Tuning free AM Receiver. Design Trick 12

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


3 thoughts on “Tuning free AM Receiver. Design Trick 12

  1. ഒരു ഇലക്ട്രോണിക് ഷോപ്പിലും കോയിലുകൾ കിട്ടില്ല. റേഡിയോയുമായി ബന്ധപ്പെട്ട ഏത് സർക്യൂട്ട് ചെയ്യാമെന്നു വെച്ചാലും കോയിൽ കിട്ടാൻ വളരെ ബുദ്ധിമുട്ടാണ്. എന്നാൽ ഒരു കോയിൽ നിർമ്മിക്കാമെന്നു വിചാരിച്ചാൽ കമ്പിയുടെ വണ്ണം സർക്യൂട്ടിൽ SWG ആയിട്ടായിരിക്കും പറഞ്ഞിരിക്കുക. മൾട്ടിമീറ്ററുമായി ഘടിപ്പിക്കാവുന്ന കപ്പാസിറ്റൻസ് മീറ്ററിന്റെ സർക്യൂട്ട് ഒരു സൈറ്റിൽ കണ്ടിരുന്നു. അതുപോലെ ഇൻഡക്ടൻസ് അളക്കാനുള്ള വല്ല മീറ്ററും കുറഞ്ഞ ചിലവിൽ നിർമ്മിക്കാൻ സാധിക്കുമോ സാർ ?.

    1. Dear Vinod
      You will get copper wires of different SWG( Standard Wire Gauge) ranging from 15 to 30 SWG from Mixi repairing shops / mixi component shops. Purchase 100gm of required wire. Inductance can be calculated using the formula and it is difficult to make a simple inductance meter. No problem in coil making. FM tuners uses 22-24 swg 4-5 turns 5mm dia which can be wound around a ball pen. AM uses 32-36 swg around 100 to 150 turns.

      1. Thank you for the reply sir. I request you to publish a FM signal booster circuit. I saw a circuit in many sites which comprises MPF 102 transistor. But i can’t find a MPF 102 in any shop. So please consider the parts availability while designing the above said circuit.

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