Here's a Project that imitates our feathered friends —you could say it mocks the mockingbird!
When you turn the power ON, you won't hear any sound from the Speaker. Now press the Key. You'll hear a chirping sound from the Speaker. Release the Key: and you'll still hear the chirping sound for a few more moments until it stops.
Take a look at the schematic and see if you can cell why this You'll see that when you press the Key a current is supplied to the base of the left Transistor. This allows the Transistor to operate and supplies current for the right Transistor.
The 10 uF capacitor charges when the Key is pressed and starts to discharge when the Key is released. When it is completely discharged, the circuit will no longer work. Try a different value of capacitor in place of the 10 uF and 100 uF ones and see what happens.
You can also use this Project as a door chime by using some of the techniques you found out about in Project Door Alarm in Oscillator Section. Can you think of some other applications?
The CdS Cell could do some interesting things if placed ... no, we'll let you figure that one out! Be sure to keep notes and schematics about what you do.