TheRadioBoard

Hi,

I posted on the board a couple of years ago, and got some good answers then - I'm hoping you can help me now.

(This is actually a similar question to my original post)

I've been given the task of teaching a group about radio receivers, and I was wondering if there's anything I can do to demonstrate the crystal radio "effect" without running a long antenna.

I guess I need to build a complementary radio transmitter that could function inside a room and put out enough signal to drive a crystal radio.

Does anyone have any suggestions/schematics on how to do this?

Steve

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http://www.stephenhobley.com/blog
Serving the Laser Harp, Tesla coil, and Killer Robot From Outer Space community since 1986

Hi Shobley,

Probably the simplest AM radio transmitter I know of is the scitoys.com Simple AM Radio Transmitter kit.

It is fixed at 1 Mhz . I have not tried it . And I am not sure how much power it puts out (probably VERY low power). You may have to really close couple it to a crystal set, but I would imagine it "should" work. So long as there is R.F. applied to the crystal set you have demonstrated the concept (IMHO).

Anybody have any experience with this kit or similar type circuits ?

K

If you can provide everything except the long antenna, then a loop antenna may be a suitable solution. If you're in a location reasonably close to powerful AM transmitters, then a loop antenna will likely be able to pick up enough signal to work with a crystal receiver.

There are a number of loop antenna discussions on this forum. A search should find them.

A short 30 ft or so antenna strung high in the room away from walls if possible with a ferrite antenna coil and variable capacitor in series will work fine for picking up all of your locals. A wire strung along the baseboard alone or hooked into the mains ground will do for the gnd connection to the set. I use the cover plate screw for gnd here occasionally.

I made a rough and extremly simple transmitter to test radios few years ago, here is the circuit.

and here is a the talk around it.
http://theradioboard.com/rb/viewtopic.php?t=1599

adding antenna should be easy just add a second one or two turn winding 10mm from primary extending the wire as "rabbitears a meter or so either side of coil should fill a room easily even with only 1.5V battery.

The crystal radio "effect" could be easily demonstrated using a standard radio repairman's signal generator as the transmitter (if one was available).
It would need to have internal modulation (400Hz usually) and most can be externally modulated with tone or audio from some other source (CD player etc.).

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Let me cast another vote in favor of adding a loop antenna. Easy to build from a hula-hoop or embroidery hoop (craft stores).

Listening to a local transmitter or sig generator will not be as impressive or motivating as listening to an actual radio station.

I just built a shortwave loop that brings in stations from all over the country on my little Integrated Circuit homebrew receiver.

http://theradioboard.com/rb/viewtopic.php?t=3970

Lots of info from Dave on building loops.

Rich

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Homebrew Radio ex-Silicon Valley

I have built a few of those + different variants and can wholeheartedly recommend them to anyone.

the issue there is with the availability of low frequency / broadcast band oscillators.

My solution was to use a crystal in the SW bands + gates as oscillators; or to use a counter / divider (HC4060) as an oscillator and pick on one of the lower frequencies from the divider.

If you use an emitter follower to modulate the power supply to the oscillator, you can get rid of the transformer too, and make the whole thing really small.

Actually, last Saturday I bought that 1Mhz crystal oscillator to reproduce scitoys.com AM transmitter As soon as the oscillator arrives I will let you know.

Besides that, I would also go for loop antenna radio! Its so simple and elegant. If you have a sound powered headphones (and impedance matching transformer) you could use it as a simple AM transmitter too! My max distance is around 50 cm

Aydar.

Today I received the crystal oscillator. My AM transmitors works too! (dont know why my photos are so big this time, sorry)

You should be able to upload your pics at a predetermined size, to your photo storage site (automatically re-sizes the pics as it loads).

This option is available with "Photobucket" and probably, most other sites.

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