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First Project - Simple FM Radio/Receiver made Complex

  1. Jul 9, 2010 #1
    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: http://www.redcircuits.com/Page150.htm

    AVC: http://electroschematics.com/479/aut...olume-control/

    VU Meter: http://www.circuit-projects.com/audi...and-lm324.html

    FM Receiver (Radio Shack Special) -

    Website is down: http://braincambre500.freeservers.co...)(1)(1).htm#aa [Broken]

    Older design: http://www.somerset.net/arm/reprints...ecial/rss.html [Broken]

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

    Thanks again.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jul 9, 2010 #2


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    Science Advisor
    Gold Member

    There's plenty there for a first project. Good luck with it.
    If you think your level is too high at some stage, why not just change the gain of that op-amp stage by using the appropriate feedback resistors (which will be there in any case -so you just need to change the values)? If you have only a low gain in that stage the feedback will make the op-amp very linear and wide band!

    If you want to use a resistive pot-down that will have no significant effect on the noise level because the signal is already high. Noise is only a problem for very low wanted signal levels.

    Have fun.
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