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A long-wave AM receiver for aircraft weather information This simple receiver uses two ICs & will pick up airport weather beacons in the LW band. Use it to receive up-to-the-minute weather reports. It runs off a 9V battery & is easy to build. By DARREN YATES If you’re interested in aircraft or flying then you’ll no doubt already have a receiver that taps into the airport frequen­cies around the country. You can always pick up interesting information, particularly during aircraft emergencies and bush­fires. Information on the strength and direction of 54  Silicon Chip the recent NSW fires was broadcast by pilots back to ground bases and to nearby airports on this band. However, there is another band which gives up-to-the-minute weather and visibility information and these stations can be found on the LW (long-wave) band and are known as Weather Beacons. They often contain Morse code information along with a recorded message about the current temperature, visibility and cloud levels and current usage of a particular airport. The weather beacon at Sydney International Airport even has a computerised voice broadcasting this information. This band is located below the AM broadcast band and ex­ t ends from around 190kHz up to 450kHz. This low-cost long-wave receiver is designed to pick up this band and the bottom of the AM broadcast band, enabling it to pick up Sydney’s ABC Radio National station at 576kHz. Its frequency coverage extends from below 200kHz up to 580kHz. It uses D1 1N4004 Q1 BC548 E C 10 16VW 100 16VW 1.5k B S1 9V A LED1 RED 470  IC1 ZN414,YS414 IN COM L1 220pF 1 VOLUME VR2 10k RF GAIN VR1 10k .033 100k 0.1 100k K 1 63VW OUT  3 8 IC2a LM358 100k Q2 BC337 B 5 1 2 6 IC2b E B Q3 BC327 1k L1 = 200T, 0.2mm DIA ENCU WIRE WOUND ON A 10mm DIA FERRITE ROD 85mm LONG 1 WEATHER RADIO YS414 IN OUT ZN414 IN COM COM 100 16VW E 7 4 100k C 10  C 8 0.1 B OUT E C A K VIEWED FROM BELOW Fig.1: the circuit is essentially a TRF (Tuned Radio Frequency) design based on a ZN414 radio IC. This tunes over the long-wave band & feeds the recovered audio to VR2. The audio stages comprise IC2a & IC2b which drives a pair of complementary emitter followers (Q2 & Q3). just two low-cost ICs and three transistors. Sensitivity of the receiver is very good – Sydney’s beacon could be picked up without an external antenna from the author’s home in Penrith and Richmond Air Base was no problem at all. Circuit description Let’s take a look at the circuit of the Weather Radio, as shown in Fig.1. As you can see, it’s quite straightforward, using a ZN414 TRF receiver in the front end and an audio amplifier to drive the loudspeaker (TRF stands for Tuned Radio Frequency). Looking at the circuit, the ferrite rod antenna L1 and variable capacitor gang VC1 form a parallel resonant circuit which tunes the frequency of interest. The tuned frequency is fed to IC1, the ZN414 . This IC contains more than a dozen transis­tors which amplify and detect the RF and then amplify the recov­ered audio. The output appears across, and is filtered by, a .033µF capacitor. The 10kΩ pot VR1 applies DC via a 100kΩ resistor to the tuned circuit which enables the IC to vary the RF gain. IC1 only requires about 1.3V at very low current to work, so the 470Ω resistor provides the load for the circuit as well as supplying the power to it. Q1 and LED 1 form a simple voltage regulator which provides a constant 1.3V output across the 10µF capacitor to power IC1 (via the OUT pin). From the 470Ω resistor, the output signal is AC-coupled to the 10kΩ volume potentiometer, VR2, and then to the audio ampli­fier. This consists of an LM358 (or TL072) dual op amp and two complementary transistors. The first op amp (IC2a) is connected as an non-inverting amplifier with a gain of 101, as set by the 100kΩ feedback resistor from pin 1 to pin 2. Pin 1 of IC2a then drives IC2b. This op amp drives complementary transistors Q2 and Q3 directly and they operate in class B mode, without any quies­cent current to minimise crossover distortion. However, the resulting harmonic distortion is low since the transistors are included in the feedback network PARTS LIST 1 PC board, code 06107941, 102 x 44mm 1 57mm 8Ω loudspeaker 2 10kΩ log potentiometers (VR1,VR2) 1 60-160pF tuning gang (VC1) 1 85mm length of ferrite rod 6 metres of 0.2mm enamelled copper wire 3 knobs 4 100mm plastic cable ties 1 plastic utility case, 158 x 99 x 53mm 1 front panel artwork 1 miniature SPDT switch (S1) 1 9V battery snap connector 1 9V alkaline battery Semiconductors 1 ZN414 TRF radio (IC1) 1 LM358, TL072 op amp (IC2) 1 BC548 NPN transistor (Q1) 1 BC337 NPN transistor (Q2) 1 BC327 PNP transistor (Q3) 1 5mm red light emitting diode (LED1) 1 1N4004 rectifier diode (D1) Capacitors 2 100µF 16VW electrolytic 1 10µF 16VW electrolytic 3 1µF 63VW electrolytic 2 0.1µF 63VW MKT polyester 1 .033µF 63VW MKT polyester Resistors (1%, 0.25W) 4 100kΩ 1 470Ω 1 1.5kΩ 1 10Ω 1 1kΩ Miscellaneous Screws, nuts, washers, solder. August 1994  55 10uF .033 IC1 100uF Q2 1uF VC1 4 5 6 7 8 9 1uF 1 100k 7 D1 IC2 LM358 8 100uF VOLUME VR2 9 3 Q3 100k 1k 0.1 4 10  1.5k 100k 1 2 100k 470  Q1 RF LEVEL VR1 5 0.1 6 1uF 1 2 3 BATTERY NEGATIVE LED1 SPEAKER BATTERY POSITIVE L1 Fig.2: install the parts on the PC board as shown here & take care with IC1 as it looks identical to a TO-92 transistor. VC1 has its two sections connected in parallel to give a range of 0-220pF, while the leads to the ferrite rod antenna must be kept well away from the loudspeaker & the rest of the circuit. of the op amp and since the overall gain of this stage is a minimum; ie, 100% negative feed­back and therefore, unity gain. To maintain high frequency stability in the complementary emitter follower output stage (comprising transistors Q2 & Q3), a Zobel network con­ sisting of a 10Ω resistor and a 0.1µF capacitor is connected across the loudspeaker. Power is supplied from a 9V battery with diode D1 providing reverse polarity protection. Note that since IC1 requires only a low voltage and because the supply voltage to the audio amplifier is not critical, you could easily run the circuit from a 6V supply. However, to do this you would need more space to fit the batteries into the case. Construction Most of the components for the Weather Radio are installed on a PC board coded 06107941 and measuring 102 x 44mm. This is then mounted inside a standard plastic case measuring 158 x 99 x 53mm. The PC board is mounted on one side of the case (behind the front panel), as shown in the photographs. Before you begin any soldering, check the board thoroughly for any shorts or breaks in the copper tracks. These should be repaired with a small artwork knife or a touch of the soldering iron where appropriate. When you’re happy that everything appears OK, you can solder in the resistors and diodes, followed by the capacitors. This done, install the transistors and IC1 and IC2. Note that IC1 has three leads and looks identical to the transistors, so check this component carefully when installing it on the board. Tuning gang Fig.3: this is the full-size etching pattern for the PC board. 56  Silicon Chip The tuning gang is a plastic dielectric type with two sections of 0-60pF and 0-160pF. These two sections are connected in parallel on the PC board to produce a variable capacitor of 0-220pF, as shown on the circuit of Fig.1. The capacitor is secured to the PC board with two 2.5mm screws and then its three tags are soldered to short lengths of tinned copper wire The ferrite rod antenna is attached to the rear of the case using plastic cable ties. Additional cable ties should be used to lace the wiring to the pots, switch, loudspeaker & LED to maintain a tidy appearance & to prevent tuning drift. which are passed through the associated holes in the board and soldered in place. Drilling the case Before you go much further, you will need to do some work on the plastic case. The board is mounted on one side of the case, as mentioned above, along with the RF Gain and Volume control potentiometers (VR1 & VR2). The loudspeaker is mounted in the bottom of the case and when the lid is attached, the whole assembly is turned upside down so that the loudspeaker faces up. You will need to drill holes for the loudspeaker grille, the PC mounting pillars, the knob shafts, power switch and the LED. The latter two items are mounted on one end. Perhaps the easiest approach to drilling the case is to use the front panel artwork (included with this article) as a template. You will need to drill a circular pattern of holes for the loudspeaker and four holes to mount the ferrite rod antenna which we will now discuss. The tuning coil L1 is wound as a single layer of 200 turns of 0.2mm enamelled copper wire on an 85mm length of ferrite rod 10mm in diameter. If you need to cut the rod to this length, the way to do it is as follows. File a nick around the rod at the point you wish to cut it and then snap it off. If you try cutting it in any other way it is sure to shatter. Start off by winding one turn around the rod about 13mm from one end and anchor it with some sticky tape. This done, continue by winding on the 200 turns. It doesn’t need to be exactly 200 turns so if you’re out by a few turns either way, it won’t matter too much. RESISTOR COLOUR CODES ❏ No. ❏  4 ❏  1 ❏  1 ❏  1 ❏  1 58  Silicon Chip Value 100kΩ 1.5kΩ 1kΩ 470Ω 10Ω 4-Band Code (1%) brown black yellow brown brown green red brown brown black red brown yellow violet brown brown brown black black brown 5-Band Code (1%) brown black black orange brown brown green black brown brown brown black black brown brown yellow violet black black brown brown black black gold brown This close-up view shows the PC board after all the parts have been installed & the wiring completed. Note that shielded audio cable must be used for all connections between the pots (VR1 & VR2) & the PC board. The loudspeaker can be secured inside the case using super-glue. Make sure that the turns are tight and close to each other. Once you’ve wound the turns, anchor the other end with some more sticky tape and then carefully cover the whole winding with tape. This done, strip the enamel from both ends of the coil and tin them with solder. The rod is attached to the side of the case opposite the PC board, as shown in the photos, and is secured using two plastic cable ties. Before the PC board can be mounted in the case, you will need to fit a suitable shaft to the tuning gang, to enable a knob to be fitted. We did this using a fairly crude but effective method – super-glue. First, we roughened the end of the tuning gang shaft with a file and did the same to a 15mm long tapped metal spacer. A dab of super-glue was then applied to the tuning shaft and the two Fig.4: this fullsize front-panel artwork can be used as a drilling template for the various controls & the indicator LED. were butted together and then put to one side to allow the glue to dry. This method works sur­prisingly well. Mount the PC board using metal pillars, screws, nuts and lockwashers. Finally, wire in the two pots and the speaker and then fit knobs to the shafts of the pots and the tuning gang. The LED and speaker can be mounted (permanently) with super-glue, while the battery can be held in place using double-sided sticky tape. Alternatively, you can make up a metal bracket to hold the battery in position. Testing Before testing the receiver, check your work thoroughly for any possible errors. Once everything is correct, connect your multimeter across the On/ Off switch. This places the multimeter in series with the supply to allow you to measure the current drain. Select a low current range (200mA) on the multimeter, then connect the battery and check the current reading. Depending on where the tuning gang is sitting, and with the volume control well down, you should hear some low-level static coming through the speaker. The current consumption should be about 10mA. Any more than 20mA and you should switch off imme­ diately and check for errors. Now advance the volume control to about half way and then advance the RF gain control as well. The static should rise markedly and the loudspeaker may even squeal, depending on the setting of the tuning gang. If everything is OK, you should be able to tune across the long-wave band and pick up one or two low frequency AM broadcast stations as well. The weather beacons will be below these. Adjust the RF gain control to increase the signal level until the circuit starts to oscillate (squeal) and then wind it back a little. Next increase the volume until it is at a comfortable level. SC RF gain 190 580 Weather Radio Volume Tuning Frequency (kHz) August 1994  59