Stopwatch 0-99 seconds (MC14518B)

Description: The circuit that has been used is shown in picture 1. Through the experimental part we will explain each of the parts function, but in order to have a notion of the basic idea, let just say, that this circuit besides the 5V power supply, is fed with a pulse which comes from a crystal. The crystal's pulse is divided properly in order to obtain the 1 Hz pulse which we need in order for the circuit to work properly, and display the seconds on the 7 segment displays, through a procedure which we will explain through the experimental part. In the present article, we will describe the function of a digital stopwatch, 0 – 99 sec. The function of the stopwatch relies in the use of 4 integrated circuits, which in this case belong to National Semiconductor. It is obvious that other integrated circuits can be used to achieve the same result, however in this case we have used the following parts: 1 x CD4060BM (14 stage ripple carry binary counter), 1 x CD4040BM (14 stage ripple carry binary counter), 1 x MC14518B (BCD counter), 2 x MC14511B (BCD to seven segment driver), 2 x 7 segment LED displays.

The generator of the circuit comprises of the integrated circuits CD4040CM and CD4060CM. A crystal oscillating at a frequency of 4,194,304 MHz is utilized. This frequency is too high for direct use in the circuit; therefore, it must be divided down to a usable frequency of 1 Hz. The CD4060 integrated circuit performs this division by powers of 2. The outputs are labeled as Q4, Q5, and so forth. By applying a pulse to the CLK input of the CD4060, an output signal can be obtained at Qn with a frequency of f/2^n. For example, by extracting the signal from Q14, the original frequency of 4,194,304 Hz is divided down to 256 Hz.

This 256 Hz signal is then fed into the CD4040, and by taking the output from Q8, a 1 Hz inverted signal is produced. The inversion does not impede the circuit's functionality; rather, it ensures that the subsequent circuit is triggered by a logical '0'. A visual indication of the counting process can be achieved by connecting an LED to the output, which illuminates with each positive pulse, allowing for easy monitoring of the circuit's operation.

The 1 Hz signal from the generator is input to the BCD counter MC14518. This integrated circuit increments its output by a logical '1' with every pulse received. The MC14518 is effectively split into two segments: one for counting the units of seconds and the other for counting the decades. The pulse from the generator is directed to the segment that counts the units since it is necessary for the display to increment by 1 every second. Conversely, the first display must increment every 10 seconds, which is achieved by grounding the CLK input and utilizing the signal from Q3 to the CKE input.

This configuration ensures that the first display increments only when the Q3 signal transitions from a logical '1' to a logical '0'. The first display thus increases every 10 seconds, resetting to zero after reaching 9, while simultaneously triggering the BCD counter for the decades. When the decades display reaches 9, the circuit resets to zero, allowing the counting to commence again.

The integrated circuits MC14511 serve as BCD to 7-segment drivers. Their function is to convert the BCD output of the MC14518 into a format that can be displayed on the 7-segment displays. The inputs for Lamp Test and Blanking are set to logical '0' as they are not required for this application. The LE (Latch Enable) input allows the display to maintain its number while the pulse continues, functioning similarly to a hold feature found in modern stopwatches.The circuit that has been used is shown in picture 1. Through the experimental part we will explain each of the parts function, but in order to have a notion of the basic idea, let just say, that this circuit besides the 5V power supply, is fed with a pulse which comes from a crystal. The crystal?s pulse is devided properly in order to obtain the 1 Hz pulse which we need in order for the circuit to work properly, and display the seconds on the 7 segment displays, through a procedure which we will explain through the experimental part.

In the present article, we will describe the function of a digital stopwatch, 0 ? 99 sec. The function of the stopwatch, relies in the use of 4 integrated circuits, which in this case belong to National Semiconductor (http://www.national.com). It is obvious that other integrated circuits can be used to achieve the same result, however in this case we have used the following parts: ?.

1 x CD4060BM (14 stage ripple carry binary counter) B. 1 x CD4040BM (14 stage ripple carry binary counter) C. 1 x MC14518B (BCD counter) D. 2 x MC14511B (BCD to seven segment driver) E. 2 x 7 segment LED displays The generator of the circuit comprises of the integrated circuits CD4040CM and CD4060CM. We use a crystal which oscillates at a frequency of 4,194,304MHz. It is obvious that this frequency is completely useless, as it is too big to be used as it is to our circuit.

What we should is devide this frequency, in a way that in its final form, the pulse will have a frequency of 1Hz, which is the desirable frequency. Initially we use the integrated CD4060, which devides the imported frequency in its input, by forces of 2.

As we can see on the integrated circuit the outputs are marked as Q4, Q5,? Qn. By importing a pulse in the CLK input of the 4060, with a frequency f Hz, we take out of output Qn, a signal which has a frequency equal to f/2n,. So, by exporting the signal out of Q14, knowing that the imported signal has a frequency of 4,194,304Hz, we take a signal, which has a frequency of 256Hz.

By importing this signal, to 4040 and by exporting the signal through Q8 we have finally taken an inverted signal, at the frequency of 1Hz. The fact that the signal is inverted, firstly doesn?t affect the proper function of our circuit and secondly is due to the inversion of the CLK input as we can see.

This inversion just causes, the following circuit to be triggered with a logical ?0?. By putting a LED on the same output, we have a visual of the counting, as in each positive pulse the diode polarizes positively, and a current passes through it. The signal of 1Hz, which we have taken from the generator, is imported to a BCD counter MC14518. This integrated circuit adds a logical ?1? at each pulse, on its output.???. .The MC14518 is virtually divided into two segment. One counts the units of the seconds, while the other the decades. As we can see in picture 1, the generators pulse is imported to the part which counts the units. This is very logical, as we want in each secont the number of the display to be raised by 1. On the other hand, we want the first display to raise by 1, every 10 seconds. This is why, we ground the CLK input, and we use the signal of Q3 to the CKE input. By using this means, we make sure that the first display will be triggered, only when we have a decreasing signal on Q3; that is, only when the signal drops from logical ?1?

to logical ?0?. As we can see, the first display increments every 10 seconds, which means that after 9 on the second display (1001 on the output of the BCD counter) the first display must be set to zero, while the first must be set to +1. That is that from 1001 à 0000, and we have a descending pulse, as the last digit descends from logical ?1?

to logical ?0? and triggers the BCD counter of the decades. When the decades display becomes 9 then the circuit goes to the next state, which is zero, and the counting begins once more. The integrated circuits MC14511 are BCD to 7 segment drivers. As its name clearly state, their sole purpose is to translate the BCD information of MC14518, to a code understandable by the 7 segment displays.

The inputs (Lamp Test, Blanking) are used to test the LEDs of the display and pulse modulate the brightness of the display. In this case we these inputs to logical ?0?, as we don?t need them. The LE input (Latch Enable) is used to keep the number of the displays while the pulse still runs. It is a HOLD function similar to the one of the modern stopwatches.