You can hear a local AM station clearly with this IC radio. It is tuning free so only the strongest nearby station will be available. Only a few components are necessary to build this Matchbox size radio and can be assembled quickly on a common PCB. No coil, No Gang, No professional touch. But still it is a Radio. See the trick to use the Gain pins of an amplifier IC as Signal receiver.
IC LM386 is used as a Signal receiver cum Audio amplifier to perform dual functions. Its Inverting input (pin2) is connected to ground through capacitor C1 leaving the Non inverting input (pin3) unconnected. IC LM386 is the low voltage amplifier IC with a gain 20 set internally. Its two gain pins (pin1 and pin 8) are used as the inputs for the radio signals captured by the Antenna and the Signal diode OA71.Usually the gain pins are connected using a coupling capacitor in Amplifier designs to set the gain. If the gain pins are left free, the gain of amplifier will be the default gain 20. If a capacitor is connected between the gain pins, the gain can be increased to 200. If a resistor is added, the gain can be set between 20 and 200. If the bypass pin 7 is connected to ground using a capacitor, the gain can be increased higher. This capacitor also prevents the oscillation of IC.
Here the gain pins are used a signal inputs and the signals are amplified and convert into audio which can be heard through a small Mylar speaker or Earphone.
Use a 12 volt Mini battery to make the unit compact. Antenna is a must to receive the signals. A long plastic wire around 3-5 meters can make the audio loud enough. Output power of different versions of LM386 is shown below.
LM 386 N-1 – 250 mW
LM 386 N-3 – 500mW
LM 386 N-4 – 750 mW