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Quality Stereo HOONUER PROJECT! Wireless Microphone or Audio Link By Ross Tester Here’s an easy-to-build stereo wireless audio link or microphone that offers long range and is very easy to build, as it’s based on a pre-assembled transmitter module. You can receive it on any FM broadcast band receiver. I count mic differences or if you require are supplied in the kit. These can t’s certainly not the first wireless different levels in each channel. be soldered direct to the PC board to microphone we’ve ever published By the way, the transmitter module make it a fully self-contained project but this one is a little different. is quite capable of operating at line or they can be attached to the board via First of all it’s stereo, providing suprislevel if you want just a line level transsuitable lengths of mini shielded coax ingly good quality sound. mitter (eg, to feed an audio program cable. A third option is to use “proper” Second, it has a really good range. around your home). Sensitivity is microphones – they can be electret or We tested it at well over 50m and it was about 100mV. Oatley Electronics, who dynamic types – but no provision has still performing very well – noise-free, designed the kit, have the transmitter been made for plugging these in. in fact – but at the time we couldn’t module available by itself if that’s what get our receiver any further away. So Construction you’re after. But more on that anon. it’s likely to have even better range You also have the choice of two Start with the smallest components than that. – eight resistors and four capaciThird, it really is simple to build tors, ensuring that the electro– the hard work (the transmitter lytic capacitors are the right way module) is already done for you. around (+ towards edge of PC It’s just a matter of assembling the . board). microphone module, which conHz 5k -1 Hz 20 .. ... ... ...... Audio response: ..... The two electret bias resistors tains the electret mics themselves, ......................... 40dB n: tio ra pa (the 4.7kW resistors closest to the preamp and level controls, and solse l ne an Ch :.................0.1% on edge of the PC board), are only rti dering the transmitter module onto sto Di ic on rm Total Ha 7.2MHz required if electret microphones it, “piggy back” style. 10 76. 10 ... ... ... y:. Output Frequenc are used – they can be left out if Finally, the transmitter module is mS ........................... 50 you use dynamic microphones. crystal-locked, so you won’t have the Pre Emphasis:....... 15V range:................ 3ge lta By the way, keep the resistor drift probles of some earlier wireless vo ly pp su DC .. 30mA <at> 9V ... pigtails when you cut the off – microphones. And just in case you ... ... ... ... ... . :. nt rre Supply Cu you’ll need some of them in a were wondering, that doesn’t mean moment! the output is locked to one particular power supply levels – 3-6V or 7-15V We’ll assume you are going to make frequency – it has a nifty synthesis DC. The latter results in a lower current the module self-contained; ie, the circuit built in to give you the choice drain. The transmitter module also has tiny electret microphone inserts are of seven different frequencies between a “5V out” rail to supply power to the soldered to the PC board. 106.7MHz and 107.7MHz. preamp module. It does matter which way around On-board preset pots adjust the senTwo electret microphone inserts the inserts go – look closely at the sitivity of each channel to take into ac- ns: o Typical Specificati 34  Silicon Chip siliconchip.com.au It’s easy to build, requires very little setup . . . and it’s cheap! In fact, the low price might turn some people off, thinking it’s low quality. Try it – and be pleasantly surprised! two pins and you will see that one of them is connected to the insert body or case. This is the negative or earth pin and goes, in both cases, to the “–” mark on the PC board overlay (towards the middle of the board). If mounting the microphones remotely, ensure that the earth braid of the microphone cable connects to the “–” on the PC board and the negative pin of the microphone inserts. If using other microphones, you will probably want to connect a couple of 3.5mm sockets to the PC board so you can plug the microphones in at will – remember, once again, that polarity matters! And if you want to use the complete kit for line level (100mV or so) inputs, you can increase the 10kW resistors at the op amp inputs to 47kW and leave out the two 4.7kW resistors as above. The tiny pushbutton switch which selects frequency can be soldered in next. The pins are slightly offset so unless you really force it in, it cannot go in the wrong way. The only other passive components to mount are the two 1MW “gain” trimpots, VR1 and VR2. These may +5V 100 µF 4.7k* 100nF 10k LEFT IN 4 4.7k 4.7k 1 IC1a 3 4.7k* FREQUENCY SELECT IC1: M5223 100 µF 760mm INSULATED WIRE +5V OUT 8 2 LEFT CHANNEL IN ANTENNA VR1 1M 10k ANT VHF FM TRANSMITTER MODULE 3-6V IN FREQ SEL 7-15V IN S1 3-6V DC OR 7-15V DC 5 100nF RIGHT CHANNEL IN SC 2005 10k 6 IC1b 7 RIGHT IN GND 0V VR2 1M 10k FM STEREO TRANSMITTER * THESE RESISTORS ONLY NEEDED WHEN ELECTRET MICROPHONES BEING USED  CHANGE THESE RESISTORS TO 47k FOR LINE LEVEL INPUTS Fig.1: all you have to build is the preamp. The transmitter is pre-assembled and aligned. Easy! siliconchip.com.au May 2005  35 The transmitter module Based on a Rohm BH1417F chip, the internal workings of which are shown above, the transmitter module was originally manufactured for use in small audio wireless devices fitted into cigarette-lighter plugs in cars, hence the strange board shape. The module also contains a DC-DC converter to supply 5V which we tap into for the microphone/preamp circuit. The front side of the module is shown below (left) approximately same size, while the underside is shown below (right). As mentioned earlier, it is complete and ready to use in its own right – it will accept line level (300mV or more) input (from a CD player, for example) and needs only power and an antenna wire to operate. The module has been typeapproved for acceptance by the US FCC so would also pass the LIPD standards here in Australia. It is not only pre-assembled, it is pre-aligned and ready to rock. The only user setting is the transmitter frequency, which is set by a push-button (so there’s no fiddly tuning!). You have a choice of seven different frequencies 36  Silicon Chip between 106.7MHz and 107.7MHz (so there should be no collisions with local FM radio stations). The default (power up) frequency is 107.7MHz. Each time you push the button, you decrease the frequency 200kHz and the on-board LED flashes once to acknowledge. When you reach the lower limit (106.7MHz) the LED flashes three times. Incidentally, the BH1714F chip is the same as used in the SILICON CHIP Micromitter (mini stereo FM transmitter) published in December 2002. This module essentially does the same job as that project, at a lower cost. In this case, though, the module only offers seven frequencies whereas the Micromitter offered 14 (another seven between 87.7MHz and 88.9MHz). ( A full description of the operation of the BH1417F chip can be found in the December 2002 article. Copies of that magazine or a reprint of the article can be obtained from SILICON CHIP for $8.80 including GST and postage. Like that project, you can use any standard FM broadcast band receiver to pick up the signal from the transmitter. If it’s a stereo FM receiver, it will receive stereo. siliconchip.com.au FREQ SEL 3-6V IN LEFT GAIN G K222 FM TX RIGHT GAIN 100nF ANT +5V OUT PIN 1 VR2 10k 4.7k 4.7k 4.7k GND 7-15V IN IC1 M5223 100 µF + RIGHT CHANNEL MIC G VR1 1 3-6V IN FREQ SEL S1 1M G LEFT CHANNEL 100 µF MIC + 10k 4.7k 4.7k 4.7k 100nF 1M 7-15V IN R L +5V 760mm ANTENNA WIRE Fig.2: the component overlay shows just 16 components and a pre-built module to go onto the PC board. It shouldn’t take more than an hour or so to build. © oatleyelectronics.com Want cheap, really bright LEDs? We have the best value, brightest LEDs available in Australia! Check these out: Luxeon 1, 3 and 5 watt All colours available, with or without attached optics, as low as $10 each Low-cost 1 watt Like the Luxeons, but much lower cost. •Red, amber, green, blue and white: Just $6 each! Lumileds Superflux These are 7.6mm square and can be driven at up to 50mA continuously. •Red and amber: $2 each •Blue, green and cyan: $3 each Asian Superflux Same as above, but much lower cost. •Red and amber: Just 50 cents each! •Blue, green, aqua and white: $1 each. Go to www.ata.org.au or call us on (03)9419 2440. This photo is reproduced significantly larger than life size so you can see exactly where the components go and, where it is important, which way around. The red and black wires at top are for power, the black wire at right is the antenna. be supplied in the kit as horizontal or vertical-mounting types; either can be accommodated on the PC board. Now we come to the final “main” PC board component, the op amp IC. It may have a socket supplied – in which case, solder in the socket. The notch in the end faces away from the microphones. Don’t insert the IC yet. If you don’t have an IC socket, carefully solder the IC itself in – again, with the notch facing away from the microphones. The insulated power supply wires can now be soldered on. The black, or ground wire, is easy; it connects to the +5V OUT PIN 1 R siliconchip.com.au Connecting the transmitter module Before soldering in the transmitter module, an antenna needs to be connected to it. This should be a length of insulated hookup wire cut to exactly 760mm long. FREQ SEL 3-6V IN G 7-15V IN point marked GND on the PC board. The red, or positive, wire, connects to the appropriate position on the PC board for your supply: to the 3-6V point if you have a 3-6V supply or the 7-15V point if you have a 7-15V supply. (Don’t connect more than 6V to the 3-6V supply point!). L Fig.3: this diagram shows the seven connection points between the transmitter module (green, on top) and the main board (grey, underneath). Fig.4: this is how to bend the wire link between pin 1 of the IC and the 18mm main 6mm board +5V 1mm position. If you start with a 765mm length and cut off 5mm of insulation for soldering, you’ll be spot on. There are actually two positions marked “antenna” on the transmitter module. Use the one closest to the LED. Trim any excess wire from under the module. All connections between the two PC boards are made using resistor lead offcuts (commonly known as pigtails). Most are just short lengths (they end up around 4mm or so), soldering directly between the main PC board and the transmitter board above. However, one connection, the +5V connection, does not have any provision on the transmitter board and must be treated differently. Cut a 25mm length of pigtail and bend as shown in the diagram below right (Fig.4). The tiny bend end solders directly onto pin 1 of the IC nearest the antenna lead you just soldered in, as shown in the overlay diagram. This IC is a surface-mount type and therefore the pins are very close May 2005  37 Parts List – Stereo FM Microphone 1 mini transmitter module (Oatley) 1 main PC board, code K222, 67 x 40mm 1 length of insulated mini hookup wire, 765mm long (antenna) 2 lengths hookup wire, red and black, length to suit (power) 1 mini-DIP (8-pin) IC socket 1 mini pushbutton switch, SPST 2 electret microphone inserts (see text) Semiconductors 1 5223 dual op amp (IC1) Capacitors 2 100mF 16V PC electrolytics 2 100nF (0.1mF) polyester Resistors (1/4W, 1%) 6 4.7kW 2 10kW 2 1MW mini trimpots together. You’ll need a fine pointed, well-tinned iron and a good eye (or a magnifying glass). It’s easiest (and safest) to solder the link wire to the outside of pin 1. The other end of this wire goes over the edge of the transmitter board and solders to the +5V point of the main board – but this should be left until the other wires are soldered to their respective points on the transmitter board. Don’t try to cut the wires to short lengths yet – this can be done after soldering. In fact, it’s probably easiest if you use complete resistor lead offcuts – or if you don’t have enough offcuts, at least half length offcuts. It makes holding them (with a pair of fine pliers!) that much easier while soldering if they are longer. Apart from the +5V connection we talked about earlier, there are six connections between the two boards. First, solder these to the transmitter module board in the positions shown in the diagram, with the majority of the lead on the underside of the board. Now pass these six wires AND the end of the +5V connecting wire through their respective holes on the main board so that the two boards end up, say, 2-3mm apart (it’s not critical). In all cases except the +5V wire, the 38  Silicon Chip And finally, a view from the opposite (transmit module) end. You can clearly see that link between pin 1 of the IC and the +5V point on the main board. sets of holes are right above and below one another. Solder all seven wires to the underside of the main PC board, being careful not to unsolder them in the transmitter board above! When you are satisfied that your soldering is OK, carefully cut the six interconnecting wires above the transmitter board and below the main board so that all you are left with is short links between the two. Trim the +5V wire on the underside of the main board. Because the holes in the transmitter board are plated-through, if necessary you should be able to touch-up the soldering on that board, from above, when completed. Testing After giving the kit a complete visual inspection and satisfying yourself it’s all good, insert the op amp IC into its socket on the main board (if you haven’t soldered it in, of course) and wind the two preset pots down to minimum. Connect power and the LED should light. With a suitable FM radio, tune to near the top end of the band – at one point (actually 107.7MHz) you should find that the radio mutes or goes quiet. This is good – it means that you are “on the air”. Now slowly adjust each of the two pots up in turn. Before too long, you should hear any sound made in the room (click your fingers, for example) coming through the radio speakers. If you go too far, you’ll probably find that the sound becomes distorted. And if you go further, you’ll find that it goes into feedback and you’ll get a howl from the speakers – exactly the same as you will hear from any microphone and speaker that are too close together. When you are happy with the level, adjust the two pots so that they are close to equal, unless for some reason you need to have one channel higher than the other. Finally, remember that you can also use this project for an audio link, fed from any line level output. Simply remove the microphones and their associated 4.7kW resistors and increase SC the 10kW resistors to 47kW. Where From, How Much? This project design is copyright (C) Oatley Electronics 2005 and kits are only available from Oatley. The complete kit of parts (everything listed in the parts list including transmitter module) sells for $29.00 inc. GST, plus p&p. (Cat K222). The fully assembled and tested transmitter module only (ie, no main PC board nor components) is available separately from Oatley Electronics for $22.00 inc GST, plus p&p. Oatley Electronics may be contacted via their website (www.oatleye. com); by phone (02) 9584 3563; by fax (02) 9584 3561; or by mail to PO Box 89, Oatley, NSW 2223. Pack & post is typically $7.00 per order. siliconchip.com.au