Worried
About Soldering? Don't Be:
For some simple ideas on solderless construction techniques have a look
at the Crystal Sets 2 page. When
constructing these small electronic projects it will be necessary to
determine the exact value of resistors, which are colour coded, and
capacitors, which sometimes have confusing numbers on them. I
have
included a table for both Resistor Colour Codes and a Capacitor
Conversion Table HERE.
TRF RADIOS (Part 3) - Readers' Radios:
Several MDS975 readers have
e-mailed their transistor and integrated circuit TRF radio designs -
and here they are!
1: AUGUSTIN'S RADIO
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Augustin from Romania
kindly e-mailed us some photographs and the details of his own
'Matchbox Radio'.
Unable to source an MK484
or ZN414 Integrated Circuit, Augustin set
about constructing a two transistor radio housed in a matchbox, and has
cleverly used a small polyvaricon type tuning capacitor, found in many
small
commercially bought pocket radios on the market, and also ingeniously
'etched' a purpose designed printed circuit board.
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Hi, I'm Augustin, I live in Romania and I made my Matchbox Radio out of
two transistors. I shall explain my circuit below:
THE COILS
(L1 and L2):
The first coil (L1) has 75 turns of litz wire. The second coil (L2) has
7 turns. You can make the coils on a flat ferrite rod or bar. After you
finish the first coil, you make the second starting from one end of the
ferrite. The second coil can be made of litz or normal enamelled
/ insulated wire. The number of turns (7) is not too critical.
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THE VARIABLE CAPACITOR (Cv):
The variable capacitor is a common type, something about 270 pf.
It is
not too critical. For miniaturisation it is best to use something that
will fit inside the matchbox. I used a 270 pf variable capacitor from
an old transistor radio.
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THE
TRANSISTORS:
Both of the transistors that are used in this circuit can be either BC
108 or BC 107 or BC 109. I used BC 108 transistors with a metal capsule
for both. These are made
in a metal capsule. The equivalents in a plastic capsule should work,
but I do not guarantee it. Don't know why but those in metal capsule
seem to work a little better.Anyway, I guess it's easy to find BC 108
devices.
Equivalents in a plastic capsule:
BC 108 - BC 172 - BC 548 - BC 238
BC 107 - BC 171 - BC 547 - BC 237
BC 109 - BC 173 - BC 549 - BC 239
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THE
RESISTORS (R1, R2, R3) :
R1 and R3 are 30 K Ohms
R2 is 1 K Ohm
THE
CAPACITORS (C1 and C2 ):
Both capacitors are 10 nF but the values are not too critical but do
not exceed 35 nF. I stuck to using 10nF devices.
The Electrolytic capacitor has a value anywhere from 2 to 33 uF.
I used 33 uF. |
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THE
EARPHONE:
I used a phone capsule with the impedance of 230 ohms. If you
want to use a crystal headphone you should solder a 1K resistor in
parallel with it.
THE POWER
SOURCE:
I power the radio from a 1,5 V AAA size alkaline battery which lasts
about 2 weeks non stop use.
If you have any questions about my radio you can e-mail me at this
address:
Best wishes, Augustin.
See MORE of Augustin's radios HERE
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2:
CHAD'S RADIO
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Chad e-mailed
MDS975 from
Woodland Hills, USA describing the MK484 radio that he has successfully
built. The circuit is slightly different to the one described
above in that the headphones are 2000 Ohm magnetic types and are wired
in series with the positive power supply.
Chad receives three local stations at good strength plus another five
at varying levels.
See the circuit details (right).
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Depending on the type and impedance of the magnetic headphones being
used the 1k resistor marked * would need to be adjusted, but should not
really be less than 470 Ohms. As in the above Matchbox Radio
circuit, there is quite a good reason to use a variable preset
potentiometer here as this can set the AGC at the best level and make
compensation for a weakened battery.
Chad's radio circuit works extremely well with 2000 Ohm magnetic
headphones and even produces a result when using 32 Ohm headphones or a
small loudspeaker!
There is perhaps room for variation with the value of the 0.01µF
(10nF) capacitor marked **, perhaps because you don't have the
exact component at hand or maybe just because you like
experimenting! Chad
used a 62pF capacitor here.
You will see that a small variation either way will effect the way the
radio tunes. You could try experimenting with a 0.02µF (20nF) or
a
0.005µF (5nF) capacitor here to see what the differences in gain
and tuning performance are.
Chad also
changed
the output capacitor from 0.1 uF, as shown in the diagram, to 0.04 uF.
As with all radio circuits it is always worth experimenting with
values, not only to see if any improvements can be made, but also just
for the educational experience!
For the very best results it is safe and best to stay with the value
specified and keep the wiring of the circuit as neat as possible and
the
connecting wires short to prevent stray capacitiy and instability that
would cause oscillations (howl) within the radio.
Thank you Chad for sending in the details of your experiments and for
the photos of your radio (right).
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3:
PETER'S RADIOS
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Peter has
successfully built several matchbox radios and wrote:
Many thanks for your
webpage, it got me interested all over again in building these
radios. See photo on the right:
The Brymay matchbox
receives Medium Wave and uses the MK484 i.c. + BC548B transistor.
The England Finest
matchbox covers LongWave and uses the same circuit, both high impedance.
The lower unit is
the
prototype with added 2N2222 for a low impedance output.
All use an on-board
1.5
Volt button cell.
In Devon these
radios
pulled in BBC
Radio 4 and RTE 1 on LW. MW reception is fine too with Virgin,
Talk-Sport, BBC Wales, etc. The best yet was the Radio Sweden English
program.
Great Fun! Good listening.
Regards,
Peter.
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4: MILAN from Slovakia:
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Hi, I´m Milan from Slovakia.
I send you the schematic for my 15 year old TRF radio for listening
only one radio station. The radio was built into a plastic case of
about 50x65x14 mm in dimension. I used a homemade PCB. The ferrite
antena had a 75 turn winding with 10x0,05 mm wire, and a 7 turn winding
with 0,3 mm wire for the connection to the BC108 transistor.
For the RF a transformer is needed with little ferrite core - see
schematic and photograph. This circuit was made in a time when the
power of MW transmitters was 10 times bigger than now in Slovakia.
This radio is easy and pleasant to use and is very simple to construct.
It hasn´t a power switch, volume controller, or tuning control.
This radio has a tuned circuit that is pre-adjusted to one favourite
transmitter.
My favourite station is Slovensko One - The Slovak First Broadcasting
Programme - on 1098 kHz medium wave.
Yours truly,
Milan from Slovakia
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5: FELIX SCERRI :
CRYSTAL SET BASED CIRCUIT PROVIDNG A HIGH QUALITY PROGRAMME SOURCE
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Felix Scerri ( VK4FUQ ) kindly e-mailed us
with an interesting circuit diagram, shown above, for a crystal
set based receiver that will provide a high quality AM programme
source.
Felix writes:
"My interest in crystal sets goes back a long way and I'm particularly
interested in using them as high quality AM program sources."
LINKS:
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