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I'd like to build a simple AM radio (so simple it will only receive a single AM channel).

What is the simplest way to build a single channel AM radio that will output to an 8ohm speaker? (e.g. 1030 AM)

My current thinking is along the following somewhat vague lines, but I'm hoping to make them more concrete.

  1. use a piece of wire as an antenna
  2. create an LC circuit to tune input from antenna to the desired frequency
  3. amplify the signal
  4. connect amplified signal to speaker

Aside from the antenna, I would prefer to use "discrete" off the shelf components such as capacitors and inductors of particular values that I could plug into my breadboard.

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It also helps to pick a strong station. I noticed mentioned 1.03 MHz. If you're near Boston MA, that's a good choice since it's 50 kW clear channel. Back in grade school, I could pick it up about 20 miles northeast with just a crystal radio and headphones. I did have about a 50 foot antenna from my window to a shed in the back yard. That helped a lot. –  Olin Lathrop Feb 13 at 22:44
    
What's with the close votes? This is certainly on topic, and the OP has shown he has at least given it some thought. It's clear enough what he's asking, and it's well enough written. What's the problem? –  Olin Lathrop Feb 13 at 22:54
    
@OlinLathrop I don't like the requirements. "Simplest" without specifying some minimum performance requirement isn't a great specification. This is one of the few cases where I think more subjective requirements are better: I'd feel better if it asked for a "good design for educational purposes", but even better would be if it asked about a specific part of the design issue. It's bordering on a "discuss with me" type question due to the lack of a clear problem, and inclusion of half a solution. –  Phil Frost Feb 14 at 2:09

4 Answers 4

up vote 4 down vote accepted

Your basic strategy to use a piece of wire for the antenna, into a L-C resonant circuit tuned to the frequency of the desired station, amplify that, then connect to a speaker is on the right track but misses at least one important step. That is to detect the AM signal, or to demodulate the amplitude modulated radio signal to get the original audio signal.

After the demodulator, you should also add a stage to drive the 8 Ω speaker with. That will be a stage meant to produce power, not necessarily amplify the voltage. To skip this, you can make just a line level audio signal and feed it into some ordinary audio amplifier, or powered speaker like you would hook up to a PC sounds card. The powered speaker is really just a power amplifier and speaker in a single box.

The simplest AM detector is just a diode. The output of the diode effectively rides the peaks of the RF signal, and goes up and down with its amplitude. You set up the impedance driven by the diode such that the output decays slowly enough so that it doesn't change much in the gap between carrier cycles, but does change fast enough to follow the modulated signal. Fortunately this isn't too hard since there is a large frequency ratio from audio to carrier. If the highest audio frequency is 5 kHz and the carrier is 1 MHz, then you have 200:1 difference.

A even better way to make a detector is to use a bipolar transistor as emitter follower. You can think of this as a powered diode in this application:

This audio out signal can probably be fed directly into a PC "powered speaker".

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Some of the simplest AM radio receivers were based on the ZN414 - a radio on a single chip in a TO-92 package. (replaced by the MK484).

enter image description here

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It is poorly suited to drive an 8\$\Omega\$ speaker directly (it won't be very loud) but you could add an LM386 amplifier as shown here.

enter image description here

If you really want to work with discrete transistors rather than ICs hiding in transistor packages, you could just follow up that tuned loopstick-capacitor circuit with a diode detector (a germanium such as a 1N34A or Schottky diode such as a BAT54 would be best) and an audio amplifier. The missing part of your list is the detector, which converts the RF envelope into audio.

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Simplest possible AM receiver that outputs to an 8 ohm speaker:

schematic

simulate this circuit – Schematic created using CircuitLab

This radio has:

  • extremely poor selectivity
  • extremely poor sensitivity
  • extremely poor volume
  • extremely poor fidelity

It is however, without a doubt, the simplest radio that meets your requirements. It works because the voice coil can pick up RF currents just like an antenna, and these currents will move the diaphragm. It will help to use a very cheap speaker, since these have worse shielding.

You will need to be quite close to the transmitter for it to work however, due to the poor sensitivity. And also far from other transmitters, because it pretty much has no selectivity. Being entirely passive and also being very sensitive, the speaker won't be very loud, and since speakers aren't really intended to be operated in this manner, I'm sure the audio fidelity will be horrible. It is, however, obviously maximally simple.

While not an especially good receiver, it is an instructive example, and it does work. It demonstrates that radios aren't magic, and electromagnetic radiation can interact with just about anything. The only thing you need to receive AM is a detector: anything that responds to current differently in one direction or another.

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Anyone care to explain how this is not the simplest receiver? Does it not work? Or is there something simpler? –  Phil Frost Feb 13 at 22:20
    
This is missing the detector. Other than something accidentally acting as a diode, this won't pick up AM. –  Olin Lathrop Feb 13 at 22:22
    
@OlinLathrop The detector is the nonlinear magnetic materials in the speaker. –  Phil Frost Feb 13 at 22:24
    
Make an (especially) crappy connection between the antenna and the speaker that acts as a copper oxide rectifier. –  Spehro Pefhany Feb 13 at 22:30
    
Even if the speaker reacts non-linearly to high frequencies, those high frequencies will still have a 0 average level, which is what the speaker will produce as sound. The magnetic material would have to be non-linear differently in each direction to act as detector. –  Olin Lathrop Feb 13 at 22:33

Here are a few that look OK:-

enter image description here

My favourite is #2.

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