Radio & Electronic Projects: A medium-wave receiver

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Introduction

Let us start off with something that is really quite simple and yet is capable of producing a sense of real satisfaction when complete - a real medium wave (MW) radio receiver! It proves that receivers can be simple and, at the same time, be useful and enjoyable to make. To minimize the confusion to absolute beginners, no circuit diagram is given, only the constructional details. The circuits will come later, when you have become accustomed to the building process. In the true amateur spirit of ingenuity and inventiveness, the circuit is built on a terminal strip, the coil is wound on a toilet roll tube (as amateur MW coils have been for 100 years!), and the receiver is mounted on a piece of wood.

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Putting it together

Start by mounting the components on the terminal strip as shown in Figure 1, carefully checking the position and value of each one. The three capacitors are all the same, and so present no problem. They (and the resistors) may be connected either way round, unlike the two semi conductors (see later). The resistors are coded by means of colored bands.

You can refer to Section 7 if you have difficulty remembering the colors and their values.


Figure 1 Terminal strip - position of components

1. Brown, Black, Yellow 100 000 ohms (R1, R5, R6)

2. Green, Blue, Brown 560 ohms (R2)

3. Red, Violet, Brown 270 ohms (R3)

4. Brown, Black, Orange 10 000 ohms (R4)

The integrated circuit (the ZN414Z) and the transistor (the BC184) must be connected correctly. Check Figure 1 carefully before fitting each device.

Now wind the coil. Most tubes are about 42mm diameter and 110mm long. Don't worry if your tube is slightly different; it shouldn't matter. Make two holes, about 3mm apart, about 40mm from one end, as shown in Figure 2. Loop your enameled wire into one hole and out of the other, and draw about 100mm through; loop this 100mm through again, thus anchoring the wire firmly. Now wind on 80 turns, keeping the wire tight and the turns close together but not overlapping. After your 80th turn, make another two holes and anchor the wire in the same way as before.

Again, leave about 100mm free after anchoring. Using another piece of enameled wire (with 100mm ends as before), loop one end through the same two holes which contain the end anchor of the last winding, wind two turns and anchor the end of this short winding using another pair of holes.

Figure 2 shows the layout.


Figure 2 The layout of the parts on the wooden base.

With some glass paper, remove the enamel from the ends of both pieces of wire which go through the same holes (i.e. the bottom of the large coil and the top of the small coil), then twist these bare ends together.

Remove the enamel from the remaining ends of the coil. The coil is now finished! The baseboard can be any piece of wood about 150mm square. Fix the coil near the back edge using drawing pins and connect the wires from the coils to the terminal strip as shown in Figure 2. Using short pieces of PVC insulated wire (and with assistance if you have never soldered before), solder one piece across the two outer tags of the variable capacitor, shown by the dotted line in Figure 2, and then two longer pieces to the center tag and one outside tag. Connect these to the terminal strip. Then solder two more insulated wires on to the jack socket (into which you will plug your crystal earpiece), the other ends going to the terminal strip. The last two wires (one must be red) need to be soldered on to the battery box, their other ends going to the terminal strip also. Make sure the red wire goes to the positive terminal on the battery, and is connected to terminal 9. The other connection to the battery goes to terminal 10.

Attach the terminal strip to the baseboard with small screws or double-sided sticky tape. The other parts can be mounted the same way.

Listening is done ideally with the recommended crystal earpiece. Don't be tempted to use your Walkman earpieces; they are not the same and will not perform anything like as well. The receiver should work without an extra aerial, but one can be attached to terminal 1 if necessary. A long piece of wire mounted as high as possible is ideal. The Audio-frequency Amplifier project will enable you to use a loudspeaker with your receiver, using the signal from the jack socket. No circuit modifications will be needed!

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Parts list:

Resistors: all 0.25 watt, 5% tolerance

R1, R5, R6 10 kilohms (kOhm)

R2 560 ohms

R3 270 ohms

R4 10 kilohms (kOhm)

Capacitors:

C1, C2, C3 100 nanofarads (nF) 500 picofarads (pF)

Semiconductors:

ZN414Z, BC184

Additional items:

12-way 2 A terminal strip

22 meters of 28 SWG enameled copper wire

A few short pieces of colored PVC-insulated wire

Crystal earpiece

3.5mm jack socket

1.5V AA-size battery and box

Toilet roll tube

Double-sided sticky tape or selection of screws

Tools required:

Small screwdriver, soldering iron.

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