Dan's Double Detector Crystal Radio Invention

Last Updated 27-Jun-2003

27-Jun-03: I still work on it a little bit at a time. Also I just bought an actual digital camera, so I can put up nicer pictures. I replaced the fuzzy picture of the antenna and added a picture of the project layed out on a board, but not wired yet. That's the current state of the project.

5-Apr-03: I just found another design on the web that uses two detectors to do basically the same thing I thought of. Bad news/Good news I guess. Bad news is "prior art" - no chance of me getting a patent. Good news is it validates my idea. I always thought if it was a good idea someone had to have already thought of it. It's just not obscure enough to be any other way. Here's a link to the other design. http://www.peeblesoriginals.com/push-pull-set.html. Looks to me like the other design uses two detectors to cut the output impedance of the radio in half instead of doubling the output voltage like my design. Either way, I'm pretty sure you get twice as much power at the headphones.

To get my set to use the "cut the output impedance in half" method I'd have to add more taps to the smaller coil and maybe get another variable capacitor. That could happen, but probably not. There's just to many other projects to do - how about a stereo, high power, class A tube amplifier?

23-Mar-03: Just put together a quick web page to show off the project. (It is called a "vanity" web site). Also, a number of people helped and they should get to see it. Keep checking back if your interested. I'll update the page as I get more done. I have to hurry and prove it actually works. The first storm we get this spring will probably blow down my antenna.

I was driving home from work one day and thought of a better crystal radio detector circuit. It's pretty simple, so it's hard to believe that I was the first to think of it. But, I've never seen it before - so I did invent it even if someone else did first. Then one trip back to South Dakota to see my family a bunch of us geeky Winkers wound the coils and patched the thing together in Dad's garage. Families argue about all kinds of things when they get together. We argue about where to put the taps on a radio coil. Alas, there wasn't a strong enough radio signal at Mom and Dad's to make it work. I took it home and it sat in my garage until about two weeks ago. Now it works!

It's not done. I got it patched together with clip leads and stuff. WCCO comes in strong. I haven't tried to get any other station yet. That would probably be a matter of filtering out WCCO more than of tuning in something else. Tuning in lots of stations really isn't the point. The point is to see if I really did invent a better crystal radio detector.

And so far that's not done. I was thinking it would just be a matter of tuning in a station and switching between normal and Dan's mode, but changing the mode also changes the tuning. You have to re-tune for maximum signal after switching modes, and re-tuning is a many step process. So, it's hard to get a side by side comparison. I'll try to get it more permanently mounted on a board. Then at least I'll be able to tune it easier. I think eventually I will be able to determine which way works better.

I have an old CB radio in the shed. The crystal radio should be able to tune CB radio frequencies. I could set the CB radio up to transmit a constant amplitude tone, then get a helper with a cell phone to go some distance away with the CB transmitter and beep the CB radio when I asked over the phone. Then I could get an accurate reading of the response on the oscilloscope and carefully tune the crystal radio once in each mode. I think that would be a pretty valid and scientific test.

The Most Recent Picture.

I think it's gonna look nice when I'm done. A lot of the parts aren't actually attached to the board yet, and it needs to be wired. Now that I have a decent camera you can see that something happened to the big coil. I'm not sure what happened. It's been like that almost ever since we made it. I might try putting on another coat of clear lacquer.

The Whole Setup The First Time It Worked

It's just in pieces with "clip leads" connecting it together. Soon I hope to make a more permanent radio to keep forever and ever. Some of the pieces you might be able to make out from left to right

My impedance bridge from before zip codes (no date on the machine, but the manufacturer's address doesn't have a zip code). This is not part of the crystal radio circuit. It's for measuring the values of capacitors and coils.
The two coils, one inside the other.
The "CSC Design Mate" circuit prototyper from about 1979. It still works Dad! It's not plugged in to any outlets here though. It wouldn't be fair to plug it in to an outlet as part of a crystal radio.
A soldering iron. That's important.
An oscilloscope. I suppose some would consider tuning a crystal radio with an oscilloscope to be cheating. Ok, I cheated. It sure saves time.

Far End Of The Antenna

Put this up in about 4 hours one Saturday (or was it a Sunday). It's 16 1/2 feet off the ground, and completely coincidentally, the "long wire" is almost exactly 100 feet long - close as I can reckon by counting fence posts. The other end of the "long wire" antenna is attached to the house at about the same level. It has survived a few storms by now. I'm really surprised it's still up (Jun 27, 03).

This Is Your Typical Crystal Radio

The diode delivers power to the headphone during the positive part of the RF cycle. The negative part of the RF cycle goes nowhere. We went to all the trouble to put up a 100 foot antenna and then waste half the energy we manage to capture.

This Is Dan's Double Detector Crystal Radio

Here the top diode delivers power to the headphone during the positive part of the RF cycle and the bottom diode delivers power during the negative part of the RF cycle.

In your typical crystal radio the voltage across the headphone comes from Det1 and ground (0 volts), so it has a magnitude equal to Det1 (the red line) in this graph. By putting the voltage from Det2 (the green line) at the other side of the headphone, we get a difference of Det1-Det2 across the headphone (the blue line). (There is a typo on the graphic. It really is "Det1-Det2", not the other way around).

Of course, this isn't exactly how it works. Really, the forward voltage drop of the diodes has a big effect on the output voltage, and all kinds of impedance interactions probably show up when you hook up another detector. But, this should get the idea across. A person could go through a lot of math to figure out exactly what happens. I think it's easier and more satisfying to just build the thing and see how good it works.

Close Up Of The Coils

These are the coils that sister Liz and I wound last time she was visiting at Mom and Dad's. It's a lot more work to wind radio coils than it looks like from here. You slide the inner coil in and out as part of the tuning.

One of the variable capacitors is just in front of the coils. The other is off to the right.

Close Up Of The Double Detector

Just a close up of the double detector, don't know if you can make enough of it out to make sense anyway.

Check Out Winker's Home Page
Home

+------------+
|   Website  |
| Created in |
|     vim    |
+------------+