APPENDIX C ELECTRONIC TRIGGER MECHANISM C.1 PURPOSE The electronic trigger mechanism (ETM) provides a meansofinitiating operation of the differential fall-out collector (DFOC) and other instruments from light source intensity of 2 suns or more or light-level differential of slightly more than 1 sun. C.2 OPERATION The ETM consists of two separate and distinct light-sensitive circuits, the phototubecir- cuit (PT) and the photocell circuit (PC). The PT circuit is the light-level-sensitive one. The light-sensitive element has three phototubes connected in parallel and so oriented that each receives light about a horizontal axis 120° from the other two. With the PT sensitivity potentiometer (Fig. C.1) set for some predetermined light level, the circuit remains inactive until that light level is exceeded. When light strikes the phototubes, a current flows through the circuit containing the three 45-volt batteries, the phototubes, and the PT sensitivity potentiometer. This current causes a voltage drop across the PT sensitivity potentiometer. This voltage drop makes point 7 (the firing anode) on the tube (OA4) positive with respect to point 2 (cathode). As the light level increases, both the current and the voltage drop aiso increase. When the voltage drop reaches approximately 90 volts, OA4 will fire and operate the phototube relay (PTR). The circuit can be made to operate at a higher light level by decreasing the resistance of PT and at a lower light level by increasing it. The PC circuit is a light-level-differential circuit and operates in almost complete darkness as long as the required differential intensity in light is available. The three photocells are connected in series along with the Sensitrol relay (SR) and a capacitor. The PC sensitivity potentiometer is shunted across the three cells. If the intensity of light striking the cells is constant, then the current outputof the cells is constant and all of it is shunted by PC and none flows through the SR coil and capacitor. However, if the intensity of light increases, current output from the photocelis increases, the capacitor charge becomes less than the increased voltage drop across PC, and current flows from plus to minus through SRand the capacitor. Thus current flows through SR only when the light intensity is changing. The more rapid the change and the greater its magnitude, the larger will be the current through SR. When this current equals or exceeds a value of 10 ya for approximately 0.5 sec, SR will operate and latch closed. The SR contacts in turn energize PTR, thereby achieving the same end function as the PT circuit. Three sensitivity adjustments are available in the PC circuit. First, increasing the resistance of PC sensitivity potentiometer increases the sensitivity and conversely. Second, two 67