【正文】 d, cleared to zero and enabled to be ready for counting the sum of money required from the first, second and third passengers, the total sum pushbutton 4 unblanks the fourth readout enabling any person to retain the readout corresponding to the total the release of the pushbutton, the fourth readout will be blanked unit also contains the main electronic circuit which will be fully described in the following OF THE MAIN ELECTRONIC CIRCUIT The general block diagram of the main electronic circuit is shown in Figure consists of five subcircuits designated by the symboles CTI up to CT4supporting circuits, these are: The number of passenger deticition circuit CTI, travelling distance scaling circuit CT2, waiting time scaling circuit CT3, circuit CT4 which generates clock pulses for the display of Passengers Detection Circuit CT1 As shown from the general block diagram, the circuit CTI has three inputs I, 2 and 3 as well as three outputs J, K and function of the circuit is to supply a high level voltage at terminals J, K or L if and only if one, two or three passengers are hiring the taxicab, term passenger, here, means one person or a group of related a passenger is getting into the cab, we simply turn on a free readout display by turning the corresponding rotary selector switch to a fully clockwise will automatically disconnect the corresponding terminal I, 2 or 3 from logical relation between various input terminals I, 2 and 3 and the output terminals J, K and L is shown in Table a binational circuit we start the design by deriving a set of boolean possible simplified boolean functions that gives minimum number of inputs to gates may be obtained from Table possible logical diagram that is based on the above derived expressions is shown in Figure consists of two inverters, four 2input AND, to3input AND two 3input OR gates Distance Scaling Circuit CT2 As shown from the block diagram of Figure 4, the circuit CT2 has four input J, K, L and E and one output function of the circuit is to supply a single pulse at the output M for a certain number of pulses generated at the output of the speed sensor(certain number of meters travelled by the taxicab), according to the number of passengers hiring the suggested fare per kilometer of travelling distance is shown in colomn two of Table circuit, in this case, should supply a single pulse at the output M for every 100, 125 or 143 pulses generated at the input terminal E according to the level of voltage at input terminale 3, K or L, circuit could be, as shown in Figure 5, three decade counters, connected as a three digit frequency divider whose dividing ratios 100, 125 and 143 are automatically selected by the voltage level at terminals J, K and L, possible circuit diagram that may verify the above function is shown in Figure consists of three decade counters type 7490, one BCDto decimal decoder type 7445, three 4input AND, one 3input ANDone 2input AND two 3input OR Scaling Circuit CT3 As shown in the block diagram, the time scalingcircuit will have four inputs J, K, L and F and one output function of this circuit and accordingto colomn three of Table 2(fare per 2 minuts of waiting time)is to supply a single pulse at the output N for every 120, 240 or 360 pulses supplied at the input terminal F from the I Hz clock according to level of voltage at inputs J, K and L, scaling circuit would be similar to the distance scaling circuit but with different diving Possible circuit diagram is shown in figure consists, in this case, of three decade counter type 7490, two 3input AND, one 5input AND, one 2input AND one 3input OR CT4 Which Generates Clock Pulses for Display Circuit The function of this circuit is to supply one, two or three pulses at the output terminal R for each pulse generated at any of the terminals N or M, according to the voltage level at the input terminals J, K or L, output P will receive a pulse for each pulse generated at any of the input terminals N or function can be performed by the circuit shown in Figure 8, it consists of one ripple counter type 7493, one half of a dual JK masterslave flipflops circuit type 7476, three inverters, three 2input AND, one 3input AND, one 2input OR and one 3input OR a pulse is generated at either input terminals N or M, a high level voltage will be generated at the output Q of the will g a t e t h e I Khz signal to be connected to the input A of the ripple counter as well as to the output terminal one, two or three pulses are counted by the ripple counter, according to the level of voltage at the input terminals J, K and L, respectively, a high is generated to reset the counter and change the state of the flipflopsuch that Q bees , the 1 KHz signal is disabled to reach the outputerminal R or the input A of the ripple order to ensure the proper function of the circuit, the flipflop should be cleared whenever a new channel is has been achieved by the input 5 and will be explained later when describing the function of the channels rotary selector Circuit As shown in Figure 2, the display panel would contain three 4digit displays that give the sum of money required from each passenger separately as well as a one sixdigit display that gives the total ine of the possible wiring diagram for the display circuit is shown in Figure any of the rotary selector switches to fully clockwise direction will supply the corresponding display by5 volts through terminals 1, 2 and 3, corresponding display will be unblanked by supplying a low level of voltage through terminals A, C and G, terminals 8, D and H, respectively, at low level will keep them reset to corresponding display is then enabled by removing the low voltage from terminals B, D, and H, respectively, to be ready for counting the sum of money required from the corresponding passenger starting from counting pulses for these three displays are supplied through terminal total sum display will be enabled w