With this simple circuit, you can secretly listen to conversations going on in a room. The circuit is very sensitive and powered by a 3V battery.
The working of the circuit is simple. When a conversation is going on in the vicinity of the condenser microphone, the weak sound signals are amplified by transistors T1 and T2. The amplified signal is fed to audio driver transformer X1, which further amplifies the signal. The transformer output is fed to a speaker through a capacitor. In brief, when the microphone senses sound in a room, the same can be listened outside the room through the speaker. Keep the speaker away from the microphone to avoid howling due to feedback. Transformer X1 is a readily available six-leaded driver transformer having a single primary winding and dual secondary windings. Adjust preset VR1 for maximum clarity and gain.
Assemble the circuit on a general-purpose PCB and house in a suitable cabinet. Connect the condenser microphone through a wire and place the cabinet anywhere in the target room. Connect output wires to a 0.5-watt speaker kept in the room where you want to monitor from. Any type of two-core wire can be used for the purpose. The circuit can pick up sound signals from a distance of several metres. Use a good-quality microphone. It can be hidden inside a 2-3cm long tube.