Make Electronics

What Next? 267

Experiment 31: One Radio, No Solder, No Power

theory

How radio works (continued)

Figure 5-71. An antenna at the top of the schematic picks up faint
electromagnetic radiation from a distant transmitter. The coil
at the left side is tapped at intervals so that its resonance can
be adjusted to match the carrier frequency of the radio signal.
Other frequencies are grounded (at the bottom of the sche-
matic). The diode passes the “top half” of the signal to the ear-
phone at the right, which is incapable of responding fast enough
to reproduce the carrier frequency, and thus filters it out, leaving
only the audio frequencies that were superimposed on it.

By adding a capacitor, you can tune the circuit. Now an
incoming pulse from the transmitter is initially blocked by
the self-inductance of the coil, while it charges the capacitor.
If an equally negative pulse is received after an interval that
is properly synchronized with the values of the coil and the
capacitor, it coincides with the capacitor discharging and the
coil conducting. In this way, the right frequency of carrier
wave makes the circuit resonate in sympathy. At the same
time, audio-frequency fluctuations in the strength of the
signal are translated into fluctuations in voltage in the circuit.

What happens to other frequencies pulled in by the an-
tenna? The lower ones pass through the coil to ground; the
higher ones pass through the capacitor to ground. They are
just “thrown away.”

The righthand half of the circuit samples the signal by pass-
ing it through a germanium diode and an earphone. The
power from the transmitter is just sufficient to vibrate the
diaphragm in the earphone, after the diode has subtracted
the negative half of the signal.

Look back at the diagram of the amplitude-modulated
signal. You’ll see that it fluctuates up and down so rapidly,
the earphone cannot possibly keep up with the positive-
negative variations—hence the need for the diode. It will
remain hesitating at the midpoint between the highs and
lows, producing no sound at all. The diode solves this prob-
lem by subtracting the lower half of the “audio envelope,”

leaving just the positive spikes of voltage. Although these

are still very small and rapid, they are now all pushing the

diaphragm of the earphone in the same direction, so that

it averages them out, approximately reconstructing the

original sound wave.

Figure 5-72 shows how the circuit can be enhanced with a

variable capacitor, to tune it without needing to tap the coil

at intervals.

Figure 5-72. By adding a capacitor to the circuit, its resonance

can be tuned more precisely. The diagonal arrow indicates that

a variable capacitor is used.

The radio can pull in the stations on the AM (amplitude-

modulated) waveband that happen to be most powerful in

your area. The waveband ranges from 300 kHz to 3 MHz. If

you find yourself interested in radio, your next step could

be to build a powered radio using a couple of transistors.

Alternatively you could build your own (legal) low-power

AM transmitter. There’s an ultra-simple kit available from

http://www.scitoys.com consisting of just two principal

components: a crystal oscillator, and a transformer, shown

in Figure 5-73. That’s all it takes.

Figure 5-73. An AM radio transmitter can be made from just

two components: a transformer (left) and a crystal oscillator

(right), available from http://www.scitoys.com.