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An FM transmitter for musicians These three FM transmitters all use the same circuit but you can vary the construction to suit your application. One unit is configured as a guitar FM transmitter, one as a handheld wireless microphone and the other for use with a lapel micro­phone. Design by BRANCO JUSTIC This circuit is not new, having been featured previously in the October 1993 issue of SILICON CHIP. However, there have been a couple of minor circuit modifications to tweak the performance, while the PC board has been redesigned to make the unit easier to build. 54  Silicon Chip In particular, the PC board is now single-sided, whereas the earlier Mk.1 version used a double-sided PC board that re­quired soldering on both sides. As with the previous design, the new “FM Microphone Mk.2” was designed by Oatley Electronics and they will be making avail­able a com- plete kit of parts. So which version should you order? Well, you don’t have to worry about that because the kit contains all the necessary parts for each version. It even includes three labels; you simply use the one that’s right for your application. The differences between the three units are really quite minor. In fact, the two wireless microphone versions are identi­cal except that the handheld unit has the microphone attached to the case while the second unit has the microphone attached to a lapel clip. Fairly obviously, the microphone is left out of circuit for the guitar FM transmitter unit and the guitar provides the input signals instead. Physically, all three versions are housed in the same plas­ tic case, which measures 125 x 40 x 24mm Fig.1: the circuit is based on three transistors (Q1, Q2 & Q3). Q1 functions as an audio preamplifier, while Q2 & Q3 form a modulated oscillator with good isolation between the antenna and the tank circuit (L1 and its parallel 1pF capacitor). (length x width x depth). As shown in one of the photos, a standard 9V alkaline battery sits at one end of the case, while the PC board occupies the other end. A miniature slide-switch on the side of the case switches the power on and off. Performance As before, this FM transmitter design features excellent frequency stability. Some FM wireless microphones can be tempera­mental devices to use, particularly as far as frequency drift is concerned and there are several causes for this. The first of these is due to a drop in the supply voltage as the battery ages. The second is due to capacitance effects between the user’s body and the dangling antenna. Third, and not usually recognised, is drift due to change in temperature. When you set up an FM wireless microphone to operate at a particular frequency, say 95MHz, you don’t expect it to drift much. If it only drifts by a small amount, the AFC (automatic frequency control) circuits of your FM tuner should cope with the change in frequency so that the signal is always received clear­ly. But there is a limit to the AFC range of any FM tuner (per­ haps ±100kHz) and beyond that, the signal will start to distort badly and ultimately, will not be received at all. That is why drift caused by body capacitance can be so annoying as it varies all over the place. This design does not have these problems. We tested it in a number of ways, including heating up the PC board with a hot air gun and even then, drift was not a problem. Nor do power supply variations worry it. In fact, drift due to supply voltage varia­ tions of 1V for a 9V supply is quoted as less than 0.03%. The operating range is quoted as better than 100 metres with a good quality tuner. Other relevant specifications are: signal-to-noise ratio >60dB; pre-emphasis 50ms; frequency re­sponse 40Hz to 15kHz. Circuit details Fig.1 shows the circuit which uses three NPN transistors. Transistor Q1 (BC549) is an NPN audio preamplifier stage which steps up the input signal from the electret microphone or from the guitar. The output from Q1 (at the collector) is then coupled via a 0.1µF capacitor and an 8.2kΩ resistor to the Parts List 1 PC board (available from Oatley Electronics) 1 electret microphone insert 1 9V alkaline battery 1 9V battery snap 1 subminiature former with core, can and base (L1) 1 SPST miniature slide switch (S1) 1 50kΩ trimpot (VR1) Semiconductors 1 BC549 NPN transistor (Q1) 2 BF199 NPN transistors (Q2,Q3) Capacitors 4 0.1µF monolithic 1 0.047µF monolithic 2 100pF ceramic 1 33pF ceramic 1 22pF ceramic 2 15pF ceramic 1 1pF ceramic (see text) Resistors (0.25W, 1%) 1 220kΩ 1 8.2kΩ 1 100kΩ 1 6.8kΩ 2 22kΩ 1 1kΩ 1 12kΩ 1 680Ω 1 10kΩ 1 270Ω Kit Availability This FM wireless microphone/ transmitter has been produced by Oatley Electronics who own the design copyright. They can supply a complete kit of parts, as follows: PC board, all on-board parts, a uni­ directional microphone with clip, a surplus plastic case, slide switch, battery clip and stickers. The price is $17.00 plus $5.00 for postage & packing. The company’s ad­ dress is PO Box 89, Oatley, NSW 2223. Phone (02) 9584 3563 or fax (02) 9584 3561. November 1998  55 This view shows how the parts are installed inside the plas­tic case. This is the handheld microphone version, which has the microphone attached to one end of the case. base of Q3 which is the lower half of a cascode oscillator circuit. The cascode configuration, involving Q2 & Q3, is the secret of the circuit’s excellent rejection of body capacitance effects on the operating frequency. The operating frequency is set by a parallel LC network comprising the 1pF capacitor and adjustable coil L1 at the base of Q3. By virtue of the cascode configuration, the components which set the operating frequency are well and truly isolated from the antenna which is connected to the collector of Q2. L1 allows the operating frequency to be set to a vacant spot on the FM broadcast band (88-108MHz). Building it Fig.2 shows the assembly details for the PC board. The assembly is a pretty straightforward process; it’s simply a matter of inserting and soldering each component in turn. The most important point to remember is to keep all the component leads to an absolute minimum length. Because the circuit operates in the FM broadcast band, even short lead lengths have signifi­cant inductance and this can prejudice the performance. If you intend building either of the FM microphone ver­sions, leave trim­ pot VR1 out. Conversely, for the guitar trans­mitter version, include VR1 but delete the 22kΩ resistor on its wiper and delete the microphone. Note that all the resistors are soldered “end-on” to save space on the PC board. The length of the antenna wire is up to you. You can have it short and unobtrusive or long and thereby obtain better range. We suggest a length of about 65-90cm for good range; any longer and the range will be reduced. Once all the parts have been soldered to the board, you are ready to test it and set the operating frequency. For this you need an FM radio. Connect the 9V battery and turn on your FM radio. Now tune across the band until the speaker squeals. The frequency on your dial is now the operating frequency of the circuit. If you want to adjust the frequency of operation, you re­verse the above process. Tune your radio to a vacant part of the band – let’s say this frequency is 99MHz. All you should be get­ting is hiss from the loudspeaker of the radio. Now adjust the slug of coil L1 until you get a continuous squeal from the radio. That’s it, the job is complete. In more detail, the tuning range of Resistor Colour Codes            No. 1 1 2 1 1 1 1 1 1 1 56  Silicon Chip Value 220kΩ 100kΩ 22kΩ 12kΩ 10kΩ 8.2kΩ 6.8kΩ 1kΩ 680Ω 270Ω 4-Band Code (1%) red red yellow brown brown black yellow brown red red orange brown brown red orange brown brown black orange brown grey red red brown blue grey red brown brown black red brown blue grey brown brown red violet brown brown 5-Band Code (1%) red red black orange brown brown black black orange brown red red black red brown brown red black red brown brown black black red brown grey red black brown brown blue grey black brown brown brown black black brown brown blue grey black black brown red violet black black brown SMART® FASTCHARGERS One charger for all your Nicad & NiMH batteries As featured in ‘Silicon Chip’ Jan. ’96 Fig.2: keep all leads as short as possible when installing the parts on the PC board. Note particularly that Q1 is a BC549 type and is different to Q2 and Q3 which are both BF199s. the wireless microphone can be adjusted upwards by removing the 1pF capacitor. With this capacitor in circuit, the tuning range of L1 will be in the lower region of the FM band: from below 88MHz to about 102MHz. With the 1pF capacitor in circuit, the tuning range will be from about 95MHz. You have to decide which portion of the band you want your circuit to operate in and then pull the capacitor out or leave it in. You then adjust the slug of L1 as described above. After you have adjusted coil L1 to your satisfaction, move the microphone well away from the radio so that the acoustic feedback squeal and distortion is no longer apparent. You should now be able to speak into the microphone and your voice should come from the radio with clean reproduction. You can now complete the construction by wiring up the on-off switch and then installing the board and battery inside the plastic case – see photos. The PC board is positioned at the top of the case and is secured using silicone sealant. You will have to drill a hole in the end of the case to accept the lead for the guitar or microphone (lapel version). If you intend building the handheld microphone version, you will need to mount the microphone on the end of the case before installing the PC board. This will involve drilling a small pilot hole (with the two halves of the case attached to­gether) and then carefully reaming the hole to size. Once this has been done, separate the two halves of the case and secure the plastic base of the microphone to the bottom half only using silicone Capacitor Codes  Value IEC Code EIA Code  0.1µF 100n 104  .047µF   47n 473  100pF 100p 101  1 33pF   33p   33  22pF   22p   22  15pF   15p   15  1pF  1p0   1 Designed for maximum battery capacity and longest battery life Charge: Power tools  Torches  Radio equipment  Mobile phones  Video cameras  Radio controlled models  Field test instruments  Lap-top computers  Toys  Dust busters  Others  The REFLEX® charger is powered from a Power Supply (optional) or from 12 or 24V batteries. AVOIDS THE WELL KNOWN MEMORY EFFECT. SAVES MONEY and TIME. Restore Nicads with memory effect to remaining capacity and rejuvenate many 0V worn-out Nicads. CHARGES VERY FAST plus ELIMINATES THE NEED TO DISCHARGE: charge standard batteries in max. 1 hour and the ‘fastcharge’ batteries in max. 15 min. Partially emptied batteries are just topped up. Batteries always remain cool, increasing both the total battery life and the useful discharge time. DESIGNED AND MADE IN AUSTRALIA For a FREE detailed technical description please Ph: (03) 6492 1368 or Fax: (03) 6492 1329 2567 Wilmot Rd, Devonport, TAS 7310 sealant. The PC board can then be installed, as before. You will also have to drill a small hole at the other end of the case for the antenna lead. Drill this hole in the bottom half of the case only, then make a small cutout for the slider of the plastic switch. This cutout can be made by drilling some small holes and then filing the hole to shape. The switch is also secured to the side of the case using epoxy resin. Be careful not to get any of the epoxy inside the switch – it will be ruined if you do. Also supplied with the kit is a small piece of 3mm-thick foam rubber. This should be attached to the bottom of the case at the end opposite the PC board. It’s there to stop the battery from rattling around. Once everything is in place, the two halves of the case can be secured together using the supplied self-tapping screw. This is installed from the back and screws into a central boss in the top half of the case. Finally, fit the appropriate front panel label and SC the job is complete. November 1998  57