replace or want to add a siren to your AT&T Home Security System, make sure the total 12volt direct current (DC) draw of the siren(s) does not exceed 3/4 ampere (amps) or you risk damage to the central controller (CC) circuit board.
Background:
The AT&T Home Security System can support 1 or 2 sirens.
A siren is not a speaker.
A siren is a device specifically designed to emit a certain loud tone or sound(s) when 12volts DC is applied to its terminals.
Sirens can be bought that have 3 wires on them. If you use one wire and the common, you get one type of sound out of the siren and if you use the other color wire and the common, you get another type of sound out of the siren.
The siren you have or buy defines the sound emitted, not the AT&T Home Security System.
Sirens are usually placed outside the house because of their volume but can also be installed inside.
My system does not use sirens.
My system has a voice annunciator circuit board installed inside the CC that sends actual human voice words to 2 installed speakers ("Fire, fire, leave immediately", "Burglary, burglary, leave immediately").
I have never found evidence that AT&T designed or sold a voice annunicator circuit board as part of their home security system and believe the unit I have was an optional add-on supplied by my home security installer.
Replacing or adding a siren:
As seen in the wiring diagram below, the AT&T Home Security System Model 8720 central controller can support up to (2) 12volt sirens but the total current draw of all sirens can not exceed 3/4 amps.
So if you want to add or replace a siren:
(1) Make sure any siren you buy as a replacement or to add to your current system does not violate the 3/4 amp draw limit or you could damage the CC circuit board or blow the circuit board installed fuse.
(2) If you must add a siren that is going to exceed the total siren current draw allowed by the CC, you are going to have to add an external 12volt DC power supply that has sufficient current to power your siren and use a small relay inside the CC's housing that switches external power to the siren when the CC senses any sort of home security event.
The diagram above shows how you would add a relay to your AT&T Home Security System to control a siren with current demands greater than 3/4 amp at 12VDC.
The main points:
- the siren terminals on the CC circuit board are connected to the relay activation or coil terminals.
- the plus (+) of the external power supply is connected to one switched terminal of the relay.
- the other switched terminal of the relay is connected to the (+) on the siren.
The idea is that when the CC senses some sort of security event (fire, intrusion...), it places 12volts DC on the siren terminals. This 12volts activates the small coil in the relay which closes a set of contacts connecting the siren to the power supply, sounding the siren. When the CC is deactivated, 12volts DC is removed from the relay and the contact connecting the external power supply to the siren is opened, silencing the siren.
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