A Crystal Radio Set

Introduction: (Initial Observation)

Radio stations broadcast their programs in the form of strong electromagnetic waves. As waves get further and further from the station they become weaker.

Radio receivers are devices that receive electromagnetic waves trough antenna, filter it to tune to a specific station, amplify the signals and send them to speakers.

If you are very close to a radio station, signals may be powerful enough to power up a speaker or an small earphone with no additional power source.

Information Gathering:

If you just want to make a simple crystal radio, make sure you see our Simple Crystal Radio page. Another sample instruction is available here if you like to see more step by step pictures. If for any reason your radio did not work, refer to check list page.

Finally if you are ready to build a radio using household items, click here.

Question/ Purpose:

How to build a crystal radio? The purpose of this project is learning about some electronic components while making a wave powered radio.

Identify Variables:

Components that we use to build a crystal radio are our variables. We can experiment different components and record the results to see which combination produces the best receiver.

Hypothesis:

Based on your gathered information, make an educated guess about what types of things affect the functionality and performance of the radio that you make. For example about antenna, your hypothesis can suggest that a taller or longer or wider antenna will receive stronger radio signals.

Experiment Design:

Any piece of metal can absorb electromagnetic waves and convert them to a very small amount of electricity or electrical waves. If this piece of metal is connected to the ground with a wire or by itself, it will send all that electricity to the ground. The first step on designing a radio is trapping and filtering these electrical signals on their way to the ground.

The simplest way to do that is using a coil of wire.

All crystal radio plans offered here will use this structure to receive and filter radio frequencies. Depending on the number of loops in the coil of wire, we can filter and catch different frequencies.

Description of material that we need:

Description of material that we may use for our experiment is as follows:

A base for the radio :
It can be just about any non conductive material. I like to use wood. a regular piece of 5″x7″ or 7″x9″ for mounting all the parts on.

A Capacitor :
if you want something that looks good you will have to buy it you will need a 10-365pf air variable capacitor Or an old transistor radio “tuning capacitor” will do just fine. they are easy to get and work well. But you will have to desolder it from the radio [that aint nothing] and it will save you about 12 bucks! plus shipping!!!!

The Coil (inductor):
For this project we will be winding the coil ourselves. Use a piece of plastic pvc pipe 2″ outside diameter coil former. cut your pvc pipe to a length of 3-1/2″ or so but not much shorter. a little longer won’t hurt, but, you want it to fit on the base.

We will have to wind the coil with MAGNET WIRE that is enamel coated wire you can buy from electronic stores. for our coil we can use from 26 gauge to 18 gauge wire. here’s the deal! lets say you use 22 gauge wire like me the set will be a little louder but it will be not as selective as if i used 26 gauge wire. but with the 26 gauge wire will have less volume but it will be more selective! so its ok to use 22 gauge wire out in the country, if you live in or near the city you can use the thinner wire like 26 gauge. but for starters i would use the 22 gauge wire it is easier to work with!! and easier to hear!

WIRE :
For the coil we will be using MAGNET WIRE that is enamel coated wire. For hook up wire we can use any thing 20 to 14 gauge stranded is preferred but if you have solid then thats ok!

Antenna :
The Antenna we will be making for this project will be a long wire Antenna and that means a piece of wire 50 foot or so long. it can be just about any piece of wire solid or stranded insulated or not. one thing, we can not let the antenna touch anything or it will work poorly or not at all! so thats why I use insulated 12 gauge stranded wire. one end is hung from a tree via an insulator that is home made out of a piece of 3/4″ pvc 6″ long with 2 sets of holes drilled one at each end. about 1″ from either end. The wire is connected to one end of the insulator and rope to the other end. thats what gets hung in the tree. ATTENTION DO NOT GET WITHIN 10 FOOT OF POWER LINES OR CROSS UNDER OR OVER POWER LINES WITH THE ANTENNA MAY RESULT IN SERIOUS INJURY OR DEATH!! The other end of the antenna gets connected to the set.

Ground :
Means just that. connected to the same electrical property as mother EARTH! this is done by driving a metal [copper preferred] rod in the ground 6 or 8 foot! one will do 2 or 3 wont hurt especially if you live in a dry climate. but 1 will do to get us started. From the radio to the ground should be as short as possible. if the wire gets too long it will act like an antenna. A cold water pipe is a good standby if you have a metal one! ATTENTION AGAIN DO NOT HOOK TO A GAS LINE IF YOU HAVE NATURAL GAS. THIS IS A NO NO!!!

Detector :
Use a 1N34 germanium Diode. the diode will rectify the radio frequency -RF- in to audio frequency -AF- which will be what we hear in the phones.

The Phones :
Here we have a little problem. we can not use a regular set of head phones like you get with your walk man! hum? well this is because the set don’t have the power to make them work! but we can either buy a set of phones for around 15 us dollars or get a crystal earpiece. they are about .50 to 1.50 dollars thats the choice we have but its a small price to pay for a lifetime of fun and its a good learning experience too. When you look for earphones, ask for Crystal earphones, high impedance earphones or Piezoelectric earphones, or read on the box to find the right one.

Experiment 1:

Instructions
In the first experiment we make a radio that uses a coil and a diode to function. This simple design does not use any capacitor for tuning. The first step in making this radio is winding the tuning coil. This is best done with two people. One person should hold the paper roll and the other the spool of wire. Poke a small hole about a 1/2″ from each end of the paper roll. Take the end of the wire from the wire spool and thread it through the hole. Pull the wire through so that it is about a foot long. Put a narrow strip of transparent tape around the end of the tube to keep the wire from slipping out during winding. Now, begin winding the coil. When winding the coil, do not overlap the turns, they should lie adjacent to each other. When you are close to reaching the second hole on the other side of the roll, unwind an additional foot of wire and cut it off near the spool. Thread the end of the wire through the hole as before and apply a strip of tape.

Attach a thin copper strip to the base board using a screw.

Now mount your paper roll to the base board using two pushpins through either end. Locate the paper roll so that the copper strip will gently rub across the top of the coil.

Add 3 more push-pins to the cardboard square as shown in the figure below. Connect the germanium diode between the screw holding down the copper strip and one of the nearby pushpins. You’ll make a better connection if you loop the ends of the diode around the screw and the pushpin. The side of the diode with the black band should point away from the copper strip.

Loop the loose wires from the ends of the tuning coil a couple of times around each of the remaining pushpins.

Using a sand paper, gently remove the enamel insulation from the last inch or so of the wires coming from the tuning coil. Also gently sand (by rubbing in the same direction) the enamel wire along the path on the surface of the coil where the copper strip rubs.

All that is left now is to hook your radio up to an earphone, antenna, and grounding system (such as a cold water pipe) and you should hear music! By sliding the copper strip along the tuning coil, you should be able to hear several weak, but distinct, radio stations. For good electrical contact, you may need to press the copper strip firmly against the coil with your finger while listening to the radio.

Experiment 2:

Instructions

In this experiment we will also use a variable capacitor for tuning the radio.

Again we will start with the coil.
we will be making a taped coil with 20 or 22 or 24 gauge MAGNET wire. 1st we will drill 2 sets of holes all the way through the former. make sure the drill bit is just a little larger than the wire. about 1/8″ will do. the holes should be 2-5/8″ apart and centered so that there is equal distant from the ends. start one end of the wire through the former leaving enough sticking out the other side to work with. 3″ or 4″ should be plenty. now get some help! lol 🙂 you have to start winding on the former. wrap 5 turns keeping the turns tight! on the 5th turn take the 1/8″ drill bit or nail or screwdriver and make a tap. for those of you who don’t know what a tap is. take the wire and place it over the nail, make 2 turns with the nail to form a loop. that is a tap and thats why we call it a taped coil! so keep winding and tapping we are not done yet! you keep winding till you get to the 10th turn then make another tap. also tap the 16th 24th 32nd 40th 52nd 65th 80th turns then keep winding to 100th turn but dont tap that one you run the end of the wire through the other set of holes on the former. WALA! your done with the coil that is! you just completed the 2nd most hardest part, getting someone to help was the hardest part! A couple of hints, use tape to hold the wire on the coil, or some other method when winding the coil. when done use some glue around the ends of the coil to hold the windings tight. and to keep the windings as tight as possible while winding. after the glue dries take an xzacto knife or some sand paper and clean the taps and both ends of the wire so you can see the copper but don’t get to crazy we don’t want to hurt the windings. the longer the coil the lower the frequency the shorter the coil the higher the frequency but 100 turns will get you in the BC band. So if your set tunes to low you will have to remove some windings. on the other hand if it tunes to high you will have to ad some turns. try 10 at a time.

Now you will want to mount the coil
on the base [ that piece of wood i told you about earlier ] but first you want to lay out the parts on the wood then figure how and where you are going to mount them. then when you are sure you got the right spot then mount the coil. remember a well planned project is a neat project! i think thats why mine are so shabby looking!

now you can mount the capacitor
and and start mounting the fahnestock clips, so we can hook up the detector and the phones and a ground lug to connect it to mother earth!

Now we are ready to wire it up the coil or L1
We will call the 100th turn end the cold end. this will also be known as ground! 1st turn will be the hot end. now you will have to hook up the coil and capacitor in parallel to form a L/C tuning circuit. the capacitor has 2 sections the stator [the part that don’t move]. they are the solder lugs on the capacitor. and the rotor [the part that moves]. This will be the outside frame of the capacitor. you hook the rotor to the cold end of the coil and you hook the stator to the hot end. this is the part of the radio that will do the tuning.

From here we will hook up the detector
[ this is where the RF gets changed to AF ]. that will come off the hot end of the coil the other end of the detector will get connected to one of the phone jacks.

Next we will hook the other side of the phone
jack to ground or cold side of coil or mother earth how ever you want to look at it!

Now we are ready to hook up the antenna.
after you have mounted the antenna outside at least 10 foot of the ground. i ran mine from the tree to the eve of the house using some homemade stand-offs or insulators or what ever you want to call them. then i ran the wire through the window well i actually opened the window 1st then ran it through. then you can hook the antenna to the set. that is why we made the taps. And i bet you thought i had you do all that work for nothing! Now hook the antenna to one of the taps and tune the capacitor slowly and you should here some AM broadcast stations. if not try another tap! one thing to keep in mind is that the connections in any crystal set must be 100 per-cent they have to be better than a DC connection. you might have to nudge a wire a little to make a good connection. so don’t get discouraged if you don’t hear something the first time check for a loose or bad connection. or try a little later in the evening. there is one rule of thumb I have found with tinker-n around with crystal radios is nothing is written in stone. so feel free once you get this one working is to use your imagination experiment and have fun. so when you get done you will be able to tell every one that you can listen to the world on a wire and it cost less than a trip to MacDonald!

List of Materials and Equipment:

Base : a piece of wood 5″ by 7″ or larger.
Capacitor : we need a capacitor in the range of 10 to 365pf, air variable.
A Coil former : A piece of pvc pipe 1-1/2 to 2-1/2″ outside diameter. and at least 5″ long.
Some Wire : About [ not less than ] 60 foot of 22 gauge enamel coated magnet wire and some hook up wire not really a whole lot. a few feet will do.
Antenna : wire 50 foot at least, 100 foot will do also. remember longer your antenna the louder it will be. 14 or 12 gauge insulated stranded is what i use.
Detector : A germanium diode (About 10 for a buck!, available at radio shack)
Phones : well the choice is up to you 15 dollars for a good set, I brought one there not to bad or 1.50 for a little ear piece. both will work but I found a pair of old military head sets for 5.00 each. and they work good!
Fahnestock clips :
We will need some way to hook up the wires together the Fahnestock clips is one way of doing it with out soldering. you will have to mail order them. well i do. You can also solder all connections for a more permanent version. I recommend at first to use the Fahnestock clips it will be easier to modify or experiment with! that’s the part i like is experimenting!!
Additional material: Depending on your design and while experimenting you may need one paper roll, one screw, small copper strip, 5 push-pins and a piece of sandpaper.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.

Calculations:

Description

Summery of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.

Conclusion:

Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If your radio did not work, check the “Things that might go wrong” page.

References:

List your online or printed references here.

Q. We need a testable question on the crystal radio (the antenna in particular) in which we would gather numerical data. Could you help?

A. The radio signals received by your crystal radio will convert to electrical waves before entering the ear piece. You can measure such electrical signals in millivolts for different antenna sizes. You will need a sensitive digital voltmeter that can measure AC millivolts.