Clock & Timer Circuit Diagrams

Digital Stopwatch Circuit

The principle, design, and operation of a digital stopwatch circuit are described in this article. A digital stopwatch is a circuit that displays the current time in minutes, hours, and seconds, or the number of clock pulses. The second type is one in which the circuit displays a count from 0 to 59, which represents a 60-second time period. To put it another way, the circuit merely shows the time in seconds. This is a straightforward device that uses a 555 timer to generate clock pulses and two counter ICs to perform the counting.


  • Digital Stopwatch Circuit Principle:
  • Digital Stopwatch Circuit Diagram:
    • Digital Stopwatch Circuit Design:
    • Working of Digital Stopwatch Circuit:
    • Digital Stopwatch Applications:
    • Digital Stopwatch Limitations:

Digital Stopwatch Circuit Principle:

This circuit works on the basis of synchronous cascade and a two-stage counter operation. The concept is to show clock pulses that count from 0 to 59, reflecting a 60-second interval. This is accomplished by using a 555 Timer IC in astable mode to generate 1 second interval clock pulses. While the first counter counts from 0 to 9, the second counter begins counting when the first counter’s count value hits nine. The counter ICs are coupled in a cascading fashion, with each counter output connected to a BCD to 7 segment decoder, which drives the 7 segment displays.

Digital Stopwatch Circuit Diagram:

Digital Stopwatch Circuit

Digital Stopwatch Circuit Design:

The astable multivibrator configuration of the 555 Timer is the first part of the design. The needed time period is 1 second in this case. We can determine the values of C using the frequency of the output signal provided by f = 1.44/ (Ra+Rb) C, assuming Ra and Rb values of roughly 10K. We have a 100uF electrolytic capacitor here.

The second step in the design process is to connect the two IC – 4510 counters in a synchronous cascade arrangement. This is accomplished by connecting the counter ICs’ clock pins to the 555 Timer’s output, resulting in parallel clock input signals. The carry out pin of one of the IC is connected to the carry in pin of another IC.

Because we want to start the second counter once the first counter hits the count value of 9, we use a basic combinational logic circuit to do so. According to the counter truth table, the appropriate binary count or status of the counter output signal for a clock pulse count of 9 is 1001. To put it another way, Q1 and Q4 are at high logic signals for a count of 9. The inputs of the AND gate IC 7408 are connected to the first counter’s pins Q1 while Q4, and the output is attached to the second counter’s U/D pin.

Here our requirement is to display the clock pulses till count of 60. This can be done by ensuring the second counter resets once the count reaches a value of 5. We achieve again by designing a simple logic circuit consisting of another AND gate IC, whose inputs are connected to pins Q3 and Q2 of the second counter.

The third part involves designing the display circuits. This is done by connecting the outputs of each counter IC to the inputs of BCD to 7 Segment Decoder. The outputs of each Decoder ICs 4511 are connected to the 7 Segment display.

Working of Digital Stopwatch Circuit:

When the typically open switch is turned to the closed position, the circuit operates. The Timer 555 alternates between high and low signals, giving in an oscillating signal whose frequency is determined by the values of two resistors and the charging capacitor. In other words, the timer 555 IC generates the needed time period clock pulses. The two-stage arrangement of BCD counters CD4510 receives this clock signal. The IC CD4510 is made up of four synchronously timed D- Flip-flops that are linked together to facilitate counting.The clock pulses are counted by two CD4510 counters in a two-stage synchronous cascaded system. As soon as the clock pulses reach IC U3, it begins counting from 0 to 9. The AND gate IC U4A creates a high logic output when the count hits 9, which is supplied to the U/D pin of IC U2. The count procedure is started by IC U2. When the IC U3 reaches its final count, the IC U2 continues counting, as does the IC U3. The AND gate U5B, on the other hand, sets the reset pin to high level once the IC U2 count hits 6. The count is presented on the BCD to 7 segment decoders CD4511-driven 7 segment displays. The clock pulses from 0 to 60 are so shown by the circuit.

Digital Stopwatch Applications:

  1. This circuit can be used as an indicator at quiz competitions.

Digital Stopwatch Limitations:

  1. The circuit does not display the actual time, but rather the count of clock pulses.
  2. The use of digital counter ICs produces a time delay in the whole operation, because of the propagation delay.
  3. This is a theoretical circuit and may require changes.

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