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Simple, fun, educational project By JOHN CLARKE Build an . . . AM Radio This simple AM radio can built in two forms. One is shirt pocket size, not much larger than an Android phone, which drives headphones or ear-buds. The other is a retro-style mantel radio with a hand-span dial and a 100mm (4-inch) loudspeaker in a basic timber cabinet. 32  Silicon Chip siliconchip.com.au S1 POWER D1 1N5819 K +8.7V CON1 A 9–12V DC IN A K 6.8k LED2  A B A LED3 Q1 BC547 D2 1N4148 E  A 1k 27k IC1: MK484 OR TA7642 1 VC1 TUNING 2 10nF 100k OUT IC1 470 F 10 100nF VR1 10k GND BIAS SET 1k 100nF 18nF AM RADIO RECEIVER 1 IC2 LM386N K 8 7 5 470 F CON2 PHONES 47nF 4 1N5819 K A 10 B E LM386N MK484, TA7642 BC547 LEDS A K SPEAKER 10 F 10 F A SC 2 470pF VR2 100k 6 3 VOLUME D2, ZD1 2011 100F 100nF IN 4 ZD1 4.7V A 2.2k 3 9V BATTERY K K 100 F K FERRITE ROD ANTENNA  LED1 K C 4 8 C OUT IN GND 1 Fig.1: the circuit is based on an MK484 (or TA7642) radio receiver IC. This amplifies and detects the tuned RF signal and drives an LM386N audio amplifier. Q1, LED2 & LED3 provide a regulated 1.4V rail for IC1. W ANT A SIMPLE radio that you, your children or grand-children can easily build? This one uses a small PCB with two ICs and not a great deal more. It’s not a superheterodyne so the alignment is very simple and you don’t need any special equipment. The pocket-sized version is housed in a remote-control case incorporating a 9V battery compartment. If you want, there is the option to power it from a 9-12VDC external supply (eg, a plugpack) and to drive an external loudspeaker. It is tuned using a rotary thumbwheel dial and has a volume control, battery condition indicator and power switch. The retro-style desktop version is designed to look a little like the old AM radios of a bygone era that took pride of place on top of the fireplace mantel. It incorporates a loudspeaker and a hand-span tuning dial. It is housed in a small timber box with an aluminium front panel and this carries the volume control, battery condition indicator and power switch. The sound from the loudspeaker is not overly loud but is quite sufficient for personal listening. AM radio IC The circuit for the AM radio is based siliconchip.com.au on a single IC that includes RF (radio frequency) amplification, a detector and AGC (automatic gain control). A similar device was originally available in 1984 from Ferranti Semiconductors and was known as the ZN414Z but is now obsolete. The MK484 replaces this and although out of production, there are remaining stocks. Additionally, the TA7642 is also now available with similar performance to the MK484. These AM radio ICs will work from 150kHz to 3MHz. Add a tuning coil, a variable capacitor plus some capacitors and resistors and the IC becomes a fully functional AM receiver. For our circuit, the receiver operates over the standard AM radio band of 531-1602kHz. The signal output from the IC is amplified to drive a pair of headphones or a loudspeaker. We tested both the TA7642 and MK484 in our circuit and found that the TA7642 has greater sensitivity than the MK484. However, its selectivity is wider, ie, it’s not as good. This means that the TA7642 will exhibit greater crosstalk (or interference) between stations that have adjacent frequencies. We did not test a ZN414Z as we didn’t have one available. Note that while the performance of Specifications Tuning Frequency: approximately 531-1602kHz Output power: ~300mW into 4Ω Operating current: typically 27mA this AM Radio Receiver is acceptable, it does not have the selectivity and sound quality that’s available from a superheterodyne receiver. Circuit details The full circuit for the AM Radio Receiver is shown in Fig.1. IC1 is the AM radio chip. We have reproduced its equivalent circuit in Fig.2 (from the TA7642 data sheet). This is a “tuned radio frequency” or TRF circuit and it combines a high-gain RF (radio frequency) amplifier and a detector, to recover the audio signal. It is not a regenerative or reactive receiver. The inductance of the ferrite antenna rod (L1) and variable capacitor VC1 form a tuned parallel resonant circuit. This has a high impedance at the tuned frequency and a low impedance at other frequencies. IC1 amplifies the tuned signal and January 2012  33 3 R3 12k 2 T1 R7 12k R8 12k T4 R2 3.3k R1 5.6k T2 R13 12k R11 12k R15 12k C4 23pF C2 12pF R4 12k C1 12pF R9 12k R6 12k R5 12k T5 R10 12k T6 R14 74.6 R12 12K T7 T8 T10 T9 T3 1 Fig.2: this diagram shows the internal circuit of the TA7641 single-chip AM radio receiver. It includes an RF amplifier, a detector and automatic gain control (AGC) – see text. then its internal detector rectifies and amplifies the resultant audio frequencies. IC1 is a 3-pin device with its AC output and DC power supply input using the same pin. One-chip AM radio The internal workings of IC1 are quite interesting. While it contains 10 transistors (and a number of resistors and capacitors), there are three RF amplifier stages. Transistor T1 is an emitter-follower to provide a high input impedance. T2 is its load and operates as a current sink, biased by T3 and R1. The signal is then AC-coupled to T4, the first RF amplifier stage. This operates as a common-emitter amplifier with a 12kΩ collector load while transistor T3 also provides its DC bias. The output is then AC-coupled to T5, the second RF amplifier stage. Again it has a 12kΩ collector load and its DC bias is provided by transistor T6. The third amplifier stage, formed by transistor T7 shares the same bias generator. The output is then AC-coupled to the detector, transistor T9. This is critically biased by transistor T8 (note the low-value resistor from its collector). The result is that it rectifies and amplifies the modulated signal, ie, the audio. This is then amplified and buffered by transistor T10, again a common-emitter amplifier which has its collector connected to the output pin. The output pin is connected to an external capacitor (18nF in our case) which filters out most of the RF carrier, leaving the original modulating signal which is the audio we want to hear. That’s all relatively straightforward but this chip also includes an automatic gain control (AGC) function and 34  Silicon Chip it’s less apparent how that operates. The point of AGC is to reduce the amount of RF amplification for strong stations, so that the audio output level doesn’t vary too much between strong and weak stations. While Fig.2 is only an equivalent schematic and so doesn’t necessarily show exactly what is going on in the IC, it seems likely the shared biasing arrangement of both T3 and T6 provides this AGC action. With stronger signals, the increased modulation on the later stages causes the bias on the earlier stages to change so that their gain is reduced. Back to the circuit While IC1 has internal AGC, its output signal amplitude still varies somewhat with station strength. Trimpot VR2 and its associated 100kΩ resistor allows the overall RF gain (and AGC) to be adjusted to suit the signal strength at your location. When VR2 is adjusted, the DC bias at IC1’s input shifts and this changes the bias on its buffer stage and thus the signal level that’s fed to the following RF gain stages. Speaking of the buffer stage, its high input impedance (around 3MΩ) minimises the loading on the tuned circuit, providing optimal operating conditions. The resonant circuit is designed with a high “Q” factor to ensure good selectivity between adjacent stations. This is important because a TRF receiver amplifies whatever signal is picked up and so there is always some risk that strong adjacent stations can “break through”. The supply voltage for IC1 is applied to its OUT terminal and this is derived via transistor Q1 and a 2.2kΩ resistor. The demodulated AM signal also appears at the OUT terminal and the 18nF capacitor to ground rolls off the audio response above 4kHz. IC1 has a limited operating voltage range of 1.2-1.6V. This is provided by a simple voltage regulator comprising Q1, LED2 & LED3. These two LEDs are infrared types and have a forward voltage of approximately 1V when low current flows through them. This forward voltage is remarkably constant for a wide range of currents. In fact, tests of several infrared LEDs from different manufacturers showed that their forward voltage is around 1.09V at 1.6mA, dropping slightly to 0.945V at 160µA. Connecting two such LEDs in series provides a reasonably stable 2V reference and these are fed with about 1mA via a 6.8kΩ resistor from the 8.7V supply rail. This 2V reference is applied to the base of transistor Q1 and so about 1.4V appears at its emitter (due to the 0.6V base-emitter voltage drop). This voltage is then used to power IC1 via the 2.2kΩ resistor, as described above. Audio amplifier stage IC1’s audio output is fed via a 10Ω RF (radio frequency) stopper resistor and a 100nF capacitor to volume potentiometer VR1. The signal at VR1’s wiper is then AC-coupled via another 100nF capacitor to pin 3 of IC2, an LM386N audio power amplifier. The inverting input (pin 2) of IC2 is grounded and the amplifier has a gain of close to 50, as set by the 1kΩ resistor and series 10µF capacitor between pins 1 & 8. The power supply at pin 6 is bypassed with a 100µF capacitor, while a separate 10µF bypass at pin 7 removes supply ripple from the amplifier input stages. IC2’s amplified output appears at pin 5 and is AC-coupled via a 470µF capacitor to stereo headphone socket CON2. This allows either a loudspeaker or a set of headphones to be used. Plugging in the headphones automatically disconnects the loudspeaker. The 470µF capacitor provides lowfrequency roll-off below 21Hz for 32Ω stereo headphones (which are connected in parallel) while for a 4Ω load, the low-frequency roll-off is below 85Hz. In addition, a Zobel network comprising a 10Ω resistor and a 47nF capacitor is connected from IC2’s pin 5 output to ground to prevent instability. Assuming a 9V power supply, IC2 can provide about 300mW into a 4Ω load. Its distortion is typically around 0.2%, rising to 3% at the 300mW level. siliconchip.com.au TO SPEAKER IN DESKTOP VERSION VR1 CON2 K + 10 F 4V7 IC1 1 4 This fully-assembled PCB is for the desktop version (ie, VC1 not installed). Take care with component orientation. 47nF 6.8k Q1 100nF ANT. VC1* 100 F 2.2k 3 27k TO VC1 IN DESKTOP VERSION 18nF 10 100k VR2 D2 1k 10 100 F 10nF 470pF (ROD ANTENNA COIL) 2 470 F 4148 IC2 LM386 100nF 12110160 CABLE TIES OIDAR MA 1k 470 F ZD1 10 F L1 S1 A LED1 100nF LED3 + 5819 A A K K D1 – LED2 * VC1 MOUNTS ON BOARD VIA 2.5MM SPACERS IN POCKET VERSION – SEE TEXT CON1 TO 9V BATTERY SNAP Fig.3: install the parts on the PCB as shown in this diagram. Note that tuning capacitor VC1 is mounted on the PCB for the pocket version only. The power output is reduced to about 160mW when using 32Ω stereo headphones but this is more than enough to provide sufficient volume. Power supply Power for the AM Radio Receiver can come from either a 9V battery or an external 9-12V DC plugpack. When the external supply is plugged into the DC socket, the 9V battery is automatically disconnected. Diode D1 provides reverse polarity protection, while S1 is the power on/off switch. Note that a 1N5819 Schottky diode is used for D1, to limit the voltage drop across it to about 0.3V. LED1 is used as a battery condition indicator at switch-on and then functions as a power-on indicator. It operates as follows: when power is first applied, current flows through LED1, 4.7V zener diode ZD1 and a 1kΩ resistor into a 470µF capacitor which is initially discharged. If the 9V battery is fresh, it provides 8.7V at LED1’s anode. This voltage is then dropped by about 1.8V across LED1 and by 4.7V across ZD1, leaving 2.2V across the series 1kΩ resistor (ie, when the 470µF capacitor is discharged). As a result, LED1 lights with about 2.2mA initially flowing through it. At lower battery voltages, there is less voltage across the 1kΩ resistor. As a result, less current flows through LED1 and its initial brightness is reduced. In fact, when the battery voltage eventually gets down to 7V, there is only about 0.2V across the 1kΩ resistor Table 1: Resistor Colour Codes o o o o o o o siliconchip.com.au No.   1   1   1   1   2   2 Value 100kΩ 27kΩ 6.8kΩ 2.2kΩ 1kΩ 10Ω 4-Band Code (1%) brown black yellow brown red violet orange brown blue grey red brown red red red brown brown black red brown brown black black brown and LED1 barely lights, indicating that the battery has gone “flat”. After switch-on, the current through LED1 is progressively reduced as the 470µF capacitor charges and so the LED quickly dims. It doesn’t turn off completely though since the associated 27kΩ resistor ensures that it just remains on, with about 80µA through it. LED1 now indicates that the power is on but the current through it is dramatically reduced to conserve the battery. When power is switched off, diode D2 discharges the 470µF capacitor so that LED1 is ready to indicate the battery condition the next time the unit is turned on. PCB assembly The AM Radio Receiver is built onto Table 2: Capacitor Codes Value 100nF 47nF 18nF 10nF 470pF µF Value 0.1µF 0.047µF 0.018µF 0.01µF NA IEC Code EIA Code 100n 104   47n 473   18n 183   10n 103 470p 471 5-Band Code (1%) brown black black orange brown red violet black red brown blue grey black brown brown red red black brown brown brown black black brown brown brown black black gold brown January 2012  35 60 0 700 800 0 90 650 RA DIO . A M RADIO 00 10 . A M RAD . 0 55 530 0 130 1600 IO . AM kHz Fig.5: the dial label for the desktop version. It can be downloaded in PDF format from the SILICON CHIP website. Fig.4: this is the drilling template for the loudspeaker grille in the desktop version. Drill and ream all holes to 5mm. Power Phones Volume 6mm 7mm 5mm 3mm Align with bottom edge of aluminium front panel Fig.6: the drilling template and control panel for the desktop version. The timber cabinet is made from 2 x 238mm and 2 x 120mm lengths of 90 x 19mm dressed pine, with cleats at each corner to secure the front panel. a PCB coded 06101121 and measuring 65 x 86mm. This PCB is used for both the pocket and desktop versions. The only difference is that for the pocket version, you will need to make the corner cut-outs at one end of the board, adjacent to VR1 and switch S1, to allow the board to clear a couple of pillars in the case. In practice, it’s just a matter of using a small hacksaw to cut away the corners and then filing the cut-outs to shape. 36  Silicon Chip Fig.3 shows the assembly details for the PCB. Before installing any parts, check that the corner mounting holes and the holes for the cable ties are all 3mm in diameter. That done, start the assembly by installing the resistors, zener diode ZD1 and diodes D1 & D2. Note that the diodes must all be correctly orientated, as shown on Fig.3. Table 1 shows the resistor colour codes but it’s also advisable to check each one using a digital multimeter (DMM) before installing it. Next, install PCB stakes at the external wiring points, followed by the MKT and ceramic capacitors, then IC1, transistor Q1 and IC2 (LM386N). The latter can either be soldered to the board or you can mount it via an 8-pin IC socket. Make sure that it goes in the right way around. IC1 and Q1 must also be correctly orientated. Fig.3 shows how to install IC1 if using an MK484 or TA7642 device. If have a Ferranti ZN414Z in your parts drawer, then this can also be used but note that its GND and OUT pins are reversed compared to the MK484 and TA7642. This means that it would have to be rotated 180° when installing it on the PCB (ie, install it with its flat side towards Q1). Installing the LEDs LED1 (red) is mounted by first bending its leads down through 90° exactly 7mm from its body. It’s then installed with the centre of its lens 6mm above the PCB and this can be done by pushing its leads down onto a 6mm-high cardboard spacer. Its anode lead is the siliconchip.com.au This view shows how the PCB assembly, tuning capacitor and loudspeaker are mounted on the back of the aluminium panel and connected via flying leads. longer of the two and the LED must go in with this lead adjacent to switch S1. The two infrared LEDs (LEDs2 & 3) are mounted by pushing them all the way down onto the PCB before soldering their leads (they simply provide a voltage reference for transistor Q1). The electrolytic capacitors can go in next and these must be orientated as shown on Fig.3. Make sure that the tops of these capacitors are no more than 12.5mm above the PCB if building the pocket version, otherwise the lid of the case will not fit correctly. Once they’re in, install potentiometer VR1, trimpot VR2, switch S1, the DC socket (CON1) and the 3.5mm stereo socket (CON2). The next step is to connect the coil with the 10Ω resistance to PC stakes “1” and “2”. You will find that one of the leads of this winding emerges from inside the coil – this is the wire to connect to PC stake 1. For the rod used in our prototype, it’s also the unmarked lead. The other lead of the 10Ω winding goes to PC stake 2 and this wire will have a blue marking. Connecting the main coil in this way will give the highest selectivity (ie, the highest Q). The other two wires (ie, in the 2Ω antenna winding) are marked red and green. These go to PC stakes 3 & 4 and can be connected either way around. Installing the antenna rod Variable capacitor VC1 is mounted on the front panel in the desktop version and is connected via flying leads (see photo). So, if you’re building this version, just solder two 100mm-long lengths of light-duty hook-up wire to VC1’s pads for the time being – see Fig.3. Alternatively, if you’re building the pocket version, VC1 is mounted Two 100mm cable ties are used to secure the ferrite rod antenna to the PCB. Once it’s in place, separate out the four wires for the two coils and find the two that have the greatest resistance. On our prototype, the main winding on the ferrite rod measured about 10Ω while the separate antenna winding measured 2Ω. siliconchip.com.au Installing VC1 on the PCB itself. It’s not just a matter of installing it flush with the PCB though – instead, it has to be mounted 2.5mm above the PCB using a couple of spacers, so that the tuning thumbwheel doesn’t later foul the bottom of the case. You can use a couple of TO220 insulating bushes as the spacers and you must secure the assembly using two M2.5 x 6mm machine screws. Don’t use screws that are longer than 6mm, otherwise they will foul the plates inside VC1 and you won’t be able to turn the tuning shaft. The battery clip lead can now be connected to its PC stakes, adjacent to CON1. Be sure to loop the leads through the two strain relief holes in the PCB. Note that if you are building the pocket version, the battery clip must first placed inside the battery compartment. Its leads are then fed out through a slot at one end and looped through the holes in the PCB. Desktop version assembly The case for the desktop version is built using a length of 800 x 90 x January 2012  37 Building The Pocket Version 90 0 700 800 60 650 the cut-out and you will need to remove material from both the top (mostly) and bottom sections. A slot is also required in the bottom section for the tuning thumbwheel. The bottom of this slot is flush with the inside base of the case and is 4mm high x 29mm wide, centred on VC1’s tuning shaft. Fig.8 shows the thumbwheel dial label. Print it out and carefully trim it to size before attaching it to the plastic thumbwheel. It must be affixed to the top of the thumbwheel and must be orientated correctly so that the full range of dial markings will be available over the 180° tuning range. The pocket version assembly can now be completed by slipping the PCB into the case and securing it to the base of the case using four No.4 x 6mm self tapping screws. These go into matching integral mounting pillars in the case. You will also have to fit the battery snap connector (see text) and the front panel label (Fig.9). 0 Preparing the case that’s used to house the pocket version mainly involves drilling its end panel, to provide clearance holes for VR1, CON2, LED1 and power switch S1. The control panel label shown as Fig.7 indicates the drilling details and can be downloaded as a PDF file from the SILICON CHIP website. Print the label out, trim off the hole size markings and attach it to the end panel using double-sided adhesive tape. Alternatively, you can print the label onto adhesive-backed photo-paper and attach it directly to the panel. The holes can then be drilled to the sizes indicated. Use a 1mm pilot drill to start each hole, to ensure accuracy. In addition, you will have to mark out and cut a hole in one side of the case for the DC connector. You can determine the location of this circular cut-out by temporarily positioning the PCB in the case. A rat-tile file is then be used to make 00 10 0 55 Power Phones Volume 5mm 3mm 6mm 7mm 0 130 1600 530 This edge view shows the slot for the tuning thumbwheel and the hole for the DC socket. 19mm dressed pine. This is cut into two 238mm and two 120mm lengths and the pieces glued together using butt joints to make a frame (see photo). A 200 x 120mm aluminium sheet (1mm thick) is used for the front panel. 38  Silicon Chip . 9-12VDC + AM Radio SILICON CHIP   Above: this is the view inside the completed pocket version but without the battery snap fitted. Note the corner cutouts in the PCB at the top, to clear the case pillars.  Fig.7 (above) shows the drilling template and control panel for the pocket version while Fig.8 at right is the dial label for the thumbwheel that’s supplied with VC1. Fig.9: this is the full-size front panel label for the pocket version. This panel is recessed by 3mm into the timber frame and attached by gluing its inside corners to cleats located at each corner. Before attaching the aluminium panel, you have to drill the holes for a loudspeaker grille, plus holes for the power switch, LED indicator, headphone socket and volume pot. Fig.4 shows the drilling template for the loudspeaker grille, while Fig.6 shows the front-panel label/drilling template siliconchip.com.au The large tuning knob used in the desktop version previously served as the lid of a fruit container. It has two timber strips glued to its inside base and the thumbwheel supplied with VC1 is glued to these strips as shown at left. (also available for download from the SILICON CHIP website). Attach this template to the panel using double-sided tape, with its bottom edge aligned with the bottom of the panel, then drill the holes to the sizes indicated. Variable capacitor VC1 is also mount­ed on the aluminium panel. It’s just a matter of positioning it so that the 84mm-diameter tuning wheel that’s used is clear of the controls and the speaker grille. You will have to drill and ream a 7mm clearance hole for VC1’s shaft plus two 2.5mm holes to accept its mounting screws. Once all the holes have been drilled, glue the aluminium front panel to the cleats, then attach the PCB assembly to the panel and do up the nuts for VR1, CON2 and S1. The mounting holes for the rear of the PCB can then be marked on the wooden base (using the PCB mounting holes as a guide). Carefully measure the locations of these holes, siliconchip.com.au then mark corresponding locations on the outside (bottom) of the case. Before drilling these holes, remove the PCB assembly to avoid accidental damage. Once it’s out, drill two 3mm holes right through the base at the marked locations and countersink these holes by 2mm using an oversize drill – just enough so that the heads of 3mm machine screws fit inside and do not protrude below the surface of the timber. That done, the PCB assembly is refitted to the front panel and M3 x 6mm tapped Nylon spacers attached to its rear mounting holes using M3 x 5mm screws. These spacers are then secured to the timber base using M3 x 20mm machine screws fed up through the countersink holes. If the top and bottom screws “collide” inside the spacers, fit Nylon or fibre washers under the top screw heads. Alternatively, if the countersinking is too deep, you can fit washers under the bottom screw heads (or you can shorten the 20mm screws). Tuning capacitor VC1 can now be secured to the front panel using the two M2.5 x 3mm machine screws supplied. It’s then fitted with its tuning wheel. For our prototype, we used an 84mm-diameter tuning wheel which previously served as the lid of a plastic fruit container. The small thumbwheel supplied with VC1 is attached to the inside of this lid by first gluing two parallel 4mm-high x 6mm-wide timber strips either side of centre and then gluing the thumbwheel to these using silicone adhesive, as detailed below. Centring the thumbwheel It’s vital to correctly centre the thumbwheel inside the lid. This is done by first drilling a small pilot hole through the centre of the lid, then enlarging this hole to about 4mm using a tapered reamer. It’s then just a matter of visually lining up the centre of the thumbwheel with this hole when the thumbwheel is glued in place. Be sure to attach the thumbwheel with its collar facing outwards. January 2012  39 This is the view inside the completed desktop version. The rear of the PCB rests on M3 x 6mm tapped spacers which are secured using machine screws. You can either use silicone to secure the aluminium panel to the internal cleats or you can drill holes at the corners and fasten the panel to the cleats using small wood screws. You should now wait 24 hours for the silicone to set before attaching the tuning wheel to VC1’s shaft. The centre hole through the lid provides access to the thumbwheel screw. Dial label Fig.5 shows the dial label and this is also available in PDF format from our website. Before affixing it to the lid, rotate the tuning wheel to its centre position. The dial label can then be glued in place with the “kHz” marking at the bottom. A sharp hobby knife can be used to cut out the centre hole to provide access to the thumbwheel screw should this later become necessary. Final wiring The loudspeaker can now be fitted and the wiring run to it and to tuning capacitor VC1. In our case, we used a smear of silicone sealant at each corner to secure the speaker to the rear of the aluminium panel. Alternatively, you could drill mount40  Silicon Chip ing holes through the panel and secure the speaker using M4 x 10mm machine screws, washers and nuts. You will need to connect the two leads from the PC stakes at the front of the PCB to the speaker. Another two leads run from the PCB to VC1. Note that the centre terminal of VC1 must go the ground connection (ie, the centre terminal for VC1 on the PCB). Finally, the battery clip holder can be secured to the base using a wood screw. It’s optional, however – leave it out if you intend to only power the unit from a plugpack supply. Testing To test the unit, apply power and check that LED1 lights when S1 is switched on. If it doesn’t, check that the supply leads are the correct way around and that diode D1 and LED1 are orientated correctly. Check also that Q1’s emitter is at about 1.4V. If everything is correct, monitor the output (ie, via headphones or the loudspeaker) and tune in a station. When you find one, adjust trimpot VR2 for best sound quality (ie, for minimum distortion and noise). This trimpot sets the operating voltage at IC1’s input so that it operates correctly, without highfrequency oscillation or distortion as can occur if VR2 is adjusted too far clockwise. On the other hand, adjusting VR2 too far anticlockwise can result in excess noise. The next step is to make some simple alignment adjustments, so that the receiver covers the correct tuning range. First, if there’s a local station at the low-frequency end of the dial (ie, close to 530kHz), check if the station can be tuned in. If it cannot, it will be necessary to adjust the set to give a lower minimum tuning frequency and that’s done by sliding the coil towards the middle of the ferrite core. Alternatively, to obtain a higher minimum frequency (eg, if stations close to 530kHz are coming in too early in the band), slide the coil towards the end of the ferrite rod. The waxed siliconchip.com.au Parts List 1 PCB, code 06101121, 64 x 86mm 1 9V battery 1 9V battery clip lead 1 miniature PCB-mount SPDT toggle switch (S1) (Altronics S1421 or equivalent) 1 10kΩ log potentiometer, 9mm square, PCB-mount (VR1) 1 100kΩ horizontal miniature trimpot (VR2) 1 knob to suit volume pot. 1 switched 2.5mm PCB-mount DC socket (CON1) 1 PCB-mount 3.5mm stereo socket (CON2) 1 DIP8 IC socket (optional) 1 tuning coil with ferrite rod (L1) (Jaycar LF1020) 1 tuning capacitor 60-160pF (VC1) (Jaycar RV5728) 2 100mm cable ties 8 PC stakes Semiconductors 1 MK484 single chip AM radio (IC1) (Jaycar ZK-8828) OR 1 TA7642 single chip AM radio (IC1) (Wiltronics X-TA7642) 1 LM386N amplifier (IC2) 1 BC547 NPN transistor (Q1) 1 3mm high-brightness red LED (LED1) 2 5mm IR LEDs (LED2,LED3) 1 4.7V 1W zener diode (ZD1) 1 1N5819 1A Schottky diode (D1) 1 1N4148 diode (D2) Capacitors 2 470µF 16V PC electrolytic 2 100µF 16V PC electrolytic 2 10µF 16V PC electrolytic 3 100nF MKT polyester paper end of the coil former may need to be trimmed if the coil needs to be positioned slightly past the end of the ferrite rod but be careful not to cut the wires. Now tune to a station at around 1600kHz (if possible). The upper tuning frequency can then be adjusted using the padder capacitor adjustment screw at the rear of VC1 (the one closest to its output pins). If you don’t have stations available at the two frequency extremes (or siliconchip.com.au 1 47nF MKT polyester 1 18nF MKT polyester 1 10nF MKT polyester 1 470pF ceramic Resistors (0.25W, 1%) 1 100kΩ 1 2.2kΩ 1 27kΩ 2 1kΩ 1 6.8kΩ 2 10Ω Extra Parts For Desktop Version 1 aluminium panel 200 x 120 x 1mm 1 90 x 19 x 800mm length of timber (pine or similar) 1 100mm 4Ω loudspeaker 1 84mm diameter tuning dial (eg, the lid from a Goulburn Valley sliced peach plastic fruit container) 1 dial label, 71mm diameter 1 9V battery clip 1 wood screw to secure battery holder 2 M3 x 5mm screws 2 M3 x 20mm screws 2 M3 x 6mm tapped standoffs 1 100mm length of green lightduty hook-up wire 1 100mm length of white lightduty hook-up wire 1 200mm length of black lightduty hook-up wire Extra Parts For Pocket Version 1 remote control case 135 x 70 x 24mm (Jaycar HB5610 or equivalent) 1 front panel label, 50 x 114mm 2 2.5mm spacers (eg TO-220 insulating bushes) 2 M2.5 x 6mm screws 4 M3 x 6mm screws or No.4 x 6mm self-tapping screws close to them), then adjust the ferrite rod coil and padder screw so that the stations tune in at the indicated positions on the dial. It’s just a matter of adjusting the coil for stations at the low-frequency end of the dial and the padder screw for stations at the high-frequency end until the best compromise is achieved. Finally, for a full list of AM broadcast stations in Australia see: http:// en.wikipedia.org/wiki/List_of_raSC dio_stations_in_Australia Est.1978 Wide beam - 60° Long life - 35,000 hours Cool operation Cool, natural & warm white colours Dimmmable 2 year conditional warranty th 5 Generation MR16 LED Replacements Dimmable using iron cored transformer and a Clipsal Universal dimmer. Will operate with most electronic transformers (non dimming). 5W (Dimmable) - 400 lumens $24.00 (10+) $22.00 7W (Dimmable) - 460 lumens (1+) $27.00 (10+) $25.00 9W (Dimmable) - 630 lumens (1+) $32.00 (10+) $30.00 (1+) 5th Generation GU10 LED Replacements Dimmable using Clipsal Universal, Trailing & Leading Edge dimmers. 5W - 400 lumens $25.00 (10+) $23.00 5W (Dimmable) - 400 lumens (1+) $28.00 (10+) $26.00 7W - 460 lumens (1+) $29.00 (10+) $27.00 7W (Dimmable) - 460 lumens (1+) $31.00 (10+) $29.00 9W - 630 lumens (1+) $33.00 (10+) $31.00 9W (Dimmable) - 630 lumens (1+) $35.00 (10+) $33.00 (1+) Prices inc GST, valid until 31/01/12 Queensland Bowen Hills Southport Ph: (07) 3252 7466 Ph: (07) 5531 2599 New South Wales Homebush Ph: (02) 9704 9000 www.prime-electronics.com.au January 2012  41