any type of switch suitable to you. A 100K variable resistor is used to adjust the speed of the numbers in calculator.

After completing the project connect the power and press 0+1 on the calculator. When we press 0+1 on the calculator and then "=" again and again, the calculator gives sequential output as 1, 2, 3, 4. But in this project the 555 IC circuit does this job automatically.

After completing the circuit enclose it in a suitable enclosure with the batteries to make it portable. You can also operate the circuit with lower voltage for example 3V by replacing the NE555 timer IC with 7555 timer.

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The figure is showing a simple stop watch project. The circuit is easy to build and using very few parts which are a 555 timer IC, a calculator and few other components. The operating voltage of the circuit is 5V to 12V DC.
The stop watch can be operated with the switch S1. For S1 you can use
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Simple Stop Watch Project Using 555 IC