Crystal Radio
KV3540/AK9823/AK23/K23
Advise:
Make sure there is correct number of components in your package. Use 30W iron, 60% solder. Take
extra caution against overheating during soldering process. Transistors and ICs can be easily damaged
by high temperature so do not leave the iron on the board for too long. Too many or too little solders
may cause a defected circuit. Please weld the IC socket first then plug in the IC chip.
Circuit Principal:
The place we live on earth is filled with electric 1112.gnetic waves, which are produced by human. Radio
station transmit all the usable information at high frequency, through the antenna we receive these
signals, then after some processing it is possible to obtain the original information. AM broadcast is
using high frequency carrier with low frequency signal. It transmits the original signal by amplitude
modulation of the high frequency carrier wave. To receive the signal from the air, we need an antenna.
Antenna for AM channel is not very strict, usually larger area and less obstruction will have better
reception. Signal received by the antenna is very weak and covers a wide band, so ,ve need to separate
the AM frequency we want and filter out the other frequency bands. LC resonance is a good filtering
circuit. We get maximum impedance when the frequency is equal to the formula. Using an inductor and
one variable capacitor we can produce different frequencies base on different LC values. We can select
different radio channels in thls way. However the channel received is still in high frequency signal range
and can't be beard by human ear, we need to remove the high frequency carrier wave. Diode conducts in
positive direction only so used with RC filter we can demodulate the original signal. With a sensitive
earphone you can receive clear AM channel. No electricity is needed! This is the first generation
semi-conductor radio invented in World War IL
If living in city area, place the antenna stick out of window, and put the earphone on, rotate the variable
capacitor to receive radio channels. Adjust the antenna and it's direction can improve reception, if still
not satisfy, get a outdoor antenna. Caution, never connect to ground, equal potential and phase will
1
cancel out the signal received. F01mula:
f = 2 ir ,jLC
Circuit Explanation:
Electric wave received by antenna pass through the coupling capacitor Cl to LC circuit (consist of
inductor Ll and variable capacitor VCl). Use an adjusting stick to slowly adjust capacitor to the
required frequency. (Fig. 1 is ANI amplitude modulated wave). The frequency signal selected pass
through the diode then through the filtering circuit R2 and C2. Finally the sound signal is send to the
earphone through the coupling capacitor C3.
Carrier
Caution:
1. The diode has polarity, antenna need to be straighten up.
2. If sound signal is. very weak, point the antenna to a
direction with less obstruction. Make close contact to
large metal plate to increase the receiving sensitivity.
3. The earphone must be tight with ear without any gap.
Fig. 1
Ve
n:.V.VJJ]].\J_V§V_l/_[V
Bln.n-rmffMffffrf[f•...f •
e,
Signal�
V3
(a)
�
�
Amplitude
Modulated
---· e:$
' .. pt
(b)
VAM '+t-H+rr-r�ct+l�++-l�c++l-1-1-..i-,;;-
(c)
(iJt
Parts list:
Remark
Component
Serial No.
Qty.
1
GnBIGnGo
5.6MO
Rl
ReBkGnGo.
1
R2
2MO
3300pF
I
332pF
Cl
1
C2
33pF
1
IOµF
C3
1
DI
Diode
Variable capacitor 1
VCl
1
Inductor (A11)
Ll
1
Earphone Earphone
1
Antenna
1
K23
PCB
Code: Bk:Black, Re:Red, Gn:Green, Bl:Blue, Go:Gold.
(a)
Sound signal
Positive
alOnly
Mid-Frequency
canfor
"2
-lc1
T
P
r
.
c_Antenna:.--.-L��_J
DIODE
VCl
TIJNING
0
Filter out
mid-frequency
carrier to get
sound signal
:AM
0
0
0
,.. ' ' ..· ·re»•,
...L..J
0
a
Lt ·.
·,,
a a
0
0