This is another circuit you can see how it works by its name - so make a guess before building this project and finding out for sure.
As you build this project, set the select switch up. Then, press S1. You'll see LED 1 light up. Now set select switch down. What does LED 1 do?
Now with the select switch down, press S1 again. You'll see that LED 1 goes out. Move the select switch up and back again. Is there any change in LED 1 ?
You can easily see where the latch circuit gets its name. The circuit can "latch" in a certain condition and keep the same output regardless of changes in the input.
We can use latch circuits to control the operations of electronic devices since it maintains a certain output until we send a signal to cause a change - we don't have to keep sending an input signal to the latch circuit continuously. This means just a touch can activate a latch circuit... just as you saw in this project.