TRF Radios 3, HAC Radio, Ladybird Radio, Denco Green Coils

1: AUGUSTIN'S RADIO

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.

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.

	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.

	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
	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.

	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

2: CHAD'S RADIO
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).
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 capacity 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).
3: PETER'S RADIOS
Peter has successfully built several matchbox radios and wrote: Many thanks for your web-page, 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.

4: MILAN from Slovakia:

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 home made PCB. The ferrite antenna 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

5: FELIX SCERRI :
CRYSTAL SET BASED CIRCUIT PROVIDING A HIGH QUALITY PROGRAMME SOURCE

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:

PAGES WITH ARTICLES BY RADIO EXPERT FELIX SCERRI:

High Quality AM Receivers:

http://www.crystalradio.net/scerri/index.shtml

http://www.crystalradio.net/

http://www.xtal-sets.com/

Hi Fi AM Receiver Designs:

http://sound.westhost.com/articles/am-radio.htm

OTHER ARTICLES

ESP by Rod Elliot - "Mad as Hell" & lead free solder directive:

http://sound.westhost.com/madashell.htm

ESP by Rod Elliot - CFL's (Compact Fluorescent Lamps)
http://sound.westhost.com/articles/incandescent.htm