First Project - Simple FM Radio/Receiver made Complex

Discussion in 'The Projects Forum' started by Voltman13, Jul 9, 2010.

  1. Voltman13

    Thread Starter New Member

    Jul 9, 2010
    Hey everyone,

    I'm taking on a fairly difficult first project. It's based around a schematic called the 'Radio Shack Special,' which is a simple, super-regenerative radio receiver that is configured to pick up FM signals (It is supposed to be capable of both AM and FM).

    The FM receiver's signal will be sent though an Automatic Gain/Volume Control (AGC/AVC) circuit, in an attempt to even out the stations. The receiver is apparently flooded by local signals.

    The output of the AVC will be directed into three things: An LED flasher (flashes with current), LED VU meter (detects change in current), and a 45W amplifier.

    The receiver, AVC, and the 45W amplifier will have dedicated power supplies.
    The LED circuits will share a single +/-12v supply.

    This is the basic layout I've come up with:

    (1) - FM receiver (Radio Shack Special)
    (2) - Automatic gain control
    (3) -- Flashing LED's
    (4) -- LED VU Meter
    (5) -- 45W Amplifier

    (2) is linked to (1), and (3) - (5) are linked to (2).

    Now here are the questions:

    All of the circuits below the FM receiver require an input voltage of about 0.7v RMS. However, the FM receiver has a built in LM386 low power op-amp. How can the output voltage of the receiver be lowered without degrading the signal? I've looked into bypassing the LM386, removing it completely, or simply lowering its output voltage. My preference would be to lower its output voltage.

    Secondly, the AVC will most likely produce high output voltages, considering that it requires a +/- 15v power supply. The AVC is difficult to modify, so the voltage will have to be reduced after the circuit. What's the best way to reduce its output voltage?

    >> I've looked into several methods of reducing output voltage.

    - Small transformers: This could lead to a washed out signal.
    - Resistors in series: Will work, but the resistors could add unwanted noise, or overheat.
    - Resistors in parallel: This is basically the same as using one resister, but would require larger resistances (eg. 3x 255k) to have the same affect of a single resistor (eg. 85k). This would lower the chances of toasting the resistors (To my knowledge).
    - Voltage dividers: This is similar to a resistor in series, but is more adjustable.
    - Linear Drop-Out regulators: No knowledge.
    - Op-amp attenuators: No knowledge (I'll continue my research, of course).

    Needless to say, I need to read some books. Any recommendations?

    Thank you very much,



    45W Amp:


    VU Meter:

    FM Receiver (Radio Shack Special) -

    Website is down:

    Older design:

    The LED flasher circuit will be designed soon, after everything else is sorted out.

    Thanks again.
  2. Audioguru

    New Member

    Dec 20, 2007
    Why don't you make or buy a "real" (super-heterodyne) FM radio?

    The RadioShack Special super-regen is not a radio, it is a simple novelty for kids. It is much too simple.
    It is easily overloaded by strong local stations.
    It has severe distortion.
    Because it does not have an FM detector its audio level varies with signal strength.
    It has only one tuned circuit so its bandwidth will be too narrow for most FM stations or will not have enough selectivity so many stations will be heard at the same time.
    It does not have the proper de-emphasis for an FM radio so its sound will have no high audio frequencies.
    Its squelch oscillator will beat with the stereo pilot, stereo subcarrier and SCA restaurant music so its audio output will be full of whistles.

    "Real" FM radios do not need an AVC circuit. They have a real FM detector. They use many tuned circuits for good selectivity. Good radios tune the signal before the RF amplifier so they are not overloaded by strong local stations. Many RF amplifiers in real radios use automatic gain control. They do not produce whistles and have proper de-emphasis. They have low distortion.