Electrical Crystal Radio question

If anybody here has any experience with building a crystal radio, I would appreciate a response.

My grandson and I are doing radio projects. This involves basic crystal build using a coil, diode, and resistor. The system is grounded and uses an antenna with no external power and a high impedance earpiece for audio.

The radio works for a few seconds when left hooked to the ground and antenna, but then the sound fades.

Any ideas?
 
Mark, welcome to PF.

Can you describe your antenna and ground connection, please?

Are there any capacitors in your circuit? A tuning capacitor and a smoothing capacitor after the diode detector, perhaps? Ideally, can you provide a circuit diagram of your crystal radio or an online pointer to the design you are using?

Is your high impedance earpiece a piezo-electric (crystal) earpiece?
 
Mark, welcome to PF.

Can you describe your antenna and ground connection, please?

Are there any capacitors in your circuit? A tuning capacitor and a smoothing capacitor after the diode detector, perhaps? Ideally, can you provide a circuit diagram of your crystal radio or an online pointer to the design you are using?

Is your high impedance earpiece a piezo-electric (crystal) earpiece?
Thanks for the response. I am on the periphery of my technical expertise...trying to get my grandson interested in something other than video games. We built one of these very in very similar fashion and it worked (sort of). I'm trying to improve the design. In answer:

I have attached a photo. There are no capacitors. The earpiece is a high impedance unit, Philmore No. 748, sold to be used with the crystal set. I have a verified ground connection, 50' of lamp wire antenna, 47k Ohm resistor, and Germanium diode.

VHJ4zcS.jpg
 

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You have a resistor connected across the earpiece, which is good. You could check the resistor is making good contact. If you have a multimeter you can check for a resistance reading of about 47 kΩ between the earpiece screws. Try the multimeter leads round both ways as in one direction the diode and coil will short out the resistor and give a low reading. If you haven't got a multimeter just check the resistor leads are clean and are firmly gripped by the earpiece screws (but not so tightly that they pull the leads out of the resistor).

How did you verify the water pipe ground? Did you make your connection to the water pipe at a point where you can check there is continuous metal pipe between your connection and the ground? Metal water pipes can be joined to plastic pipes underground and hence offer poor earth connections. Try pouring some water on the pipe where it enters the ground and see if that changes the behaviour. You could also try making your own earth connection with a tent peg or by driving a couple of feet of copper water pipe (available from DIY stores) into some soil.
 

Klystron

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I built crystal radio kits as a child; so long ago that I cannot write down the circuits but one did have paper capacitors and tunable pizeo-electric crystal that could pick up AM radio stations and other signals. While grounding indoors on cold water pipes (hot water pipes did not work due to the water heater) picked up faint signals; a strong alligator clip on an external metal fence brought in clear signals. We sunk the steel fence posts ~12" (< 1/3 meter) in the ground connected with wire mesh fence.

The best rig had a solenoid type coil tuned by sliding an internal plunger IMS (if memory serves) not unlike your sliding rig. The solenoid arrangment made for a more compact unit and also reduced static (noise). The entire unpowered receiver fit in my shirt pocket. Replacing the small ground clip, the kind used for soldering, with a stronger clamp used to secure ground straps from a clothes drier, really improved reception. As did polishing oxide coating off critical connections as mentioned above.

Bravo for teaching children this technique. Crystal sets helped me understand electrical ground and electromagnetic radiation. The next project I helped my father build a vacuum tube powered HAM radio transmitter-receiver with Morse and voice, then re-built a black-and-white television receiver which I modified with "stereo" speakers and a photo-electric cell for remote on/off switching via a flashlight. Great fun.
 
This is mysterious, and thanks for the responses.

I am securely grounded to a cold water pipe. This is continuous copper, and I checked it by testing voltage to ground. I have a background in electrical work, but not in this area. I know that all my connections are secure, tested everything thoroughly.

I am learning this along with my grandson. We started off with this simple design, wish to expand to more advanced, but this one needs to work. Do you think I am OK with my antenna?
 

berkeman

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The radio works for a few seconds when left hooked to the ground and antenna, but then the sound fades.
I have a verified ground connection, 50' of lamp wire antenna, 47k Ohm resistor, and Germanium diode.
Do you think I am OK with my antenna?
When you can hear a channel, can you tell what the call letters are so you can check the frequency? Is it in the AM band?

How have you run your antenna wire? Does it go up into a tall tree nearby? The more vertical it can be, the better it will work in the AM radio band (for the US).

How many spots on the crystal have you tried with the whisker point? Have you tried cleaning the crystal surface and letting it dry before trying probe points?

And I would think your house ground should work fine. Is your house electrical system well-grounded?
 

Klystron

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This is mysterious, and thanks for the responses.

I am securely grounded to a cold water pipe. This is continuous copper, and I checked it by testing voltage to ground. I have a background in electrical work, but not in this area. I know that all my connections are secure, tested everything thoroughly.

I am learning this along with my grandson. We started off with this simple design, wish to expand to more advanced, but this one needs to work. Do you think I am OK with my antenna?
I'm sure you understand ground. As a child the concept was quite mysterious to me. Even as an adult radar tech, proper ground could be elusive. The soil in South-East Asia was so saturated with water at one site, we hammered in copper spikes ~45' (<15 meters) deep and still had ground problems particularly during wet monsoon season.

I seem to remember using a small coil of insulated wire that came with the crystal set as antenna. This was in a large valley in Northern California surrounded by radio transmitters on the hills and on Mount Hamilton, so reception was excellent. IMS we hooked the HAM radio via insulated antenna wire to a mod on the TV antenna. Reception was so good, I made a simple FM antenna by splitting a short section of the same wire into a "Y" tacked to an inner wall for a stereo receiver.

You could experiment with different antennas. I seem to remember using an extendible automobile AM antenna clipped to the crystal set a friend later gave me. These days there are probably Internet sites you can consult about reception in your area.
 
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I am securely grounded to a cold water pipe. This is continuous copper, and I checked it by testing voltage to ground.
Do you think I am OK with my antenna?
...this one needs to work.
0 V to ground might just mean an open circuit rather than a good connection. Some plumbing connections, especially fast-fit ones, can act as insulators though this obviously won't be a problem with continuous copper pipe.

The 50' antenna wire sounds plausible. Whether it's adequate depends on how far away is your nearest radio station in the band the receiver is tuned for. Keep the antenna off the ground and reasonably straight. If you can't get it largely vertical, as suggested by berkeman, try swinging it round horizontally by up to 90° to see if you can pick up other stations.

When I've tried low-end crystal or similar radio designs I've found that tuning is limited to varying the strength of the nearest strong radio station rather than selecting between multiple stations as expected on a modern commercial radio. Whilst crystal sets can be great fun if you can get them working, radio conditions have moved on a long way since their hey-day so don't give up if you can't get good results. A kit or design with built-in amplification may be easier to get to work. The next step in radio design is a TRF (tuned radio frequency) design. This will have an amplifier which frees you from the requirement for a good antenna and earth as well as having a reasonably local transmitter.
 

davenn

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have attached a photo. There are no capacitors. The earpiece is a high impedance unit, Philmore No. 748, sold to be used with the crystal set.
there should be a capacitor (variable) else you don't have a tuned circuit

a basic easiest circuit ....

n06cv.jpg
 

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How many spots on the crystal have you tried with the whisker point? Have you tried cleaning the crystal surface and letting it dry before trying probe points?
The photo caption shows a 1N84A germanium diode (partially hidden by the brass wiper) so these aren't issues.
 
there should be a capacitor (variable) else you don't have a tuned circuit
The coil is tuned against the self-capacitance of both the coil and the antenna wire. This does make it rather hard to predict quite what the radio is tuned to (hence berkeman's question about what is being heard).
 

berkeman

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The photo caption shows a 1N84A germanium diode (partially hidden by the brass wiper) so these aren't issues.
Oh, shoot, I misunderstood. Thanks for clarifying that. I saw "crystal radio", and made assumptions that was what the project was. What's a more accurate thread title -- just a simple unpowered AM radio receiver?
 
I saw "crystal radio", and made assumptions that was what the project was. What's a more accurate thread title -- just a simple unpowered AM radio receiver?
'Crystal set' is still standard usage among ham and other hobby radio users for unpowered (strictly, signal-powered) radio designs even if they use commercial diodes as detectors so the thread title is fine.

If you're sufficiently hard-core you can still track down a cat's whisker and galena crystal to make your own diode detector:
https://en.wikipedia.org/wiki/Galena#Galena_uses

There are hobby groups for making high-performance crystal sets. The designs can be more complicated than building powered radio sets with amplification.
 

davenn

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The coil is tuned against the self-capacitance of both the coil and the antenna wire. This does make it rather hard to predict quite what the radio is tuned to (hence berkeman's question about what is being heard).

yeah, very hard
@Mark Palmer would be much better off using the more standard style of circuit I posted as it is going to give
better and more easily/predictably obtained results
 

Tom.G

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The fact that it works for a short time then fades shows it is at least tuned to a transmitter, the antenna (and likely Ground) is adequate, the crystal is functional, and the earpiece functions.

Since the earpiece is labeled as High Impedance, it is probably of piezoelectric construction (about the only type that works with a crystal radio!). Being piezoelectric, to the outside world the earpiece presents as a capacitor, which will charge up, acting as a filter cap, from the rectified receive signal. That is the purpose of the 47k resistor, to discharge the DC on the earpiece. Without the resistor, the earpiece charges to the peak voltage of the received signal and the diode (crystal) is biased off.

Possible causes:
  • Earpiece not connected correctly
  • Resistor faulty
  • Resistor not connected correctly
Conclusion: replace the resistor

Cheers,
Tom
 
Thanks, all. I did not expect such assistance.This is nothing more than a simple AM unpowered receiver unit if there was any question on it.

I think the earpiece is OK. I ordered a couple of extras and the problem seems to be be present with all of them. I have checked all the connections and am satisfied they are correct. I have not been able to receive a signal strong and long enough to identify what is coming thru the set. I am not happy with the antenna, but I figured it would work well enough to get something consistent and it has not so far. As far as the ground is concerned, I tested it with AC voltage to make sure. I show a full 120 VAC going to ground on the cold water pipe I am using. The pipe is copper all the way to the street, buried to the main a distance of 50', about 48" deep.

I have some fundamental questions based on ignorance.

1. Does the use of schedule 40 pipe for the coil have any impact on the performance of the coil? Most of these are depicted using paper tubing or such.
2. If the earpiece acts as a capacitor as mentioned earlier in the thread, is it discharging to ground thru me to ground when I put it in my ear, thus causing loss of signal?
3. I am using 22 AWG wire on the coil. Is this OK?
4. I see different schematics for this type radio, some of which shown very specific number of winds on the coil. I just wound this coil until I ran out of room. What should work best?
5. The subject of the ground has been raised by a couple of people. If there was a ground issue, what symptom of this would be present?
6. I am using a single strand of insulated stranded lamp wire (maybe 18 gauge) for the antenna. Could this be a problem as to the type?

This is really interesting stuff, and I appreciate the feedback. My grandson is excited about the project. If we can get this one going I want to go on to a more advanced design.

Thanks again.

Mark Palmer
Louisville, KY
 

jim hardy

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i can't quite make out the color code on your 47 KΩ resistor.

At 600% it looks like it could have a yellow multiplier and that'd be 470 K Ω
upload_2019-2-18_13-10-15.png


make sure it's yellow-violet-orange not yellow-violet-yellow.

my two cents, and overpriced at that..

i still remember my crystal radio - what fun. It'd only get Miami's clear channel station, WINZ , "fifty thousand watts of music and news"" was their jingle.

old jim
 

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i can't quite make out the color code on your 47 KΩ resistor.

At 600% it looks like it could have a yellow multiplier and that'd be 470 K Ω
View attachment 238944

make sure it's yellow-violet-orange not yellow-violet-yellow.

my two cents, and overpriced at that..

i still remember my crystal radio - what fun. It'd only get Miami's clear channel station, WINZ , "fifty thousand watts of music and news"" was their jingle.

old jim
 
It was sure bought and labeled 47k. Try this high res pic. I don't know the color bands.

DR6nHvM.jpg
 

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Klystron

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Consider wrapping a/o soldering or otherwise securing the components. Wood screws into woodblock could loosen as you work the slider, etc.

(Mind you, thus from the worst solderer in the USAF. :cool:
 
Consider wrapping a/o soldering or otherwise securing the components. Wood screws into woodblock could loosen as you work the slider, etc.

(Mind you, thus from the worst solderer in the USAF. :cool:
I actually thought of that. I think the connections are secure. I have been over them. You can't tell from the photo, but the connections are caught between brass washers on the wire and components. The wiper bar connection is spring loaded.
 

Klystron

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I actually thought of that. I think the connections are secure. I have been over them. You can't tell from the photo, but the connections are caught between brass washers on the wire and components. The wiper bar connection is spring loaded.
Yes, but brass is not a conductor. I have a set of brass tools for tuning sensitive circuits to avoid impedance changes from inserting tools. Not sure about the screws. Stores such as Fry's Electronics or other online sources sell inexpensive circuit boards with conducting wrap posts (apologies for my poor nomenclature) for DIY soldering projects.
 
Yes, but brass is not a conductor.
That could explain a lot. Let me look at this again.

Brass contains copper and it will read continuity. Is it insufficient for these small value transmissions?
 

Klystron

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Allow me to defer to @jim hardy and other practical engineers.

(BTW, I like your avatar. "The Dude, abides.")
 

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