To make an AM radio, we need to have five basic parts:
1) An antenna, to receive the electromagnetic waves and convert them back to electrical signals
2) A tuner, to select out the particular carrier frequency that we want, corresponding to a particular radio station that we are interested in listening to
3) A detector (diode) , to get rid of the high-frequency signal but keep the low-frequency part.
4) An amplifier, to make the signal bigger
5) A speaker, to produce the sound that we can hear
Here are descriptions of the individual parts:
1) The antenna: To make an antenna, all we need is a long piece of wire. Ideally, this should be very long (like 50 feet), but something shorter will work pretty well. We will connect this antenna to a coil of wire that is wound around a piece of plastic tubing. The photograph below shows one that is almost exactly what yours will look like. This coil is about 3 inches high and 1.5 inches in diameter.
2) The tuner: The job of the tuner is to select which radio station we want to listen to. Each radio station broadcasts at a different carrier frequency. Our tuner uses a coil of wire (called an inductor) and a capacitor. The combination of inductor and capacitor makes something called a “resonator”-- it is a circuit that throws away all the unwanted signals, and keeps only the one that we want. The resonator resonates (obviously...) at a particular frequency that is determined by the size of the inductor (its "inductance", measured in "henries") and the size of the capacitor (its "capacitance", measured in "farads") . The little figure below shows that electrical symbol for the inductor and capacitor that make up the tuner.
In our radios, we will use a capacitor that has a fixed size, and we will “tune” the radio to different stations by changing the size (the "inductance") of the inductor. The photo below shows the inductor (coil), which is about 5 inches long. It looks a lot like the antenna coil, except that it is longer.
The tuner coil has about 200 turns of wire with a thin red insulation on it. The insulation is scraped away on the top. To tune the radio, we will use a brass strip with a contact on the end that will make electrical contact at different points along the coil, where the insulation was scraped away. By pivoting the brass strip about a screw, the sliding contact will move across the coil, changing its inductance. When the contact is near one end the inductance will be very small, and the circuit will tune in radio stations whose carrier frequency is very high. When the contact is near the other end, the inductance will be large and the circuit will tune in radio stations whose carrier frequency is very low.
The photograph below shows a blowup of the sliding contact.
The detector: The detector is something called a “germanium diode”. The germanium diode lets electricity flow in one direction but not in the opposite direction. The germanium diode looks like a little glass cylinder with metal wires coming out each end.
The electrical symbol looks like an arrow with a line at the end. The electricity through a diode flows in only one direction – the direction the arrow points in. The diode itself has a little black line or a dark stripe at one end to tell you which direction the electricity flows in. The photo above shows the electrical signal for the diode at the left, and a photograph of a diode exactly like the one you will be using on the right. The diode is quite small, not much bigger than the size of a grain of rice.
4) The amplifier: The signals that we pick up with the antenna and tuner are very small -- maybe only a few thousandths of a volt. (A regular flashlight battery is 1 ½ volts). So, the amplifier makes the signal bigger. In our radios, we will two little things called “integrated circuits”, or “chips”, to make the signal bigger. There are a lot of different kids of integrated circuits. The kind that we will use are called “op-amps”, or “operational amplifiers”. Each op-amp looks like little spiders with 8 legs ! We will use 2 of them in each radio.
They will be mounted on a circuit board that will look sort of like
the one below.
This board is about 2.5” long and 1.5” high. The electrical symbol for an op-amp is a triangle with two wires on one side (these are the inputs) and a line coming from the point of the triangle, which is the amplified output signal.
5) The speaker. The speaker is the thing that actually
makes sound. It takes the electrical signal and uses a
tiny electromagnet to move a piece of plastic or paper back and forth, making sound that we can
hear. The speakers that you will use a just small
versions of the speakers that you might have in a radio, stereo, or television in your home. The
figure at below left shows the electrical symbol for a speaker, while the photograph a below left
shows the real thing.
When we connect all these together in the right way, we get a complete radio !
To summarize the entire process: The antenna picks up the signal and brings it to the antenna coil. The antenna coil is brought close to the tuner, and the electrical signal in the antenna coil transfers to the tuner coil. The inductor (coil) and capacitor that make up the tuner select out the particular carrier frequency of the radio station we want to listen to. The detector (diode) gets rid of the very high frequency, but keeps the low-frequency signal that corresponds to the “sound” that we want to hear. The amplifier makes the signal bigger, and the speaker converts the electrical signal back into sound!