In order to convert very faint electrical signals into sound, we need a very sensitive earphone.The kind of earphones used in transistor radios or CD players will not do.
The most difficult part of building a crystal radio is building an efficient earphone that can convert the tiny electrical signals into tiny sounds that our ears can hear.
Our first radio used a telephone handset for an earphone, and that works quite well.
When substituting a piezoelectric earphone for an earphone made with a coil of wire, we must provide a way for some current to bypass the earphone.
We do this by putting a resistor or a coil in parallel with the earphone (parallel means that the resistor or coil is attached to the same two places that the earphone wires are attached).
Those are meant to be driven by a signal loud enough to drive a speaker, and are not sensitive at all.
We will talk later (in the scientific part of this chapter) about impedance and what it means.But another type of earphone is available that fits in the ear so you don't have to hold it.It is also more sensitive than the telephone handset.A sensitive earphone built around an electromagnet (we will build one of these later) might have 2,000 ohms.The telephone handset earphone is of this type, although it has only a few hundred ohms of impedance, and will not be as loud as a more sensitive device.Some crystals such as quartz, and Rochelle's Salt are piezoelectric.Some ceramics (such as those made with barium titanate) are also piezoelectric.The crystal earphone we will play with in this section has over a million ohms of impedance, and is very sensitive.A crystal earphone (more properly called a piezoelectric earphone, pronounced pee-zo) is made of a material that changes its shape when connected to a source of electricity.We now carry this diode in our catalog, to make it easier to obtain.Radio Shack used to carry them, but they no longer have them in their stores. First let me warn you that this first little radio may not work in your location.