Figure 9-10.--Motor-operated horn.

sound-producing diaphragms. Figure 9-10 is an

illustration of a motor-operated horn.

Sirens

Sirens are used in very noisy spaces or to sound

urgent alarms. The sound is produced by an electric

motor driving a multiblade rotor past a series of ports

or holes in the housing. The air being forced through

the ports gives a siren sound, the frequency of which

depends upon the number of ports, the number of

rotor blades, and the motor speed. Figure 9-11 is an

illustration of a siren.

Electronic Signal Units

27.301

The IC/E3D2 electronic signal unit (fig. 9-12) is

designed for use with alarm switchboards. The unit

Figure 9-11.--Siren.

consists of a solid-state oscillator and an amplifier.

The unit is capable of generating three distinct tones:

a wailing or siren tone, a steady tone, and a pulsating

tone.

VISUAL SIGNALS

Visual sig nals ar e used with many alar m,

safety , and warning sy stems to prov ide an

additional means of identifying the alarm

being sounded. Audible and v isual sig nals are

often used tog ether . In noisy spac es, audible

signals are supplemented by visual signals;

140.120

Figure 9-12.--Type IC/E3D2 electronic signal

unit.

and in brightly lighted spaces, visual signals are

supplemented by audible signals. In many

instruments, the same audible device is used in

combination with several visual signals. The

principal type of visual signals are lamp

indicators.

Standard watertight lamp indicators are

designed as single-dial, two-dial, four-dial, and

six-dial units. Two 115-volt lamps are connected

in parallel and mounted behind each dial. The

use of two lamps in parallel provides protection

against the loss of illumination in case one lamp

bums out. A colored-glass disk and sheet-brass

target engraved with the alarm identification

are illuminated from the rear by the two lamps.

The 115-volt lamps are in parallel with

Figure 9-10.--Motor-operated horn.

9-10