The circuit is designed to use very little current to prolong battery life so that it can be left on permanently. A superbright’ red LED is used because this provides a bright flash with a low current.
If you want to use 4.5V supply by connecting 3 Alkaline cells or any other source, change the resistor along with LED from 3.3k to 1k for a better flash.
Note that AA cells will last longer than a 9V PP3 battery 😉
To flash two LEDs alternatively we have to increase the clock pulse speed to possibly fastest to have exact proper alternation.
- 100k potentiometer
- 10k and 3.3k
- 555 Timer
- 9v Battery