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Jerry, Just a coincidence but I've also been searching for an interesting project for young builders to tinker with radio circuits.? AA7EE's receiver sketch might serve the purpose.? Would there be any interest in this group to move this design to plug and play modules that when implemented would result in a functioning 40m receiver?? The target audience would be kids in the 8-13 yr range.?? Best to center the operating frequency on ARRL's W1AW broadcasts at 7.0475 MHz.? If not this radio, any suggestions?? Must be cheap and reproducible.? By the way, have a browse through the local dollar store where you will occasionally find really affordable electronic stuff.? I recently snagged a few of their powered speaker modules that could easily amplify the HiPerMite headphone output to comfortable desk-top audio.? Howard, n3fel

The Rugster from AA7EE we've been discussing here uses the HiPerMite audio filter,?
which is a very good CW filter but I think a beginner's receiver should just let them hear
the whole audio spectrum.? ?Then they can also hear SSB or even AM stations if the thing
is tuned to an appropriate band segment.

W7ZOI's book EMRFD starts out with a nice simple DC receiver in figure 1.9 on page 1.8.
Just an SA612 plus an LM386, plus a minimum of other parts.
This is probably the circuit I would build first, and compare anything else with.
The variable cap might cost a bit more than a 1n4001, but more linear, and less sensitive to supply noise.

AA7EE also posted this other DC receiver for 80m back in 2012, similar to the EMRFD 40m receiver.
And the whole thing is simpler than putting together a HiPerMite.
? ??

Both the EMRFD 40m receiver and the AA7EE 80m receiver have a single tuned input filter.
AA7EE mentions in the Rugster post that he lives in a built up urban area, and so wanted to try a double tuned circuit.

There is also a lot of variation in the local oscillator.
AA7EE's Rugster uses a varactor diode in an LC circuit.
But his 80m DC receiver uses a ceramic resonator plus varactor diode.
The EMRFD design uses a nice traditional variable cap in an LC circuit.


If you are designing "modules" to make building a receiver easier, I guess that's an SA612 and an LM386?
that get plugged together like two lego blocks.? I'm not so sure I see much point in that.
I'd tend to just give them an etched PC board and a sack of parts.
For small hands and sharp eyes, 1206 and SOIC surface mount parts may not be an issue.
Have them put together the audio amp first and get that working, it can be useful all by itself.
Have a scope on hand (a clone DSO138 kit is now $8 on ebay!) so they can see the audio signals.
The SA612 section could be a bit more tricky to get working correctly,
make sure you have that fully figured out before turning it loose on a bunch of kids.??
Include a good write up, Dave Benson's SW40+ manual is a great example of good.

Note that the SA612 local oscillator for a DC receiver is at the receive frequency.
So you could let the kids play with sending morse code across the room by keying that oscillator.
And if there's more than a couple kids in the group, they will learn what QRM is.


If you really want a CW filter on this DC receiver but think the HiPerMite is a bit too complicated,
then steal the filter and audio amp from the SW40+.??? ?
? ? ?

You probably don't need the FET switch though,?it is there to mute the receiver during transmit.??
So remove it, and short across where Source and Drain were.
Otherwise, just follow his lead on everything from the SA612 audio output pins to the headphones.
The op amp he uses probably won't drive a speaker, but at least in my household
nobody else wants to hear a lot of CW.

Jerry, KE7ER

Howard,

On second thought, your modules could be a fine way to teach radio.
Modules start to make a lot of sense as radios become more complex.
Many who design radios for a career have created their own modules to play with new concepts.

As you suggested, a DC receiver could be busted down some.
Perhaps demo a very simple diode detector with hi-z phones for local AM stations..
Then a bare bones SA612 (with 9v battery, coil+cap for LO, hi-Z phones, wire antenna).
An inductor tuned with a brass screw for the LO is cheap and approachable.

Same rig could hear your 40m transmission by adjusting the LO coil and cap.
Then add an audio amp, RF gain pot on the antenna, audio gain pot, RF filter,
RF amp, AF filter, a back-to-back diode audio limiter, a stable si5351 vfo.
From there, perhaps a second SA612 and a crystal IF filter, agc,? audio into an RPi.
Spend the remainder of the hour showing how to code DSP algorithms in python.

I might want to play with all that once you're done with it.

Jerry, KE7ER

Here's an unexpected curiosity for those with way too many LM386's in the junk box.
A single chip regenerative receiver.? ?
Using only an LM386:

? ??

Jerry, KE7ER

Most anything that resonates with your cap, assuming you have a bunch of wire out the window for an antenna.
The coil becomes critical when there is no wire antenna, then the bigger the ferrite rod is the more signal you scoop up,
I assume via magnetic coupling, no longer thinking microvolts into a 50 ohm antenna connector.

Back about 55 years ago, I found toilet paper rolls to make fine inductor cores for AM radio projects of this sort

Jerry, KE7ER

On Sun, Dec 15, 2019 at 11:50 AM, Jerry Gaffke wrote:

Then a bare bones SA612 (with 9v battery, coil+cap for LO, hi-Z phones, wire antenna).
An inductor tuned with a brass screw for the LO is cheap and approachable.

Jerry, Give me a few weeks to work on this, but a simple, modular approach makes a lot of sense for the younger kids, while the joy of assembly might appeal to the older group.? Maybe a few DIP headers with partitioned circuits and a plug board to tie them together.? Making up those plug-in modules from SMD chips and passives could be a project for the oldest members of the group.

Howard, n3fel