140. Vibration Detector

a. General. The vibration detector (fig. 181) can be used to detect underground tunnel-ing operations, to operate as an acoustic mine obstacle, and to detect radio detonated, remote controlled mines and mines set with clock-work delay fuzes.

Figure 181. Vibration detector.

b. Description. The complete apparatus (fig. 181) is housed in a black cast-metal case of extremely strong and moisture-resistant construction. It consists of two sounding heads, a control box, a 40-ohme headset, and a 4 1/2-volt dry cell battery.

(1)

Sounding head. Each sounding head (fig. 182) has a horizontal copper plate dia-phragm, 5 1/2 inches in diameter. Its vibrations are are transmitted through a verti-cal air column to a double-button microphone. Each button has resistance of about 200 ohms. The sounding head weighs about 5 pounds and is 3 3/4 inches high and 5 3/4 inches in diameter at the widest part of the base. Two terminals are provided for connecting by cable to the control box.

Figure 182. Sounding head for the vibration detector.

(b)

Control box. The wooden control box (fig. 183) contains the headset, which is per-manently attached to the box, and the battery. The total weight, including the head-set and the battery is about 1 pound. The box is 6 inches long, 3 inches wide, and 1 1/2 inches high. It is equipped with three cable terminals, the center one being a common return for both sounding heads. Two white push buttons enable the sounding heads to be used independently of each other.

Figure 183. Interior of the control box.

c. Wiring. The two sounding heads are connected in parallel to the control box where they are joined in series circuit with the battery and the headset through the white push-button switches (fig. 184).

Figure 184. Wiring diagram of a vibration detector