THE CONSTRUCTION AND TESTING OF AN AM RADIO AND AUDIO AMPLIFIERBy Tanisha Leigh Guy, Heather Girling, Bianca Hamilton-Clarke and Laura Hammond

INTRODUCTION

Our group were given the task of, over the course of several weeks, creating a basic working AM radio receiver and Audio Amplifier; demonstrating the broadcast of an AM radio transmission. The purpose of this report is to detail the processes undertaken in creating both of the units, as well as providing information on our results and findings.

THE CONSTRUCTION AND TESTING OF AN AM RADIO RECEIVER

Equipment Ferrite Rod 2.5m of Enamelled copper wire Insulating tape 150pF Variable tuning capacitor Transistor BC548B IC MK484 Five resistors: two 100k, two 1k and one 270R Ultra miniature slide switch Four capacitors: two 100nF, 10nF, 47uF Headphone socket AA Battery Holder AA Battery Printed Circuit board

AERIAL

First the ferrite-rod aerial was constructed using a ferrite rod and copper wire.

The copper wire was wrapped around the rod 55 times, ensuring that none of the coils overlapped, this was then secured into place using electrical insulating tape.

The coil is the aerial and the ferrite rod is used to concentrate the signal gathered from the area surrounding the aerial, providing a better reception

THE COMPONENTS AND COMPLETED AERIAL

Aerial Circuit Board

The trailing ends of the copper wire were stripped of their enamel and soldered onto the circuit board of the Radio receiver, allowing the signal to pass into the radio circuit.

Five resistors were included to limit the current flowing to certain parts of the circuit.

Capacitors were used to work alongside the resistors, building up a certain amount of current before allowing it to continue to flow through the circuit.

Once the resistors and capacitors were included, a variable tuning capacitor was then soldered into place.

VARIABLE TUNING CAPACITOR

The variable tuning capacitor is an essential component in the construction of a radio receiver. The user tunes the radio into a station by adjusting the tuning capacitor, usually by turning a dial or knob, which adjusts a resonator within to vibrate at a chosen frequency. Once the tuning capacitor is tuned to resonate at a specific frequency (one that matches the frequency of the radio transmission the user wishes to tune in to) all other frequencies are ignored apart from the specifically tuned frequency.Image source:

http://www.scitoyscatalog.com/Merchant2/graphics/00000001/variable_capacitor.jpg

THE MK484

Next the MK484 component was included into the circuit.

This works as a demodulator, extracting the original broadcasted information from the tuned radio transmission signal (the audio) before passing the signal into the amplifier component.

Image source:http://www.scitoyscatalog.com/Merchant2/graphics/00000001/mk484.jpg

BC548B TRANSISTOR

The BC548B transistor was then incorporated into the circuit as an amplifier, working to enhance the audio signal extracted from the radio sine wave by the demodulator.

The transistor cannot produce sound itself but rather enhances the audio signal ready to be passed through a speaker which converts the amplified signal into audible sound.

Image Source:http://quarndon.co.uk/images/Transistor(t).jpg

OTHER COMPONENTS

In order for the amplified signal to be heard as sound, a headphone socket was included, so that the user may plug headphones into the receiver, or connect it up to an external amplifier and speaker.

A battery holder was also soldered into its place in the circuit to provide power to the radio receiver. A switch to regulate the power, turning the radio off and on at the will of the user, was also included.

RESULTS We found that we were able to tune the radio

receiver in to two separate radio stations, which we listened to until we were able to detect the names of the stations: BBC Asian Network and Sabras Sound.

Using an online frequency finder (http://frequencyfinder.org.uk) and selecting “Leicester” from the location section, we scrolled through the list of AM Radio Stations until we found the two stations we had found. This website provided us with the frequency of the radio stations we had tuned into: BBC Asian Network on frequency 834 and Sabras Sound on frequency 1260.

The reason that these two stations were the only ones we were able to tune into, even though the frequencies were so different, was due to the fact that they were in the closest range to our radio aerial.

THE CONSTRUCTING AND TESTING OF AN AUDIO AMPLIFIER

Equipment IC TBA 820M IC Socket Six resistors: three 1k, two 470R, one 1R Six capacitors: two 330uF, one 10uF, one 150pF, one

100nF, one 47nF 8ohm speaker 100mm speaker cable Slide Switch PP3 Battery snap PP3 Battery 3.5mm plug to plug cable 3.5mm stereo jack socket Printed Circuit Board Red LED light

METHOD

First the resistors and capacitors were soldered into their required positions.

We then included red LED light into our circuit; making sure that the positive leg was soldered into the correct positive slot and the negative leg was soldered into the negative slot.

The LED was included to ensure that the user would know when the audio amplifier was switched on and receiving power.

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IC SOCKET

The IC socket was then soldered into place, without the IC TBA 820M connected into it, making sure that the indented side matched up to the image on the printed circuit board to enable the different pins to function correctly.

Image source:http://base1.googlehosted.com/base_media?q=http://cpc.farnell.com/productimages/farnell/standard/42268235.jpg&size=19&dhm=494ddba&hl=en

SPEAKER

The next stage was to solder the speaker cable to the speaker itself, before connecting the cable to the respective negative and positive sections on the circuit board, relating to the marked positive and negative sections of the speaker.

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BATTERY

A PP3 battery snap was soldered into place, once again ensuring that the positive and negative wires were soldered into their correct positions, marked out on the printed circuit board

Along with the battery snap, a slide switch was soldered onto the circuit board to enable the user to turn the amplifier off or on at their choice.

Image source:http://www.maplin.co.uk/images/full/ne19.jpg

STEREO JACK

A 3.5mm stereo jack was next soldered into its place on the circuit board.

This jack socket is compatible with the 3.5mm plug to plug cable, which can be connected to the AM Radio Receiver via the receiver’s headphone socket to provide sound via the audio amplifier rather than by using a set of headphones.

Image source:http://t0.gstatic.com/images?q=tbn:Vo2QgdvvWNBEMM:http://rocky.digikey.com/weblib/CUI%2520Inc/Web%2520Photo/New%2520Photos/SJ-3525N.jpg

IC TBA 820M

Lastly, the IC TBA 820M microchip was slotted into the IC socket after making sure that the battery was not connected and that all of the soldered sections were cool.

The ICA TBA microchip, if not fitted correctly or if exposed to heat, may fail.

It is an 8pin IC that creates little distortion when the audio signal is amplified.

Image source:http://base1.googlehosted.com/base_media?q=http://cpc.farnell.com/productimages/farnell/standard/42268235.jpg&size=19&dhm=494ddba&hl=en

RESULTS Once the Audio Amplifier was completed, and the

battery was in place, the 3.5mm plug to plug cable was then connected via the 3.5mm stereo jack socket, which was then connected to the radio receiver via its headphone jack socket.

When both the radio receiver and audio amplifier were connected and switched on, our audio amplifier only provided us with a brief moment of sound before it cut.

In order to rectify this problem we discussed a number of possible faults and found it to be the speaker that was not working.

We disconnected the cables connecting to the speaker and then soldered them back onto the circuit, making them more secure. When it came to testing it again the audio amplifier began to function correctly, providing us with an audible transmission.

CONCLUSION

Using a range of provided resources and equipment, we successfully built a working AM Radio Receiver and Audio Amplifier which we were able to use to demonstrate the broadcasting capabilities of an AM transmission, and to find the limitations of the devices that we had created. Because we had worked so well as a team, we were able to delegate certain roles to each member, enabling us to complete the tasks in a quick and efficient manner, leaving enough time to rectify any problems that arose.

IMAGESOf us at work