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R1is a potentiometer that controls the sound volume.

C6 sets the voltage gain of IC2 to 200. (Therefore, the voltage out will be

200 times the voltage in.)

C7improves the stability of the LM386 amplifier to prevent problems

such as oscillation, which can turn your signal into an unintelligible

garble of sound.

C8removes any DC offset from the output of the LM386 amplifier.

C9 acts as a current bank for the output. This capacitor drains when

sudden surges of current occur and refills with electrons when the

demand for current is low.

168 Part II: Sounding Off!

Picking up the right signal

Ferrite,used for the coil core in this project, is

composed of crystalline iron oxide and is typi-

cally used at radio frequencies. For coils used

at low frequencies, such as in power trans-

formers, you can use iron as the core; however,

at high frequencies, the high conductivity of

iron allows eddy currents to be created in the

core. These eddy currents can decrease the

level of signal induced to the coil, thus weak-

ening the signal. Because these eddy currents

don’t occur in the lower conductivity ferrite

core, you get a much more efficient coil at radio

frequencies.

When a radio signal comes across a ferrite rod,

the radio signal creates a magnetic field in the

rod. When you wrap a coil of wire around the

ferrite rod, this magnetic field — which is

changing direction at high frequency (the fre-

quency of the radio signal) — induces (hence

the name inductor) an electric current in the

wire coil. A magnetic or electric field can cause

electrons to flow in a wire; if the field is chang-

ing direction frequently, like one created by a

radio signal does, the direction of the electrons

changes with the magnetic field. Voilà!You now

have a radio frequency signal traveling through

your wire coil.

Because the inductor and capacitor are con-

nected, this radio frequency signal also travels

through the capacitor. When you have an

inductor and capacitor in parallel, like in our

AM radio circuit, the values of the inductor and

capacitor determine a resonance frequency. A

signal at the resonance frequency is then

blocked from moving through the inductor/

capacitor circuit. This resonance frequency is

the frequency of the radio station that you tune

into when you change the value of the variable

capacitor by turning the knob on your radio.

Signals from other radio stations that aren’t at

the resonance frequency flow through the

inductor/capacitor circuit and through C1 to

ground; basically, these signals are thrown

away.

Only the signal at the resonance frequency

doesn’t go to ground through C1; rather, that

signal is available to your circuit to be

processed back into the music your radio sta-

tion sends out.