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I'd like to be able to provide my kids with the opportunity to build something a little better than a passive crystal set, but still very, very simple in terms of individual electronic parts count.

It seems to me that a CW receiver should be the easiest type to build, if I'm incorrect and an AM is easier, or just as easy, let me know.

What is the lowest parts count receiver one can build that covers some portion of the HF spectrum likely to have CW signals in it?

I'd prefer low cost parts, but low parts count is still the most important requirement. Ideally it could be breadboarded, but point to point wiring is fine. It doesn't have to be great audio quality, or even very selective and immune to noise. It just needs to be able to pick up enough of a signal that a careful listener will notice the CW in the static.

Is there a receiver design that needs 20 parts or less? 10 parts? Fewer?

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    $\begingroup$ This feels like a code golf question... $\endgroup$ – Adam Davis Jan 31 '14 at 21:05
  • $\begingroup$ Fixed frequency is fine if it's in the CW portion of one the bands in the US band plan, but as I only have a general license there are a few places you couldn't go if it's not popular and I need to transmit some code to demonstrate their receivers. I'd prefer the receiver to be good enough that they'll receive distant CW at high power, though. $\endgroup$ – Adam Davis Jan 31 '14 at 21:09
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    $\begingroup$ Does an SDR count as "simple"? Or does the attached computer count against the parts count? $\endgroup$ – Phil Frost - W8II Jan 31 '14 at 21:25
  • $\begingroup$ Are ICs allowed? Or are you counting each integrated component as a "part"? $\endgroup$ – Phil Frost - W8II Jan 31 '14 at 21:26
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    $\begingroup$ It sounds like you are of the opinion that easily googleable questions should not appear on stack exchange sites. I suppose that's a fundamental disagreement between your view of the site and my view of the site. $\endgroup$ – Adam Davis Feb 1 '14 at 14:21
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For CW, a simple regenerative receiver will produce an audio sidetone. Such radios were actually usable and used by radio hobbyists to receive HF CW. So I would search for a design of a 2 transistor regenerative receiver (2nd transistor used to isolate or drive some headphones) in vintage hobby electronic magazines and kits dating from the earliest decade of the availability of transistors to hobbyists. Probably somewhere between 10 and 20 parts, depending on how you count them. You might have to wind your own inductor coil(s).

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You might take a look at the Pixie 2-style transceivers, even if you need to leave the key plug disconnected for lack of license or desire to transmit. They are a very simple circuit, and if you'd like there are even kits available for import via online auction sites at extremely low prices (e.g. search eBay for "qrp pixie").

A post on The Chinese Pixie 2 QRP Transceiver gives a little more history of the design (and also mentions some slightly more complicated alternatives), and Circuit Swamp's Pixie 2 Transceiver page offers a bit more explanation of the circuit itself. It is a direct conversion receiver in this case fed by a simple oscillator circuit, mixed with the incoming signal using a transistor, and then amplified by a common IC.

In my experience, it was a fairly easy build — my kids helped "pick" the components while I did the soldering to PCB included with the kit we bought, but it should be simple enough to breadboard too. Especially with a mismatched antenna (100ft of wire thrown out the window) it picked up more local AM stations than it did 40m signals, but even under those circumstances we could still hear occasional Morse code at various pitches.

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