Figure 8-28. - Motion detector rotating up to 180 for maximum coverage.

The basis of this type of system is a gas-detection tube using the Geiger-Mueller principle to detect radiation wavelengths extending from 2000 to 2450 angstroms (Å) (1 Å = 10^-8 cm). Figure 8-34 displays the radiation sensitive area of the tube and compares this area to other forms of radiation. It should be noted that visible radiation does not extend into the sensitive area of the detector. Similarly, radiation from artificial lighting sources does not extend into the sensitive area of the detector.

Figure 8-29. - Audio detector.

Welding arcs and lightning strikes, however, will generate radiation to which the detectors are sensitive and precautions must be taken to minimize these effects.

The ultraviolet-radiation detector's focus of sensitive points is a 60-degree spherical cone whose apex lies at the detector tube. Figure 8-35 indicates the relationship between viewing angle and relative sensitivity. The sensitivity of the detector tube is a characteristic of its cathode material and is fixed, but its voltage-pulse output rate varies both with flame size and flame viewing distance. The pulse output rate is directly proportional to flame size; that is, it increases when larger flame fronts are presented to the detector. The pulse output rate is also inversely proportional to the distance of the flame front from the detector tube-the pulse output rate decreases as the distance from the detector tube to the flame front increases.

To illustrate, a l-foot (0.09 m² ) hydrocarbon fire will cause a pulse output rate of 3 pulses per second at a viewing distance of 30 feet (8 m). This same fire will cause a tube pulse output rate of 20 pulses per second at a viewing distance of 20 feet (6 m). In a like manner, 1- foot (0.09 m² ) flame front must be located at a distance of 5 feet (1.5 m) to create a pulse output rate of 30 pulses per second; a 16-foot (1.4 m² ) fire will create the same pulse output rate at a distance of 25 feet (7.6 m), and so forth.

Telephone Dialers

A schematic wiring diagram of a typical telephone dialer is shown in figure 8-36. The two cooperating