In the kit Adjustable 1-10 Minute Timer Circuit a timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass.
The kit Adjustable 1-10 Minute Timer Circuit 555 starts timing when switched on. The green LED (D3) will glow to show that timing is in progress. When the time period is over the green (D3) LED will turns OFF, and the yellow (D2) LED turns ON and the beeper sounds. The time period is set by adjusting the variable resistor. It can be adjusted from 1 to 10 minutes (approximately) with the parts shown in the diagram. You can mark the times on a scale drawn on the box. Please note that the range of time periods is only approximate. With perfect components the maximum time period should be 4½ minutes, but this is typically extended to about 10 minutes because the 220µF timing capacitor slowly leaks charge. This is a problem with all electrolytic capacitors, but some leak more than others. In addition the actual value of electrolytic capacitors can vary by as much as ±30% of their rated value.