6 



pen arm should be recording 'wet' within 7 to 10 minutes. The 

 contact plate in the LWI relay will return to the 'dry' position (Fig. 

 5-A) when the sensing element has dried and slackened sufficiently. 



Sensor indicator assembly. This assembly consists of the sensor 

 indicator switch and an LED (Light Emitting Diode) (Fig. 4). It 

 provides a means to quickly check the electric circuitry connecting 

 the sensor relay, unijunction assembly, and the LWI pen arm relay 

 to see if it is 'open' or'closed'. The sensor indicator switch (f, Fig. 4) is 

 pressed (1) when conducting an operational check of the LWI cir- 

 cuitry to make sure it is functioning properly, (2) when calibrating 

 the LWI, or (3) when it is necessary to know if the LWI sensor relay is 

 in the 'wet' or 'dry' mode. If the sensor indicator switch is pressed 

 and the LED glows, then the circuitry is 'closed'. This means that 

 the unijunction assembly is receiving an electric current from the 

 battery and the LWI pen arm should be reading 'wet'. Remember 

 that the unijunction assembly must be activated for 7 to 10 minutes 

 before the first response of the LWI pen arm will occur. If the switch 

 is pressed and the LED does not glow, then the circuitry is 'open' and 

 the LWI pen arm should be recording 'dry'. 



Unijunction accelerator switch. The unijunction accelerator 

 switch (Fig. 16) hastens the time required for the unijunction 

 assembly to discharge and activate the LWI pen arm relay. It is a 

 'momentary on' switch, which means that it will only be 'on' when it 

 is pushed and held in the 'on' position. This feature is desirable since 

 it eliminates the possibility of forgetting to return the switch to the 

 'off position, which would result in a draining of the battery and a 

 false tracing of wetness on the chart. The purpose of the unijunction 

 accelerator switch is to reduce the time required for the unijunction 

 assembly to 'fire' the LWI pen arm from about 8 minutes to only a 

 few seconds. This will save a considerable amount of time when 

 conducting an operational check of the LWI circuitry. 



Unijunction transistor oscillator assembly (unijunction assem- 

 bly, Figs. 16, 17). The unijunction assembly provides a drive circuit 

 for activating the LWI pen arm relay. Its extremely low current 

 consumption and inherent pulse-type output several minutes apart 

 are what make possible the use of only one 12 V lantern battery for a 

 period of several months or longer. 



Operation of Unijunction Assembly (Fig. 6). The operation of 

 the unijunction circuitry revolves around the slow-charging and 

 quick-discharging of a 500uF electrolytic capacitor (Cj). A 200 ohm 

 resistor (Rj) provides a small current for charging Ci, which takes 

 several minutes. In this charging state, it is as if the connection to 

 the emitter (E) of the transistor (T^) were not present. Then, when the 

 voltage across Cj reaches approximately 8 V, the emitter to base 2 

 (B2) junction of the transistor breaks down to nearly zero resistance 



