Introduction: (Initial Observation)
I like radios and I have bought a few old radios from garage sales. Any time that I turn on a light in my room, my radio makes a click sound. It seems that the electric switch is broadcasting a “Click”. The first time that I heard that I thought “It should be easy to build a two way radio”. Later I noticed that when I try to change the station on my portable radio, at certain points the larger electric radio makes a beeping sound. It was obvious that one radio is broadcasting some signals and the other is receiving them. In this project I want to build a radio (receiver only) and if possible I will also make a transmitter.
Information Gathering:
Find out about the radio and how it works. Read books, magazines or ask professionals who might know in order to learn about the radio, it’s construction, it’s history and it’s components.
A kit for crystal radio (pictured here) is available at MiniScience.com that contains some information and material.
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.
How Simple is Radio?
Near the end of the 19th century, it was discovered that an electric spark sent out electrical energy which traveled through the air without wires and could be detected at a distant point. This discovery made it possible to build wireless telegraph and communicate without wires.
How simple can it get? Here’s an example:
- Take a fresh 9-volt battery and a coin.
- Find an AM radio and tune it to an area of the dial where you hear static.
- Now hold the battery near the antenna and quickly tap the two terminals of the battery with the coin (so that you connect them together for an instant).
- You will hear a crackle in the radio that is caused by the connection and disconnection of the coin.
By tapping the terminals of a 9-volt battery with a coin, you can create radio waves that an AM radio can receive!
Your battery/coin combination is a radio transmitter! It’s not transmitting anything useful (just static) and it will not transmit very far (just a few inches (cm), because it’s not optimized for distance). But if you use the static to tap out Morse code, you can actually communicate over several inches with this crude device!
Why does it work?
Any flow of electricity creates a magnetic field. Opening and closing the circuit makes an electromagnetic pulse similar to radio waves. If you use a coil instead of a coin, and if you connect your circuit to an antenna the electromagnetic pulses can travel further.
If you want to get a little more elaborate, use a metal file and two pieces of wire. Connect the handle of the file to one terminal of your 9-volt battery. Connect the other piece of wire to the other terminal, and run the free end of the wire up and down the file. If you do this in the dark, you will be able to see very small 9-volt sparks running along the file as the tip of the wire connects and disconnects with the file’s ridges. Hold the file near an AM radio and you will hear a lot of static.
In the early days of radio, the transmitters were called spark coils and they created a continuous stream of sparks at much higher voltages (e.g. 20,000 volts). The high voltage created big fat sparks like you see in a spark plug, and they could transmit farther. Today a transmitter like that is illegal because it spans the entire radio spectrum, but in the early days it worked fine and was very common because there were not many people using radio waves.
How Does a Radio Work?
Suppose the weatherman is talking. The vocal sounds he makes into the microphone at the broadcasting studio are converted into electrical signals. After going through various stages of electronic circuits, the treated signals are fed into the transmitting antenna. There they are radiated in all directions as electromagnetic waves of a frequency belonging to that station (whose frequency is different from that of any other station in your area; otherwise you’d hear all stations at once).
As the incoming waves cut across the antenna of your radio receiver, they induce signals of that station’s frequency in the antenna. The induced signals enter the receiver, which converts them back to sounds that are almost identical to those made by the weatherman.
HOW TO BUILD A SIMPLE WIRELESS TELEGRAPH SET:
The simplest wireless telegraph set consists of a means of generating and controlling a spark which sends out radio waves into the air, and a receiver or detector to detect the radio waves.
Probably the simplest way to generate and control a spark is to use a switch (called a telegraph key) to turn on and off a buzzer which generates sparks.
The simplest way to receive or detect the radio waves generated by the buzzer is to use an AM radio tuned to a place on the dial where there are no other stations.
Here is the simple wireless transmitter which is basically an electrical circuit consisting of 3 parts, all hooked together by a WIRE.
A BATTERY supplies the electricity or voltage.
A KEY is used to complete or break the circuit.
A BUZZER is used to generate the sparks and the radio waves.
The BUZZER can be a door buzzer which can be found in any hardware store. You can also make a buzzer by removing the bell from a doorbell.
Since a buzzer is just an electromagnet which breaks the circuit which is activating it when it is activated, you can make your own buzzer by winding about 100-200 turns of wire around a nail and arranging it so that activating this electromagnet pulls on an armature which opens a set of contacts and breaks the circuit to the electromagnet. As soon as the circuit is broken, a spring returns the armature to it’s original position and the circuit is made again. This cycle of break-the-circuit and make-the-circuit continues and makes the armature vibrate or buzz for as long as a voltage is applied.
INDUCTORS – Inductors are usually made with coils of wire. The wire coils are wound around iron cores, ferrite cores, or other materials except in the case of an air core inductor where there is no core other than air. The inductor stores electrical charge in magnetic fields. When the magnetic field collapses it induces an electrical charge back into the wire. The unit of Inductance is Henry (Joseph Henry, American physicist(1797-1867) is a pioneer in electromagnetism that studied this important effect.). One Henry is the amount of inductance (L) that allows one volt to be induced when the current changes at the rate of one ampere per second.
CAPACITORS – A capacitor is a device that stores an electrical charge when a potential difference (voltage) exists between two conductors which are usually two plates separated by a dielectric material (an insulating material like air, paper, or special chemicals). Capacitors block DC voltages and pass AC voltages. They are used as filters, AC coupling capacitors and as by-pass capacitors. They are also used in conjunction with resistors and inductors to form tuned circuits and timing circuits. A capacitors value C (in Farads) is dependent upon the ratio of the charge Q (in Coulombs) divided by the V (in volts). Common capacitors come in values of microfarads or Pico farads. Often you will have to convert between Pico farads and micro farads. A chart is provided below to assist in the conversion. Measuring capacitance requires a capacitance meter. This is separate piece of test equipment. There are attachments for multimeters that allow measurement of capacitance directly.
CAPACITOR Value Conversions:
Some capacitors may be marked in micro farads and others of the same capacitance value marked in Pico farads. One Pico farad equals one micro-micro farad. You may need to make conversions between the two equivalents.
| Prefix | Power of 10 | Example |
| Mili | 10-3 | .001 |
| Micro | 10-6 | .000001 |
| Nano | 10-9 | .000000001 |
| Pico | 10-12 | .000000000001 |
Inductors are associated with circuit capacitance and can form a tuned circuit and resonate at a particular frequency.
Inductors are associated with circuit capacitance and can form a tuned circuit and resonate at a particular frequency.
Resonance Frequency Calculator
Question/ Purpose:
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this project is to build a simple radio receiver in order to get familiar with fundamentals of radio technology.
My Specific question is that how does the height of antenna affects the strength of received radio signals?
Identify Variables:
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
Independent variable is the height of antenna.
Dependent variable is the strength of radio signals. You measure this as AC voltage using a voltmeter capable of measuring millivolts.
Controlled variables are weather conditions, so all experiments will be performed at the same time and at the same weather conditions.
Constants are the shape and size of antenna, length of antenna wire and measuring instrument (voltmeter).
Hypothesis:
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
Following is a sample hypothesis:
Any piece of metal that is connected to the ground, is constantly receiving radio signals from the air and sending them to the ground in the form of a faint electric current. If the piece of metal used as an antenna is placed higher above the ground, the strength of signals it receives will also be higher because less trees and buildings will block the radio signals.
Experiment Design:
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Experiment 1: Test the effect of Antenna height on the strength of radio signals.
In this experiment I will use a sheet of metal or even a long wire as an antenna. I will place this antenna at different elevations during my test. Then I use a wire to connect the antenna to the ground or water pipe that goes to the ground. I will make sure that my antenna is fully insulated and is not touching any other object on it’s way, otherwise the radio signals may find other ways to travel to the ground. I will then use a volt-meter to see if there is any electric current in the wire.
Procedure:
Place a metal sheet or a metal can on a piece of plastic on an elevated area and away from any large metal structure such as refrigerator. Remove the insulation from one end of a long wire (50 to 100 feet, single strand number 24 solid wire) and attach the wire to the metal sheet. Set your multi-meter to measure AC voltage. Connect the other end of your long wire to the red wire of your multi-meter. Use the black wire of your multi-meter to touch a water pipe. Read the multi-meter.
Move your antenna up and down at different elevations and record the millivolts shown in your multimeter.
Your results table may look like this:
| Antenna height | Strength of Radio signals (Millivolts) |
| 0 feet | |
| 5 feet | |
| 10 feet | |
| 20 feet | |
| 50 feet |
Results: If your volt-meter is a high precision volt meter, you will see a voltage of about 0.5 volt (500 millivolts). Low precision volt-meters will not show such voltage at all. This experiment shows that radio signals that are electromagnetic waves can induce a very small electric current in any piece of metal.
Experiment 2:
In this experiment you will make a working radio. With this radio you can use a high impedance headphone, so your radio will work without any battery or electricity. You can also use an amplified speaker that needs battery or regular 110 volts electricity.
Things You Need
Tuning Coil: spool of #16 magnet wire.
Core: wood dowel 1” diameter by 5” or strong paper tube.
Slide Arm: stiff piece of metal about 5” by 3/8”.
Base: wood, 8” by 8” by 3/4”.
Capacitor: mica capacitor, 0.002 mfd.
Crystal Detector: germanium diode. IN34A
Antenna: wire, 50 to 100’ long (use whatever kind you have on hand, bare or insulated . . . if bare, you’ll need a glass insulator at each end of the antenna).
Headphones or speakers: a high-impedance pair of 2000 ohms headphones or a set of amplified computer speaker (Or both)
Minor Items: 4 Fahnestock spring clips, 5 screws, 2 washers, 6 small nails, 2 tin-can strips 1” by 1-1/2”, hook-up wire.
How to Build It
Remove the voltmeter in previous experiment and replace it with a simple radio receiver circuit. A simple radio receiver circuit includes an inductor and a capacitor linked together in parallel. Also use a set of amplified computer speakers to amplify and convert the received radio signals to sound.
Refer to the main drawing and start putting the parts together. It doesn’t make any difference which way you insert the capacitor. The same goes for the crystal diode. If at all possible, make as many connections as you can by soldering. Weak signals can be lost through poor connections.
When installing the slider, be certain the copper wire underneath touches the coil throughout the full swing of the arm. Also, take a piece of sandpaper and remove all the insulation from the top of the coil . . .right down to the bare metal. The slider must be able to make contact with each turn of wire on the coil. The reason that we use a slider is that we don’t know how many turn of wires is needed to tune to one of our local radio stations. By moving the slider we can place as many turns as we need in to the circuit until we find a station.
This is the same method that we usually make a crystal radio. (Crystal radios often don’t need the capacitor either.) The only difference is that instead of earphone or headphone, we connect amplified speakers. Remember that the built-in amplifier of computer speakers are not very powerful. The reason is that another amplifier already exists in computers. As a result you will not get a loud sound with that.
When we change the slider-arm position, we change the receiver’s sensitivity to the frequency of the station we were listening to. At the same time, however, the moving slider arm makes the receiver sensitive to other frequencies. And we can now pick up stations broadcasting on those frequencies . . . provided, of course, these stations are in the area and transmit a fairly strong signal
More Details
TUNING COIL AND CORE. If you can’t find any 1” wood dowel for the core, there are other things that will work as well. . . maybe even better. A piece of 1” outer diameter rubber hose would be excellent. So would a stiff plastic tube or rod. Or, take a piece of “one by two” wood, which actually measures 3/4” by 1-1/2”, and saw it lengthwise. This will give you a piece 3/4” by 3/4”.
After selecting your core and cutting it to the 5” length, measure off l/2” from each end. Then drill (or pierce) a small hole through the core at both marks. These holes will keep the, magnet wire in place. Thread about 4” of wire into one of the holes, and begin winding the coil. Keep the turns close to one another. It will take about 70 turns to reach the other hole. When you reach it, thread the wire through and cut off all but 4”.
Assuming you’ve used wood as the core, make supports for it with the 1” by 1-1/2” tin-can strips folded lengthwise for stiffness. Mount the tuning coil and core as shown in the main drawing. If you’ve used tubing as the core, simply lay it flat and nail it in place.
THE SLIDER ARM. Bend the 5” by 3/8” metal strip as shown in the sketch. A thick piece of bare copper wire will have to be soldered to the underside of the front. This wire allows the arm to make contact with no more than one or two turns of wire on the coil. That’s important for good tuning. You can use a piece of magnet wire for this purpose if you scrape the clear insulating coating from it. Finish the slider by wrap- ping some tape around the front edge. The tape prevents your touching the bare metal, which could weaken the signals.
AMPLIFIED COMPUTER SPEAKERS. Computer speakers have a jack that needs to be removed. A pair of wire that connects this jack to the speaker is actually two pairs. (Each wire from inside is a separate pair.) Each pair is for one speaker. If you only want to use one speaker, you can use one pair and ignore the other. If you want to use both speakers, you can connect these two pairs together. To do this remove all insulations from both pairs. You will notice that each pair has an insulated wire and a bare wire. First connect bare wires together and then remove about 1″ insulation from insulated wires and connect them together too.
Ground and antenna: To get the most out of your crystal set, you must have a good ground connection and a good antenna. Cold-water pipes make excellent conductors to ground. Make sure the pipe has been sanded or scraped clean where you plan to make connection.
As a starter, string up a temporary antenna to see what your radio set can do. Use about 100’ of wire if you can, and locate it as high as is practical. But don’t run the wire under or near power lines or leave it up when not using the set (and never use the set during a storm). Should you decide to erect a permanent antenna, you’d better use a lightning arrester and get some authoritative advice on installing the antenna properly.
Materials and Equipment:
Material that you need depends on how you want to construct your radio. Following is a sample list of material for constructing a simple crystal radio.
30 gauge enamel magnet wire
2 brass nails
1 large paper clip
1 35mm plastic film container
1 470 pf capacitor
1 crystal earphone
1 germanium diode IN34A
1 alligator clip
1 multi-strand connecting wire
1 double sided tape 1 1/2″ wide
1 bare single strand wire
1 solder
1 block of pine wood
Results of Experiment (Observation):
With everything assembled tightly, the ground and antenna wires connected, and headphones clipped in, we’re ready for the big moment. Move the slider slowly until you pick up a station. Then adjust for loudest sound. Since the set we have built is a rather simple one, it won’t receive too many stations. And it’s possible more than one station will come in at the same time. But the sounds will be clear and thrilling, and the set won’t cost us a penny to operate. So, happy experimenting. . . and good listening.
While you are testing your radio, you may find important points that should be considered during assembly or the radio will not work. Write all such points in your experiment results.
Calculations:
You will not need any calculation for this project. However if you ever want to make more advanced radios, you may want to calculate the size of capacitor and inductor in order to tune in a certain frequency. In this case visit Resonance Frequency Calculator for such calculations.
Summary 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 you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
References:
Visit your local library and find books related to electronics and radio. Many such books can be used as references for this project.
Question:
I put together my crystal radio using the crystal radio kit ubt I have not been able to pick up any radio stations, just a little static. I checked all of my connections and they seem to be correct. Any suggestions?
Answer:
Wire used to make the coil is insulated wire. After you make the coil, you must remove the insulation from the ends of the coil wire and from the surface of the coil, where the tuning rod will touch the coil.
If you have done these and it still does not work, you may not have a radio station close to your town. In this case, I try to test your radio at night. Radio waves travel a longer distance at night.
