Cosy MUTO,
JH5ESM
30 Nov., 2008
I have written an article on handcraft radio in Eleki-Jack magazine (and also in my WEB), where toilet paper cores play most important role.
However, the one-chip radio IC LMF501T (compatible with MK484) has already been discontinued and will become hard to find it.
NR4K (abbreviate for “New Radio for Kids”) is designed for LMF501T replacement. It is a workshop material for Radio Workshop for Kids held in Hibikinosai, annual open festival at Kitakyushu Science & Research Park.
Fig.1 NR4K schematic (click here to download PDF version).
Fig.1 shows the schematic of NR4K. It is a straight radio featuring differential RF amplifier and dirct coupling throughout the circuit.
The resonator consists of handcraft loop antenna made from empty
tissue box, handcraft variable capacitor made from toilet paper cores
and a fixed inductor. These handcrafts are shown later.
Of course, standard AM bar-antenna and polyvaricon can be used as well as ancient tube era's antenna coil and air-varicon.
In orthodox transistor radio, the base node of the first transistor is connected to resonator's secondary winding or inductor tap. However in this radio, simplicity is required and thus we use high input impedance differential amplifier (Q1, 2).
Single ended output from the differential pair is then amplified again by common-emitter circuit Q3.
Q4 amplifies signal again, however, its operating
point is biased to near the supply voltage, not the center of the load
line. Therefore, the collector current of Q4 only flows during positive
half-cycle of the input signal.
The envelope detection is carried out at the collector of Q4
by connecting an integrating capacitor (0.1[μF]) and it drives a stereo
headphone (monaural earphone is also available by connecting headphone
jack connection as shown in Fig.1).
Supply voltage is 3[V]. Manganese dry batteries are sufficient because its quiescent current is only approx. 2[mA]. It can be operated under 1.5[V] supply but less sensitivity.
Supply line for Q1 to Q3 should be decoupled by 100[Ω] and 100[μF], otherwise unacceptable motorboating will occur.
 |  |  |
| (a) Parts layout | (b) Mounted solderless breadboard | (c) Making the loop antenna |
The variable capacitor is made from two toilet paper cores and aluminum foil. Look at pictures and instructions for details.
Look at following step photographs to make the loop antenna;
Step 1: Tear off the top plane of an empty tissue paper box.
Step 2: Make two rectangles from the top plane.
Step 3: Fold those rectangles into two, to increase their flexural strength.
Step 4: Cross and staple those rectangle to make the loop frame body.
Step 5: Cut off the frame arm edges in “vee” style.
Step 6: Make two small holes (approx. 0.5 to 0.8[mm] diameter) on one of the frame arm.
Step 7: Thread one of the holes by using 8[m] length AWG28 (0.32) wrapping wire or UEW, by approx. 0.4[m].
Step 8: Wind 10 times the wire.
Step 9: After winding, thread the other hole the rest of the wire.
Step 10: Tape the frame edge to prevent wind off the wire from the frame, and then tape a half-split chopsticks as a stand.
The inductance of the loop antenna is approx. 50 to 60[μH]. By connecting a 330[μH] inductor in series, we get AM broadcast coverage.
The rest of tissue paper box will become an enclosure :-)
Fig.3 Completed NR4K.
