Here is a simple but effective intercom circuit that is based fully on transistors.The circuit is based on a three stage RC coupled amplifier. When the pushbutton S2 is pressed, the amplifier circuit wired around T1 & T2 becomes an astable multivibrator and starts producing the ringing signals. These ringing signals will be amplified by the transistor T3 to drive the speaker. When the push button S2 is released the circuit will behave as an ordinary amplifier and you can talk to the other side through it.
This is a low cost simple radio circuit based on op amp which can be used to listen to nearby stations.
A simple op-amp radio basically consists of a crystal radio along with a fairly sensitive audio amplifier. The audio amplifier is able to receive strong stations around Los Angeles with a minimal 15 foot antenna. Stronger signals are always provided by longer antennas.But the main disadvantage is that in the case of long antennas the stronger signals provided may produce a noise as it could be heard in the background of the weaker signals.