Crystal radio troubleshooting advice

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Crystal radio troubleshooting advice

Postby Daddy E » Sat Dec 01, 2012 11:38 am

My eighth grade child choose the "Build Your Own Crystal Radio," project. I have an accounting degree but worked as an industrial electrician for over 12 years so I have some common sense and can follow directions and/or diagrams. However, we've follwed the instrcutions as provided and we are not getting any sound after hooking up the ground and the antenna to the radio today. We're both very disappointed in our results thus far and are really hoping that someone can provide a clue that will help us get the results we desire?
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Project Question: Trouble shooting crystal radio science fair project
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Re: Crystal radio troubleshooting advice

Postby rmarz » Sun Dec 02, 2012 1:24 pm

Daddy E - Tell me more about the components you used in the radio. Did you use the Science Buddies kit? There are a few very critical choices to make if you are starting from scratch.

1) Is the earphone you used a crystal or ceramic unit? A magnetic earphone that you typically use in your MP3 player or smartphone will not work because it is a low impedence, magnetic type.

2) Is the diode a germanium diode? You need a very low voltage drop device to get any decent rectified signal from your experiment.

3) The antenna and ground are very important. The antenna should be a fairly long wire. The ground is also critical and should be something like a water pipe in your house (metal, all the way to the earthen ground pipe.

The last thing to consider is your proximity to a close, strong AM radio station. There are fewer and fewer these days as FM broadcasting has pretty much become the standard. If you are located 25 miles in the country or suburbs, you will find it difficult to get a very loud signal.

Hope this helps.

Rick Marz
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Re: Crystal radio troubleshooting advice

Postby Daddy E » Sun Dec 02, 2012 5:40 pm

Yes, we purchased the "Science Buddiies," kit and used the material's that came with the kit.
1) The earphone according to the materials list is made out of ceramic with high-impedance.
2) Yes we did use the germanium diode
3) The antenna is approxiamtely 60 ft in length, made with the 20 guage galvinized wire, and approximately 30 ft in the air, that's as high as I could climb into the 2 trees. We do not have acess to a water pipe for a ground so I went and purchased a 3 foot gnd rod, drove it into the gnd 2 feet and then attached the gnd wire from the radio to the gnding rod.
4) We live within the city limits and have acess to several AM radio stations.

So far your questions have not really helped me to troubleshoot my issue but I truly thank you for your post. And I be really interested in any other thoughts you have regarding a possible fix for our project.

Many thanks,

Daddy E
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Project Question: Trouble shooting crystal radio science fair project
Project Due Date: December 14, 2012
Project Status: I am conducting my experiment

Re: Crystal radio troubleshooting advice

Postby rmarz » Mon Dec 03, 2012 6:57 am

Daddy E - The importance of a good ground cant be overemphasized. You mentioned you drove a ground rod about 3' into the soil. The soil should be damp for best performance. Likewise, the antenna should be insulated from whatever you mounted it to, I assume you used the PVC pipe insulators in the kit (it shouldn't touch anything else). Finally, although it is a simple circuit, sometimes trying to wire the the connections using a pictorial diagram can cause confusion. Attached is a simplified schematic of how your elements should be connected.

Rick Marz
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crystal set.pdf
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Re: Crystal radio troubleshooting advice

Postby Daddy E » Tue Dec 04, 2012 10:57 am

rmarz - Thank you for the helpful hints and the pdf diagram. We'll try again and this time I'll ensure the gnding rod area is wet and yes I did use the pvc pipe connectors for the insulators as indicated by the instructions. I'll try to wet the area where the gnding rod is and see what happens.

Thanks so much! I'll try again and post the results!
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Project Question: Trouble shooting crystal radio science fair project
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Re: Crystal radio troubleshooting advice

Postby Craig_Bridge » Wed Dec 05, 2012 4:34 pm

If you could provide the state and county where you are trying to utilize your crystal set, we can look up what the strongest AM broadcasting stations are in your area.

The PDF that Rick Marz supplied shows a fixed inductor ("L") and the circuit relies on "parasitic" capacitance to tune the resonant frequence instead of a variable physical capacitor. If the impedance of the "tank" circuit (inductor and parasitic capacitor) at the frequence of the strongest AM radio station is less than the resistance of R1 in parallel with the headphones, the amount of audio signal detected won't be enough to listen too.

If the capacitance between the antenna and the lead in wire to ground is too large, the Radio Frequency energy across the inductor maybe too small even if the impedance of the inductor is reasonable.

Where are you placing this circuit and what are you building it on? In the early days of AM Radio, many of these kinds of circuits were literally built on a kitchen bread board. Effectively a dry piece of wood and the ground lead and antenna lead in wires were kept as far apart as possible. Others were built on the lid of a an empty shoe box by punching holes through to keep the leads separated. The finished assembly on the lid was then set on top the box to raise it off of any potentially conducting surface.

People often wound their own inductors on a round Quaker Oats oatmeal box or a glass drinking glass and they put taps every few turns and so they could change the position of where the diode was attached to as a crude means of tuning different stations at night when signals from longer distances could be received.

Hope this helps. Without having access to an AM modulated signal generator, trouble shooting this kind of circuit is trial and error.
-Craig
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Re: Crystal radio troubleshooting advice

Postby Daddy E » Tue Dec 11, 2012 10:40 am

Thanks everyone. rmarz and Craig_Bridge, for your posts and helpful suggestions to my questions, however, we were not able to get any noise from our crystal radio after pouring water on the gnd rod and driving it deeper. Both of us were quite upset at the lack of any sound coming through the ear piece but we live and learn. Now I've explained to my daughter that not all science projects are successful but on the other hand that's why it's called "experimentation." We both learned a lot and I hope that she especially came away with some positive take-aways from our seemingly lack of success?

Well now it's on to writing the "experimnetal paper," due this Friday, where she must describe in detail what was done and what she thinks could have have been done differently. I hope she doesn't get a failing grade becasue the radio did not work, we'll see....

Daddy E.
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Re: Crystal radio troubleshooting advice

Postby rmarz » Tue Dec 11, 2012 2:11 pm

Daddy E - Sorry you have had a bad experience with this project. Perhaps we can salvage something from the effort that will help you write your report as a 'post mortem'. If you have access to a digital multimeter (can be purchased for under $10 at most DIY stores), we can test the various components that are critical to the radio working. The germanium diode may be an 'open circuit' for example. The resistor could be an incorrect value (47Ω instead of 47KΩ) or the earphone isn't functioning. Here is how you can test the critical parts.

1) The germanium diode. Using the 200 KΩ range, measure the front-to-back resistance and reverse the leads to check resistance in the other direction. The diode will only conduct current in one direction. If you don't get a reading in either direction, the diode may be 'open'. You should see a near infinite resistance when the diode is reverse biased, and a nominal resistance (I measured about 40KΩ on a similar 1N34A diode) when forward biased. If you get something like these differing readings, the diode is probably OK.

2) Use the multimeter in the 200KΩ range to measure the resistor. It should read 47KΩ, +/- 10%. It isn't too critical, it should just be something in that range.

3) Check the earphone by touching the leads to a small 1.5V battery. As you brush the wire against the battery you should hear a faint crackling in the earphone.

If all these fail to pinpoint a bad component, at least you can add your attempt to debug the radio to your report.

Rick Marz
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Re: Crystal radio troubleshooting advice

Postby Daddy E » Thu Dec 27, 2012 3:42 pm

Thanks Rick for the trouble shooting tips. In my former life I was an electrician so I have a volt/ohm meter. I'll try your tips so that we can add these details to my daughter's final paper.

Thanks again for all your advice!

Daddy E.
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Project Question: Trouble shooting crystal radio science fair project
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