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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?

Adam Davis
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    This feels like a code golf question... – Adam Davis Jan 31 '14 at 21:05
  • 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. – Adam Davis Jan 31 '14 at 21:09
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    Does an SDR count as "simple"? Or does the attached computer count against the parts count? – Phil Frost - W8II Jan 31 '14 at 21:25
  • Are ICs allowed? Or are you counting each integrated component as a "part"? – Phil Frost - W8II Jan 31 '14 at 21:26
  • Meta: How can I build a _____? – Phil Frost - W8II Jan 31 '14 at 21:36
  • @PhilFrost No computer, no SDR, but ICs are fine and count as one component each. You can throw a micro in there if you want - the micro, an Si570, a rotary encoder, an LCD, and an NE602 only take up 5 parts, plus maybe a bypass cap or two. – Adam Davis Jan 31 '14 at 23:39
  • I don't propose those parts as the best solution, I haven't built a receiver before and don't know, I'm hoping that it's possible, though. I don't want to spend too much time on software, and don't want it to require a computer, which is why I don't want SDR. Can the LO be built with fewer components than the LCD, micro, and Si570? Probably. I don't know, which is why I'm asking... – Adam Davis Feb 01 '14 at 00:00
  • @AdamDavisKD8OAS well, you could put an amplifier on your crystal set, and now it's no longer a crystal set but a direct conversion AM receiver. Does that count? If you are willing to give up some selectivity, you can even strip down the crystal set. A diode, a cap, and a long wire works if you are close enough to an AM station that it's significantly louder than all the others. Get close enough and you don't even need the long wire. 3 parts, including antenna, if you count the headset. The answer to your question is "yes". We can discuss it all day beyond that, but it's discussion, not Q&A. – Phil Frost - W8II Feb 01 '14 at 03:44
  • @PhilFrost Can you offer a suggestion as to how to narrow this question down so it meets your standards? This comment section is full of discussion, but the question is a question and there exist one or more answers for it. It's not intended to start a discussion. – Adam Davis Feb 01 '14 at 05:24
  • Sure, but the point is the question by nature starts a discussion. You could ask any number of questions about how a particular design works. You could ask if a part in a specific design is necessary. You could ask if a CW receiver is simpler than an AM receiver. In any case, I think the best thing to do is research a little bit, find a design of interest, then ask about it. Google image search is great for finding schematics. There are whole books on simple radio projects. What SE brings to the table is specific answers when the broad resources don't cover something you need. – Phil Frost - W8II Feb 01 '14 at 12:20
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    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. – Adam Davis Feb 01 '14 at 14:21
  • Unfortunately, there's already a view of this site that trumps either of our opinions, because it's a stackexchange site. So don't take my word for it: Question [Closed]… and it’s probably best that way – Phil Frost - W8II Feb 02 '14 at 03:01

2 Answers2

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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).

hotpaw2
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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.

natevw - AF7TB
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