The Integration of Multiple Different Circuits to Form a Miniaturized One Integrated Circuit (IC)

The integration of multiple different circuits to form a miniaturized one integrated circuit (IC) - has hugely boosted the efficiency of devices being developed today. For instance, the current digital clocks that make use of ICs are more efficient compared to their analog counterparts. Thus, we will also resort to integrated circuits, LM555 and MM74C926 to build our digital stopwatch. The LM555 is a timer, while the MM74C926 is a 4-digit counter having a multiplexed 7-segment LED display. Essentially, this subsection provides insight on the project implementation, expounding on how the system components are interconnected while also giving clear illustrations of how the digital stopwatch is built. Further, we will also explain the role played by each component as well as how the stopwatch functions.

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Implementation

As mentioned earlier, the IC MM74C926 has a multiplexed 7-segment LED display specifically D1, D2, D3, D4, whereby the counting is made to start at D4. Just like the name suggests, the counter is used to effect the counting operation. On the other hand, the LM555 timer produces the necessary clock pulses. Ideally, a stopwatch is expected to display a span of 60 seconds; therefore, with our timer generating pulses at an interval of one second, and the MM74C926 counting from 0 to 59, ours will also be able to give a count of the clock pulses from 0 to 59. It is thus imperative to understand how the implementation is achieved. First of all, the IC MM74C926 comprises an internal output NAND latch, which enables the counting operation to be successful. The cascade arrangement of the counters further allows the clock pulses to be consecutively counted (Digital Stop Watch, 2011). Second, the IC consists of an NPN transistor that sources drivers for the common cathode as well as an internal multiplexing circuitry, which consists of four multiplexers. The multiplexing circuit stage also includes an inbuilt free running oscillator and does not rely on any additional external frequency generating network. Thus, when the LM555 timer generates low and high signals at a specified interval, an oscillating signal is also produced. The counter is negative edge-triggered meaning that its state changes only at the falling edge of the clock pulse generated by the timer. The count is made to display on the 7-segment LED display every time a clock pulse is produced. More importantly, just like any other low-power devices, the proposed digital stopwatch will as well be powered using a 5V DC input. The Fig.2 indicated below shows the circuit diagram of the 4-digit digital stopwatch.

Operation

The entire circuit will be enclosed in a metal box, which has provisions for four 7-segment displays, start/stop switch S2, and a reset switch S3. The IC LM555 will produce the clock signal and then transfer it to pin 12 of the MM74C926 counter. As the IC continues to receive the pulses, it also proceeds to count from 0 all the way to 9. Immediately it reaches 9, MM74C926 produces a high logic (1) output, therefore, triggering reset pin 13 of this IC MM74C926 to reset it to low (0) logic. In essence, the reset pin 13 is connected to the high (+5V) using a reset push-on-switch S3. If a user presses the start/stop switch S2 while the stopwatch is running, the count figure is momentarily rendered to zero logic, after which the NPN transistor T1 responds with a trigger, thereby resetting the timer. As a result, this forces the count operation to begin in a situation, where switch S2 is in an off condition.

On the other hand, applying a zero logic signal on the enable input pin 5 of IC MM74C926 latches the count, which in this case is the output of the counter module. If a user switches ON the start/stop switch S2, the enable input is forced to change to low (zero logic) and, as a result, the count figure at that instance is saved in the latch section of the counter (Theraja et al., 2002). Afterward, pin 6 that acts as the display-select pin ascertains whether there is a need to indicate the pulse or the stored count on the 7-segment display. For instance, in case S2 sets pin 6 to low (zero logic) the figure output in IC's latch section is displayed; however, when S2 sets pin 6 to a high logic (logic 1), the count initially stored is indicated on the 7-segment display. To inhibit the oscillator from producing clock pulses, the NPN transistor T1 is coupled to the reset pin 13 of MM74C926 by switching on switch S3. Thus, the timer and the counter are synchronized to enable the counting operation to be effective. Essentially, to start the stopwatch, a user switches ON S2, which links the base of the PNP transistor T2 with the ground, thereby setting the stopwatch in an operation mode. Consequently, the display-select pin 6 of IC MM74C926 triggers the emitter of transistor T2 to produce an output. After switching it on, a user should first reset the module such that D1, D2, D3, D4 of the 7-segment display shows 0000. To initiate the counting operation, he or she should disconnect switch S2 by pressing switch S3. Finally, to stop the entire process and shut down the display, the user just turns OFF the start/stop switch S2. Fig.3 below indicates how the circuit can be easily assembled on a general-purpose PCB or breadboard to come up with the prototype of the 4-digit digital stopwatch, which is made efficient by combining the combinational logic circuits, such as the multiplexers and decoders with sequential circuits, such as flip-flops, timers.

5.0 Project Components

5.1 7-segment LED display

As its name suggests, the 7-segment LED display is a display device that outputs digits only. Thus, this component is fit for this project since we are only interested in showing time in terms of numbers. The display will be wired as follows: first, we will ground one GND pin leaving the other as both of them are shorted together; afterward, 5V will be applied to the DP pin, which is connected in series to a resistor measuring between 510 W to 1 kW. There are two types of the 7-segment LED display common cathode and common anode however, the project relies on a common cathode; thus, the LEDs anode side is connected to the seven a,b,c,d,e,f,g pins of the display. Ideally, for a CC, the LEDs light when a positive voltage is applied to any of the seven pins, and its display color is black. If the relevant pins of a LED segment are forward-biased, the LEDs in that particular section should light implying that the screen is properly functioning.

5.2 IC MM74C926

The MM74C926 CMOS counter is an 18-pin integrated circuit comprising a 4-digit counter, 4-bit internal output latches, an internal multiplexing circuitry with four multiplexing outputs, as well as a BCD-to-7-segment decoder and an NPN transistor output sourcing drivers for display. The multiplexing circuit has its own oscillator, which produces frequency signals at a particular interval, and requires no external clock. Since the counters advance on the falling edge of the clock pulse, they are thus negative-triggered. The IC is such that if a user applies a high logic (logic 1) signal to the reset input, the counter is reset to zero, and, as a result, the carryout is also set to logic zero. As mentioned earlier, a logic zero signal applied to the enable input will latch the output. On the other hand, the pin 6 of the IC plays a significant role in determining whether to display the output of the latch or the initially stored figure. It is also important to note that once the 4-bit latch produces an output, it is immediately transferred to the driver of the BCD to 7-segment display, which displays the numerals 0 through 9.

5.3 IC LM555

The LM555 is a highly stable 8-pin integrated circuit, which is meant for producing accurate time delays. The timer can be designed to operate in monostable, astable, or bistable modes. However, for the case of this project, the timer will be designed to run in the astable mode, which will allow us to precisely control the free-running frequency as well as duty cycle using two external resistors and one capacitor. Of the 8-pins, some additional ones are provided to enable easy triggering or resetting of the timer if a need arises. For instance, the circuit can be reset and even triggered in case of falling waveforms. The LM555 timer is widely used in precision and sequential timing, pulse-width and pulse-position modulation, time delay generation, among other applications.

5.4 Variable resistor

Typically, a resistor is used in a circuit to control its resistance and, consequently, the current or voltage as the three are connected using Ohm's law. However, a variable resistor varies with a standard resistor in that one can adjust its value to fit the required one. In this project, the resistor will be connected in series with the 7-segment display LEDs. More specifically, the project will make use of a variable resistor having a resistance range of 50W~2MW and a tolerance of 10%. In addition, it will be able to withstand 640V AC and possess an absolute minimum resistance of 10W. Thus, with the specifications mentioned above, it will perfectly suit this project.

References

Digital stop watch and digital timer circuit. (2011). Retrieved from http://www.circuitstoday.com/digital-stop-watch.

Theraja, B. L., et al. (2002). A textbook of electrical technology: In S.I. system of units. New Delhi: S. Chand & Co.