This Project is an all-electronic version of the signal you've seen many times at train crossings. You can also use this circuit as an alarm or "surprise" Project as well.
You can figure out how this Project works by looking at the schematic. LED 1 and LED 2 are connected to the outputs of a J-K flip-flop, and they go on and off as the outputs switch between 0 and 1.
An audio oscillator supplies sound to the Speaker. The circuit that generates the clock signal for the J-K flip-flop is called a Schmitt trigger. A Schmitt trigger is a bistable pulse generator (bistable means the circuit has two steady operating conditions it can be in).
A Schmitt trigger takes a slowly changing signal and converts it into one with sharp changes (much like the on-off output of an astable multivibrator).
In this Project the Schmitt trigger takes the output of the blocking oscillator and uses it form the clock signal for the J-K flip-flop. You can see this circuit in action just by setting the power to ON.
The sound and flashing LEDs make this ideal for other applications. Can you figure out ways to use this Project for similar purposes?