Sirens

There are a wide variety of sirens available for alarm systems which ore becoming the most popular form of audible warning device due to their low price and their low current consumption.

There are two distinct types of sirens:-

• Electronic sirens
• Motorised sirens

ELECTRONIC SIRENS

These are by far the most popular for a number of reasons, the first being the variety of sounds that can be produced using solid state electronics, the second reason is their low current consumption. Finally they are relatively inexpensive as compared with other types of sounders.

Below is a breakdown of the various sections in a typical siren. The first section is a tone generator and there are generally two of these, each developing a different sound (a low and a high tone). An oscillator is then used to switch from one sound to the other. This gives a sound like a police siren. The speed of this switching process can sometimes be varied by a “preset” on the circuit board itself. Finally this signal is amplified and fed into a speaker.

TYPES OF SIRENS

EXTENSION SPEAKERS
Because of the simplicity of electronic sirens many control panels will have their own built into the control box itself and will also provide an output for extension speakers. This self-contained siren may also double as an exit entry sounder.

MOTORISED SIRENS

These sirens give a high-pitched droning sound with a distinctive start up and slow down period.

Operation

Two hollow cylinders, with a number of air vents cut in the side, are arranged such that one fits inside the other. The outer cylinder is stationary while the inner cylinder is rotated at high speed by a 12V motor. The result is that air is pushed through the vents and a high-pitched hum is produced.

These sirens are quite loud but have the disadvantage that they require a large current to start up, typically about 1.25 amps. Most domestic and commercial control panels have a power supply that provides 1 to 1.5 amps. As a percentage of this power will be used to run the panel itself, and other auxiliary equipment, these sirens may not be suitable unless other power supplies are used.

MAINS OPERATED  SIRENS

These sirens are extremely loud and are useful for premises such as warehouses and factories. They are run from the 240VAC mains supply which is switched by a 12VDC operated mains relay. This relay is contained within the siren itself and is wired to the 12VDC bell output on the control panel.