Radio & Electronic Projects: A medium-wave receiver using a ferrite-rod aerial

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Introduction

This design came from the Amateur Radio Club, and enables you to build a simple Amplitude Modulation (AM) receiver for frequencies between 600 kHz and 1600 kHz. It should take you around 2 hours to build, and can be used with Walkman-type earpieces. Figure 1 shows the circuit diagram.


Figure 1 Circuit and block diagrams of the radio


Figure 2a The PCB, solder side; Figure 2b The PCB, component side

Description

The whole circuit is built on a 50mm by 50mm printed circuit board (PCB) designed to fit on the inside of the lid of a plastic box, and is stuck there using sticky pads, the shaft of the variable capacitor going through a hole in the lid. Only two pairs of leads are soldered to the board - one pair goes to the 1.5V battery in its holder, and the other to the earphone socket. Figures 2a and 2b show the printed circuit and the component layout double size for clarity. You are not obliged to build the circuit on a PCB.


Figure 3 Details of coil and headphone socket

Building it

1. Check and identify components. Tick the parts list.

2. Carefully unwind the wire. Use paper to make an insulating tube (called a 'former') around the center of the ferrite rod and secure it with Sellotape. Now, close-wind all the wire (leave no gaps between adjacent turns) around the paper former. Secure the winding with more Sellotape, leaving 50mm of wire free at each end for connection to the circuit. See Figure 3a.

3. Solder in VC1.

4. Solder in the integrated circuit holder . There is a notch in one end of the holder; this should face VC1. Solder also the wire link and the capacitors. Be careful to avoid solder 'bridges' between adjacent tracks on the PCB.

5. Solder the battery leads. These must be connected properly - the red battery lead to the + (positive) area and the black lead to the - (negative) area.

6. Strip bare 1 cm of insulation from the ends of two wires. Solder them between the PCB and the headphone socket (see Figure 3b). Use the end tabs on the socket. Using another pair of insulated wires connect the ON/OFF switch to the PCB tabs shown in Figure 2b.

7. Fix the elastic band. This goes through the holes at the top of the PCB, with the ferrite rod being slipped through the two end loops. (Note: although the coating on the copper wire is designed to melt away during soldering, it is quite common for difficulty to be experienced in obtaining a good soldered joint; to be on the safe side, remove the coating before soldering (with a small piece of sandpaper).) Carefully place the wire ends of the coil through the PCB just above VC1, and solder on the track side.

8. Fit IC1 into its holder . This should be done carefully, making sure that all the pins are located above their respective clips before applying any pressure! Make sure also that the notch on the IC (as shown in Figure 2b) matches the notch in the holder, and faces VC1.

9. Put battery in its holder . Listen for some noise in the headphones as VC1 is rotated. Make sure the headphone plug is fully inserted into its socket.

10. Fix the working board to the lid. Use the sticky pads and apply gentle pressure. Fit the tuning knob, the ON/OFF switch and the earphone socket.

11. Test again. If all is still working, fit the lid screws and admire your completed radio!

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Parts list

Capacitors:

C1, C2 0.01 microfarad (uF)

C3, C4 0.1 microfarad (uF)

VC1 500 picofarads (pF)

Semiconductor IC1 ZN416E

Additional items:

Plastic box (recommended size 76 × 64 × 50mm internal)

8-pin DIL socket for IC1

Printed circuit board

Tuning knob for VC1

Wire link for PCB

2m of 30 SWG copper wire, self-fluxing

Piece of paper 25 × 50mm, to make the coil former

Ferrite rod 70mm long by 10mm diameter, approximately

Battery, AA size 1.5V, with holder and attached wires

Miniature earphone socket (3.5mm stereo jack)

ON/OFF switch (push-button SPST latched or slide switch)

4 off 100mm insulated connecting wires, for jack socket and ON/OFF switch

Pair Walkman-type earphones

Elastic band, to attach ferrite rod to PCB

4 off sticky pads for securing PCB to box lid

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