1. Project Concept: A Retro Stopwatch
The purpose of this project is to build a simple digital stopwatch that counts seconds up to 59 and can be manually started, stopped, and reset. The stopwatch uses discrete digital components, with the TC74HC175AP flip-flops playing a key role in storing and transferring timing data. The display is handled using 7-segment LED modules, and the user interface includes tactile push buttons.This kind of stopwatch doesn’t rely on modern microcontrollers or programming. Instead, it showcases the power of combinational and sequential logic built using flip-flops, counters, and decoders. It’s ideal for hobbyists who appreciate the tactile, hands-on feel of electronics and want to gain a better understanding of digital logic.
2. Understanding the TC74HC175AP
At the heart of this project lies the TC74HC175AP, a high-speed CMOS device with four D-type flip-flops. Each flip-flop in the IC has a data input (D), a clock input (CP), a direct clear (CLR), and complementary outputs (Q and Q’). In simple terms, this chip stores binary data and outputs it based on clock pulses. In the stopwatch, these flip-flops help latch and hold counter values, enabling consistent and synchronized time-keeping and display transitions.These flip-flops work in cooperation with clock generators, binary counters, and decoder drivers to generate the timing sequence and manage visual output.
3. The Core Modules of the Stopwatch
The stopwatch consists of five primary modules, each constructed from off-the-shelf components, with the TC74HC175AP playing a critical role in the logic control and memory module.1. Clock Generator
This module creates the foundational timing signal — a steady stream of electrical pulses that drive the counting process. A basic astable multivibrator using a 555 timer or a crystal oscillator circuit can be used to generate 1 Hz pulses (one pulse per second).2. Counting Circuit
The counting circuit takes input from the clock generator and increments the value accordingly. This is divided into two parts: one counter for seconds (0–9) and one for tens of seconds (0–5). The counters count from 00 to 59 before rolling over.3. TC74HC175AP Latching and Memory
Here, the TC74HC175AP flip-flops are used to temporarily hold the current value when the stopwatch is stopped. When the start button is pressed, the flip-flops are clocked to update with the counter values. When stopped, the flip-flops hold the values and prevent changes until reset.4. Display Module
The output from the flip-flops and counters is passed to 7-segment display drivers. These drivers convert binary signals into outputs that illuminate the appropriate segments of a 7-segment LED display. The stopwatch shows two digits — seconds and tens of seconds — for a maximum of 59 seconds.5. Control Module
Three push buttons are used: Start, Stop, and Reset. These buttons control whether the clock signal is passed to the counters and whether the flip-flops update or reset.4. Building the Circuit: Step-by-Step
Step 1: Clock Generation
You begin by constructing a 1Hz clock generator. Using a 555 timer IC in astable mode with suitable resistor-capacitor values will give a pulse per second. This output will be used as the main input to the counter circuit.Alternatively, for higher precision, you can use a quartz crystal oscillator circuit. This ensures the stopwatch is accurate to real-world seconds.
Step 2: Setting Up the Counters
Next, connect two binary counters — one for units (0–9) and another for tens (0–5). Each counter will increment upon receiving a clock pulse. The units counter overflows every 10 pulses, sending a carry-out signal to the tens counter. This results in a rolling sequence from 00 to 59, after which it resets to 00.Use simple binary counters like the CD4029 or 74LS90 series. Wire them in cascade to ensure proper overflow behavior.
Step 3: Integrating TC74HC175AP
Now comes the crucial part — integrating the TC74HC175AP flip-flops. You’ll use them to latch the counter values during runtime. When the stopwatch is running, clock pulses continuously update the flip-flops with new values. However, when the Stop button is pressed, the clock input to the flip-flops is halted, and the current values remain latched, allowing you to read the stopped time.Each TC74HC175AP has four flip-flops. Two of these ICs can be used to handle the binary output of the units and tens counters. Connect the counter outputs to the D inputs of the flip-flops, and the flip-flop Q outputs to the display drivers.
When the Start button is pressed, a clock-enable signal is sent to the flip-flops. As long as this signal is active, the flip-flops will latch new values on each clock pulse. When the signal is removed (Stop button), the flip-flops stop updating, freezing the display.
Step 4: Display Connections
Each set of 4-bit outputs from the TC74HC175AP flip-flops corresponds to a digit (0–9 or 0–5 in binary). These outputs are connected to 7-segment decoders like the 74LS47 or CD4511, which convert the binary values into segment control lines for the display.Connect the segment outputs from the decoders to the appropriate pins on the 7-segment LED modules. Ensure that you use current-limiting resistors to prevent LED burnout.
Mount the two 7-segment displays side by side, labeling one as “Tens” and the other as “Units” to help interpret the time.
Step 5: User Interface
Install three tactile switches:● Start Button: Enables the clock signal to reach the flip-flops and counters.
● Stop Button: Disables the clock signal, causing the flip-flops to latch their last value.
● Reset Button: Clears the counters and flip-flops, setting the display back to 00.
The logic behind these buttons can be built using NAND or NOR gates, with flip-flops controlling the clock gate signal. Debounce the buttons using RC filters or Schmitt trigger buffers to ensure clean signals.
5. Assembling the Stopwatch
After all modules are tested independently, assemble them onto a single prototype board. Arrange the power supply circuit to provide a stable 5V to all components. Use headers or jumpers to make parts replaceable or adjustable.Label the major components and organize wiring carefully to prevent errors. A neatly laid out circuit helps in troubleshooting and ensures signal integrity.
If you have access to a soldering station and enclosure materials, you can transfer the prototype to a soldered perfboard or custom PCB. Create a simple casing using acrylic or wood to house the display, buttons, and electronics. This will give your stopwatch a polished, finished appearance.
6. Testing and Calibration
Power on the stopwatch and press the Start button. Observe the display as it increments every second. After some time, press Stop. The display should freeze at the exact count. Press Reset to return to 00.Check for:
● Accurate second intervals (compare with a reference clock).
● Smooth transitions between digits.
● Clean button responses without bounce.
If the stopwatch advances too quickly or slowly, adjust the clock generator's timing components or crystal frequency. If buttons respond erratically, add or adjust the debounce circuit.
7. Enhancements and Variations
Once the basic stopwatch is operational, consider adding extra features:● Minute Counter: Add two more counters and two more TC74HC175AP chips to count up to 59 minutes.
● Lap Timer Mode: Add a second set of flip-flops to store intermediate times.
● Buzzer Alarm: Connect a piezo buzzer that sounds when a certain time is reached.
● Battery Operation: Use a 9V battery with a voltage regulator for portable use.
● Backlight: Add LED backlighting for low-light readability.
These additions can make your stopwatch more functional and visually impressive while continuing to rely solely on hardware logic.
8. Educational Value
Beyond the enjoyment of building a retro stopwatch, this project teaches valuable concepts:● How D-type flip-flops work and how to use them in practical circuits.
● Timing control through logic rather than programming.
● Basics of binary counting and decoding for displays.
● Circuit integration using modular design.
It also demonstrates the reliability and flexibility of CMOS components like the TC74HC175AP, which remain relevant despite advances in programmable logic.
Final Thoughts
Creating a digital stopwatch using the TC74HC175AP is not just about timing seconds; it’s about stepping into the fascinating world of digital electronics through tactile, hands-on learning. This project is perfect for hobbyists who enjoy logic puzzles in hardware form, students who want to deepen their understanding of digital systems, or anyone intrigued by how things work beneath the surface of modern electronics.While today’s projects often lean heavily on microcontrollers and coding, building a logic-based stopwatch revives the foundational practices that built the electronics world we know. There’s a certain magic in watching numbers appear on a screen because of nothing more than silicon chips, wires, and a clever arrangement of logic — and this project captures that beautifully.
US 14347 
GB 12604 
CA 10440 
AU 8967 
BR 6412 
IE 5913 
NZ 2240 
SG 1136 
