# STATIC CONTROL OF MACHINES 247

STATIC CONTROL OF MACHINES 247 static control of machines 247

STATIC CONTROL OF MACHINES 247 In the AND gate circuitry when any of inputs A , B or C is low the voltage at the base of transistor Q1 is low (0) as most of the + 5 volts drops across resistance R1 . Transistor Q1 is therefore in cut off state . Thus voltage at collector of transistor Q1 i . e . , at point X is high and this voltage is coupled to the base of transistor Q2 . Thus base of transistor Q2 is at higher voltage with respect to its emitter . Transistor Q2 therefore conducts and is in saturation state . Voltage at point Y on the collector side of transistor Q2 is therefore low . When all the inputs A , B and C are + 5 V high i . e . , + 5 volts , the voltage at the base of transistor Q1 is also high and therefore transistor Q1 conducts , and the voltage at point X or at the base of transistor Q2 is low , thus it does not conduct . If Q2 is cut off then voltage at point Y is high (+ 5 V). Thus it is clear that output Y will be high (+ 5 V) only when all the inputs are also high . Now , referring to the OR gate circuitry , when any of the input is high , the base of transistor Q1 is also high with respect to its emitter , as a result the transistor conducts . This leads to low voltage at point X and thus transistor Q2 remains cut off . As Q2 is cut off , the voltage level at point Y is high . If all the inputs a e low then Q1 will be cut off and Q2 will conduct . This gives low voltage at Y . Thus we see that this circuit fulfills the conditions described for an OR gate . In practice the high and low values for static devices are not rigidly fixed at + 5 V or 0 volts . For example with 7400 series TTL devices any input voltage between 0 and 0 . 8 V is considered low and any input voltage between 2 and 5 V is considered a high input . This is so ,