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FM radio intercom for IDotorbikes This motorcycle intercom provides communications between rider and pillion passenger or between riders on separate bikes. Because it is a 2-way radio system, it allows you to communicate with more than one rider at a time and can even tune in your favourite FM station when not being used in the intercom role. By JOHN CLARKE Motorcycle intercom systems have always been popular but until recently, have been restricted for use between rider and pillion passenger only. Communication between separate bikes has been left to special sign language such as sticking both arms out to ask "shall 20 SILICON CHIP we turn left or right?" and other more subtle communication forms. These sign languages are often misinterpreted. The above rider with the outstretched arms was later to discover why he received no response - the following rider assumed he was just showing off. This problem can be overcome by using a radio intercom. These are available commercially but cost a fortune. With that in mind, we set about designing a unit that you can build yourself and save quite a lot of money. Before going further though, we should point out that this is not really a project for the beginner. There are a number of coils to be wound during construction and the PC board is fairly closely packed with parts, so you will need to have some experience with kit construction to successfully tackle this project. Also, depending on where the kitset suppliers have sourced their parts, one of the integrated circuits may be a surface mount type. These are a lot smaller than conventional ICs and so are harder to mount. It's not just a matter of assembling all the parts either. You must also be able to follow the alignment instructions. Have we put you off? We are sure we haven't. Main features The SILICON CHIP Radio Intercom has a number of features which are not normally found in intercom systems. Because it is a radio intercom, it can be used for communications between riders on separate motorcycles. And because the intercom transmits and receives on the · same frequency, any number of riders can listen in to a transmission at the same time (provided they also have intercoms or FM receivers). In use, the Radio Intercom normally operates in receive mode so that it can pick up any transmissions from other riders. But when you speak into the microphone, the intercom automatically switches to the transmit mode due to its voiceoperated switching (VOX) circuitry. This feature greatly increases safety and convenience since it eliminates the need for a PTT (press-to-talk) switch. As with a CB radio, only one person can transmit at any one time. If two people try to speak at the same time, nothing will be heard by either party since the receiver is switched off during transmission. This means that some form of agreed procedure must be followed when using these units. For example, most radio operators use the word "over" to indicate the end of transmission and that the other party can speak. Apart from that, the unit operates very much like a CB radio except that it operates in the commercial FM band (88 to 108MHz) and on one channel only. Noise cancelling mikes Because of the high ambient noise from a motorcycle, the SILICON CHIP intercom uses special noise cancelling microphone circuitry. This involves using two separate microphones which are connected so that noise signals are largely cancelled by the following circuitry. In practice, the microphones are set up so that the rider speaks only into one of them and this provides the signal to be transmitted. All the circuitry fits neatly into a compact plastic case from Dick Smith Electronics. When not used in the intercom role, the unit can be used to tune your favourite FM station. Because the intercom operates in the commercial FM band, it can also be used as an FM radio receiver. Tuning is by means of a small thumbwheel on the front panel. To use the unit as an intercom, the thumbwheel dial is simply set to the 88MHz position (at one end of the dial travel). A small slider switch adjacent to the thumbwheel selects between FM, Intercom and Off. We don't recommend that you try tuning the unit while the bike is in motion, however. That would be dangerous to say the least. The unit should be set for one mode or the other before starting off and should only be tuned when the bike is stopped. Handy case As can be seen from the photographs, all the electronic circuitry is housed in a small plastic case. This can be easily fitted into the pocket of a motorcycle jacket and the short wire lead antenna clipped to the jacket collar. A coiled cord connects the intercom to the helmet which is modified to accommodate a miniature loudspeaker and the two electret microphones. Power for the intercom is derived from three AA cells, thus making the unit completely portable. Other applications Just because we have described this project as a motorcyle intercom OCT0BER1989 21 FM RADIO INTERCOM - CTD does not mean that it is suitable only for this use. We invisage that it would also be suitable for use by hang glider pilots, power boat racers (for communication between the crew), rally car drivers, in helicopters and open-cockpit and ultra-light aircraft. How it works The Radio Intercom is built around four ICs, all of which are low voltage, low current devices suitable for battery operation. On the receive side, we used a Philips · TDA7000 IC which is virtually a complete FM radio receiver on a single chip. Its output drives a National Semiconductor LM831 power amplifier IC which in turn drives the loudspeakers (or headphones). The transmitter section is based on a Rohm BA1404 stereo transmitter IC (the same as used in our Stereo Minimitter project in the October 1988 issue). An LM324 quad op amp and three transistors are used for microphone amplification and for the voice operated switch (VOX). Fig.1 shows the circuit details. We'll start with the microphone amplifier and VOX switching which is built around IC1. Differential microphone amplifier Both voice and noise signals are picked up by two small electret microphones mounted inside the motorbike helmet. These are supplied with power from the + 4.5V rail via separate 1.2k0 resistors. The signal from each microphone is AC coupled to signal attenuators VR1 and VR2 and then fed to the non-inverting inputs of ICla and IClb. ICla functions as a non-inverting buffer amplifier with a gain of 1 + 1/47 and amplifies the signal from the wiper of VR2. The output appears at pin 7 and is coupled via a 1kn resistor to the inverting input (pin 2) of IC1 b. IClb is wired as a differential amplifier. It functions as an inverting amplifier with a gain of - 47 22 SILICON CHIP for signals from ICla and as a noninverting amplifier with a gain of 48 for signals from the wiper of VR1. Note, however, that the overall gain for signals from the wiper of VR2 is - 47(1 + 1/47) = - 48. This means that there will be no output from IC1 b when the two microphone signals are the same and thus noise signals common to both are cancelled out. On the other hand, speech signals will be amplified since the microphones in the helmet are arranged so that only one is near the rider's mouth. DC bias for ICla & IClb is derived via VR1 & VR2 from a 15k0/10k0 voltage divider across the supply rails. This voltage divider also provides DC bias for op amp stage IClc in the VOX circuit. The VOX circuit IClc is wired as a Schmitt trigger with its inverting input (pin 9) biased via a 47k0 resistor. A 3.3k0 resistor at the non-inverting input and a 220k0 positive feedback resistor set the hysteresis of the Schmitt trigger. This stage squares up the output from IC1 b and couples the resulting square wave signal to a charge pump circuit consisting of a 4. 7µF capacitor, diodes Dl and D2, a 47µF capacitor and a 4 .7k0 resistor. When a speech signal is received, the 47 µF capacitor is rapidly charged towards the + 4.5V supply rail. Following the charge pump circuit is another Schmitt trigger stage based on ICld. This stage compares the voltage across the 47µF capacitor with the voltage at its inverting input as set by the 100k0/10k0 voltage divider. The 150k0 feedback resistor and the 10k0 resistor in series with pin 12 set the hysteresis level of the Schmitt trigger. This hysteresis is necessary to ensure that pin 14 of ICld switches cleanly from one state to the other instead of dithering about at the threshold point. The output of ICld drives transistors Ql and Q2. When the output is high (ie, when speech is present), Ql and Q2 are turned on via 6.8k0 current limiting resistors. This does two things. First, when Ql turns on, it powers up the transmitter (IC2) by connecting the ground rail of this stage to the negative line from the battery. At the same time, when Q2 turns on, it turns off Q3 which then disables the receiver and audio amplifier (IC3 & IC4). Conversely, when the output of ICld is low, Ql and Q2 are off and the transmitter (IC2) is disabled. Thus, when no speech signal is present, Q3 turns on and powers up IC3 & IC4. The .OlµF capacitor between the collectors of Ql & Q3 provides RF grounding for the circuits at all times. Transmitter stage The output of the microphone differential amplifier !Cl b is coupled to the modulator input (pin 12) of IC2 via a voltage divider (5.6k0 & 4.7k0) and a O.lµF capacitor. The .001µF capacitor filters any RF on this input. IC2 is a Rohm BA1404 stereo FM transmitter IC but is used here in mono mode only. Because of this, only the RF section of the IC is used in this circuit. Those sections of the chip associated with stereo signal multiplexing are simply left unused. No 38kHz crystal is required. The circuit operates by mixing the audio signal on the modulation input (pin 12) with a local oscillator. The resulting RF output signal on pin 7 is then fed to a tuned filter stage and used to drive the antenna via balun L5. The oscillator output at pin 10 is tuned using L2 and a 39pF NPO capacitor. Similarly, the RF output at pin 7 is tuned using L1 and the series 56pF and 120pF NPO capacitors. These capacitors are connected in series so that an antenna can be connected at their junction without loading the tuned Fig.1 (right): the complete circuit ► diagram. ICla & IClb function as a differential amplifier for the microphone signals, while IClc, ICld & Ql-Q3 form a VOX circuit. IC2 is the transmitter stage, IC3 the FM receiver & IC4 the audio output stage. ~ t,.,') CX) '° '° ....... :::0 tri t:c 0 '-l 0 n 3 0 •• .0 1_;i 1k , _.j _ &M324 7 lk T1 • NPO * NPO • 56pF ■: VC1 20-60pFJ~ ELECTRET w ~ .,. ~ ECTRET MICS 100 + 47k~ r1 / ,c 10k 1 MUTING 2NO I 6 +---'tVCO 5 V+ * G,.:Jg 4 LOOP FILTER .0033 12 18 u ls ~ ls !;;; B MIXER GNO g •CERAMIC 16 GNO 15pF ---< 47k ■■ 0.1 •· * + 1 AF OUT MIXER RF IN 0 IF FILTER I~ .ot22 :,: 2 14 L4 13 4.71! 39 pF * NPO: = * 18k 47 F N~O == rgt VR3 ■■ * ~-~ .t1: = K VOLUME ,_ .0039 == .,__ I I I II 11 •~ii ,1 ., :: ••~If +2. 5V * :· " + .,. - - .,. ',:; VANTENN~ ~K - 22k \2,1 22011 - + 10 : = 12 . INTERCOM 6.8k 6.8k ) 220n 10: = + AV(2) 2 47 : ;_ + BYPASS 16 IC4 LM831 OUT 1 BRIDGE 1 · . t1:: _ I E 5 ::4 811 + m::- +4.5V VIEWED FROM BELOW E • • • C B 0 .,.E _ Bll + + ~eli=~ - 47 ~F. - o 33: : LOUDSPEAKERS OR HEADPHONES V02F-- - - - +--e,a--+--<e-- - _ , 8 VOl 10 I 12 BSP GND BSP ~6 E... 9 +::- ------+-. _ S1b • Ol 8C548 . . - - - - - C B • ~ B ~c 03 BC548 :1. 4.5V½ iP INTERCOMe FM... OFF FMi OFF . . - Bro:: C 02 BC548 2 2k . 2.5T 0.8mm ECW ON 5mm FORMER F29 FERRITE SLUG 2T 0.8mm ECW ON 5mm FORMER F29 FERRITE SLUG 2.5T 0.8mm ECW ON 5mm FORMER F29 FERRITE SLUG 6.5T 0.8mm ECW 5mm INSIDE DIAMETER, 5mm LONG NEOSIO F29 BALUN FORMER 2 BIFILAR TURNS O.Bmm ECW . _ - ,..: 22I V IT ·--~ 14 t----. 150k IC1d ----11""3..1 _·-/ -.:- 22k r o *l L1 L2 L3 L4 L5 - 100k 10k 13 IN- (1) AV(1) 15 14 ,...... IN +(1) . .J,; . f 47 1 ....---, IN+ (2) 1N4~~8 ... -- K 02 1N4148 Al:;\K ,Gv +!·;_ TRi~~~IT€9 8 A ~ MOTORCYCLE RADIO INTERCOM 11 IF FILTER ~ .0033:: • NPO:: 15pF 120oF.l .,.11 .,'j(~•NPOh L1 56pF • NPO L' !r] 10 / -;..,_ci + 9 ~4 IClc 220k .Ol _1t ~ :J * NPO: = 15 12 CURRENT IF SOURCE LIMITER o*1:: 15tpF== IC3 TDA7000 ~ lg 2I 17 ls !;;; 15 ::~== IC2 RF OUT 7 BA1404 osc 10 4 MOO IN 22~pF: = 33~pF: : .01:: 1ST INTEGRATOR ..---..---- INTEGRATOR * ~t.J .01 * • • .001• ■ 0*1 :: 0.15== 4.7k 5.6k .,. ,.•_.,_.- __________________, 22I"".. 01 ~= _e:--:: T- 2~ 47k ICla >,_."'""W.-+-"1- _~ v -+-V 1 1b 5 R2~---+-5 Dk + ·- ; _ / ' 15k *,:: 3.3k O.l ,______.,__-411---<11-----------------il-----_. L - - - - - - -- l 2 .,. 471 + V R 1 ~ - - + - - - - + - - - - + - - - - - - - - -.....' 2 R 50k •• ::> 33 F" ■ ::> p ■ ■ tj + 1.2k -~15 ~ < 4701) -------~~-------------------------.------------......---------------,---.1 l ls1, A great deal of the circuitry operates at RF so keep all leads as short as possible. The circuit board is mounted on two plastic pillars on the case bottom and secured using self-tapping screws. Note the metal shield between coils L1 & L4. circuit. This means that the overall capacitance across 11 is about 39pF. The three .OlµF capacitors between the + 4.5V rail and ground provide decoupling for the circuit. Note that the positive supply for, IC2 is fed via LED 1 which lights whenever a transmission is in progress. That's handy but the real reason for the LED is to drop the supply to IC2 by 2V so that it is within the specified 1-3V. FM receiver IC3 (Philips TDA 7000} and its associated components form the FM receiver. The TDA7000 is virtually a complete FM tuner on a single chip and is easy to get going since only the local oscillator requires adjustment during alignment. All the other components are fixed. Incoming signals are picked up by the antenna and fed to the RF in24 SILICON CHIP put (pin 13} of IC3 via balun 15 and a bandpass filter consisting of 14 and two NPO capacitors (39pF & 47pF). This filter covers the entire FM broadcast band and does not require any adjustment . The .0022µF capacitor on pin 14 provides RF grounding for the internal mixer circuitry. IC3 functions pretty much as a conventional superheterodyne tuner. This means that the incoming signal is mixed with a local oscillator signal to produce an intermediate frequency (IF). The IF is then filtered to remove any mixer artifacts and demodulated to produce an audio signal. There's just one deviation from normal practice. The majority of FM receivers use an IF of 10. 7MHz whereas the TDA7000 uses the very low intermediate frequency of 70kHz. The advantage of such a low IF is that it can be filtered with standard active op amp filter circuits instead of coils or ceramic filters. Normally though, a low IF results in really bad distortion when it is used with wide deviation FM; ie, the normal broadcast FM which has a maximum deviation of ± 75kHz. This is why we used the TDA7000 instead of the Motorola MC3362 narrowband FM receiver which was featured in our March 1989 issue. The TDA7000 manages to demodulate wideband deviation of up to ± 75kHz while only having a 70kHz IF by a special bit of skullduggery. What happens is that the recovered audio is actually used to modulate the local oscillator so that the received FM deviation is always less than ± 15kHz. In effect, the recovered audio signal has been compressed to reduce its dynamic range. In a hifi FM receiver this would not be desirable but in this application, on a noisy motorbike, it is ideal. Distortion is kept reasonably low too, and is less than 2.3% at ± 75kHz deviation. 13, VCl and the fixed 56pF and 33pF capacitors form the tuned circuit for the local oscillator. By varying VCl (and thus the oscillator frequency), the receiver can be tuned over the entire FM broadcast band from 88-108MHz. The remaining capacitors on the circuit are used for decoupling, filtering and demodulation. To prevent instability, a 4. 70 resistor is included in series with the positive supply rail. The audio output signal appears at pin 2 where the necessary 50µs de-emphasis is provided by the 18k0 resistor, the .0039µF capacitor and volume control VR3 (50k0). Audio amplifier The output from the volume control is fed to pin 3 of IC4 which is a National Semiconductor LM831 low voltage dual power amplifier. For this application, the two amplifier stages have been connected in bridge configuration (ie, the two amplifiers drive the loudspeakers in antiphase ). This eliminates the need for output coupling capacitors and increases the available power output to about 400mW. The 2200 resistors at pins 2 & 15 of IC4 set the gain of each amplifier stage to about 54. The audio output signals appear at pins 8 and 10, with each output driving one side of the loudspeaker load. In addition, the outputs are connected to the bootstrapping inputs at pins 5 & ,12 via 47 µF capacitors. This gives the absolute maximum peak-to-peak swing from the amplifier outputs. The amplifier outputs at pins 8 & 10 drive two 80 loudspeakers connected in parallel to give a 40 load. In addition, a 0.33µF capacitor has been included between each amplifier output and ground as a stability measure. Mode switching Switch S 1 is used as an on/off switch and also selects between the two operating modes (FM or Intercom). When FM is selected, power is supplied to the FM receiver and audio amplifier ICs only (IC3 & IC4), PARTS LIST 1 PCB, code SC06111891, 82 x 92mm 1 front panel label, 90 x 43mm 1 dial scale label 1 plastic case, 95 x 45 x 145mm, DSE Cat. H-2503 1 microphone cable with 4 conductors plus one shield, Tandy Cat. 278-355 1 miniature tuning gang, DSE Cat. R-2970 1 Neosid balun transformer core, type 1050/2/F29, DSE Cat. L-1352 3 Neosid 722/ 1 coil formers (L 1,L2 ,L3), DSE Cat. L 1010 3 Neosid F29 slugs, DSE Cat. L-1307 1 DP3P slider switch, DSE Cat. S-2030 2 electret microphone inserts 2 miniature 80 loudspeakers 1 4 x AA square battery holder 3 1.5V AA cells 1 battery snap connector 7 PC stakes 1 cord grip grommet 1 6.5mm stereo line socket 1 6.5mm stereo plug 1 6 .5mm mono line socket 1 6.5mm mono plug 1 650mm length of 0.8mm enamelled copper wire 1 7 50mm length of stiff hookup wire for antenna 1 tinplate shield, 22 x 12mm 2 1.6mm dia. x 4mm screws 2 2mm dia. x 3mm screws 2 50k0 trimpots 1 50k0 log potentiometer Semiconductors 1 LM324 quad op amp (IC 1) 1 BA 1 404 Rohm stereo FM transmitter (IC2) 1 TDA7000 Philips mono FM radio (IC3) while the transmitter stages (ICl & IC2) are disabled. In the Intercom mode, the + 4.5V supply rail is connected to both the transmitter and receiver stages, with the ground rails now switched by the VOX circuitry as described earlier. Construction Most of the parts for the Motorcycle Radio Intercom are mounted 1 LM831 dual audio amplifier (IC4) 3 BC548 NPN transistors (01-03) 2 1 N4148, 1 N914 signal diodes (D1 ,D2) 1 3mm red LED (LED 1 ) Capacitors 1 4 70µF 1 OVW PC electrolytic 1 1 OOµF 16VW PC electrolytic 5 4 7µF 1 6VW PC electrolytic 2 22µF 16VW PC electrolytic 2 1 OµF 16VW PC electrolytic 1 4 . 7µF 16VW PC electrolytic 2 0.33µF 16VW PC electrolytic 1 0 .15µF metallised polyester 6 0.1 µF monolithic ceramic 2 .015µF metallised polyester 5 .01 µF ceramic 1 .01 µF metallised polyester 1 .0039µF metallised polyester 2 .0033µF metallised polyester 1 .0022µF ceramic 1 .001 µF ceramic 2 330pF ceramic 1 220pF ceramic 1 180pF ceramic 1 1 50pF ceramic 1 120pF NPO ceramic 2 56pF NPO ceramic 1 4 7pF NPO ceramic 2 39pF NPO ceramic 1 33pF NPO ceramic 2 1 5pF NPO ceramic Resistors (0.25W, 5%) 1 220k0 1 5.6k0 1 150k0 1 4.7k0 1 100k0 1 3.3k0 3 47k0 1 2.2k0 2 22k0 2 1.2k0 1 18k0 2 1 kO 1 4700 1 15k0 2 10kQ 2 220Q 2 6 .8kQ 1 4 .7Q on a small PC board coded SC 06111891 (82 x 92mm). This is housed in a clip-together plastic case measuring 95 x 45 x 145mm. The completed prototypes were given a professional finish by fitting Scotchcal labels to the front panels and thumbwheels. Before installing the parts on the PCB, it is necessary to enlarge some of the holes and do some work on the case. Fig.2 shows the locations OCT0BER1989 25 POLYESTER AND CERAMIC CAPACITORS □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ No. 6 2 6 1 2 1 1 2 1 1 1 1 2 1 2 1 2 Value 0 .1 uF .015uF .01uF .0039uF .0033uF .0022uF .001 uF 330pF 220pF 180pF 150pF 120pF 56pF 47pF 39pF 33pF 15pF IEC 100n 15n 1 On 3n9 3n3 2n2 1n0 330p 220p 180p 150p 120p 56p 47p 39p 33p 15p EIA 104K 153K 103K 392K 332K 222K 102K 331K 221K 181K 151K 121K 56K 47K 39K 33K 15K Fig.2: this diagram shows the locations of all holes on the PCB that are larger than 1mm in diameter. The holes for the three coil formers (D) also require matching keyways. RESISTORS □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ No. 1 1 1 3 2 1 1 2 2 1 1 1 1 2 2 1 2 1 Value 220k0 150k0 100k0 47k0 22k0 18k0 15k0 10k0 6.8k0 5.6k0 4.7k0 3.3k0 , 2.2k0 1.2k0 1k0 4700 2200 4.70 of all holes on the PCB that are larger than 1mm diameter. Follow this diagram carefully and drill all holes as shown. Note that the holes for the 5mm coil formers also require small matching notches. These can be made using a small file so that the formers are a tight fit. 26 SILICON CHIP 4-Band Code red red yellow gold brown green yellow gold brown black yellow gold yellow purple orange gold red red orange gold brown grey orange gold brown green orange gold brown black orange gold blue grey red gold green blue red gold yellow purple red gold orange orange red gold red red red gold brown red red gold brown black red gold yellow purple brown gold red red brown gold yellow purple gold gold 5-Band Code red red black orange brown brown green black orange brown brown black black orange brown yellow purple black red brown red red black red brown brown grey black red brown brown green black red brown brown black black red brown blue grey black brown brown green blue black brown brown yellow purple black brown brown orange orange brown brown brown red red black brown brown brown red brown brown brown brown black black brown brown yellow purple black black brown red red black black brown yellow black blue brown The case must be modified by filing down the inside sections of the clip pillars on the base so that the PCB will sit directly on the mounting pillars (see photo). Try mounting the PCB inside the case and you will quickly understand what needs to be done. In addition, the two catches on the lid must be filed off so that the clip pillars can slide past the PCB when the lid is attached. To understand what is required, temporarily mount the PCB in the base and then try putting the lid on. The front panel also requires drilling and filing to accommodate the thumbwheel dial, volume con- Fig.3: begin construction of the PCB by installing all the parts as shown here. Be sure to orient all parts exactly as on the diagram and note that the .01µF bypass capacitor for IC3 is mounted on the copper side of the board. Do not use IC sockets as these could . upset the circuit performance. Fig.4 (below): install the tuning capacitor, coils and LED as shown in this wiring diagram. The LED is mounted on the underside of the board so that it protrudes through a matching hole in the front panel. Similarly, all the wiring to switch S1 and to the battery should be run directly to the copper side of the PCB. - 1 : ,MICROPHONE GNO 2 : MICROPHONE 1 + 3 : MICROPHONE 2 + 4,5 : LOUDSPEAKERS ~ ANTENNA 750mm LONG trol pot, slide switch, LED and cord grip grommet. This can be done by using the front panel artwork as a drilling template. Check that all items fit correctly by installing them in position. Figs.3 & 4 show the assembly details for the PCB. Begin by installing PC stakes at the 1-5 wiring points and for the antenna lead, plus two more to support the metal shield (see Fig.4). Once this has been done, the 5mm formers for L1 , LZ and L3 should be glued to the board using an epoxy adhesive. The remaining parts can now be installed on the PCB as shown in Fig.3. Note that most of the resistors and all of the diodes are mounted end on. Take care with the orientation of polarised components. These include the ICs, transistors, diodes and electrolytic capacitors. The 0. lJ.LF capacitors are all monolithic types. Note that the O,lJ.LF capacitor adjacent to IC2 must be laid flat against the PCB, as shown in Fig.3. This is important because otherwise the capacitor body will affect the tuning of LZ, particularly with changes in temperature. OCT0BER1989 27 ~4 I 5mm 1.. L2 L3 fillID a A I! I I L4 L1 Fig.5: coils L1-L3 are close wound on 5mm coil formers using 100mm lengths of 0.8mm enamelled copper wire (ECW). L4 is made by winding 6 turns of 0.8mm ECW onto a 13/64-inch drill bit. c£=b ~ L5 i ~ Fig.6: the balun transformer is bifilar wound using two 100mm lengths of 0.8mm ECW. The finish of one winding connects to the start of the other to form the centre tap. The .0lµF decoupling capacitor for IC3 is mounted on the copper side of the PCB. This was done so that the capacitor can be connected as close as possible to the IC supply pins. Coil winding Above: full size artwork for the PC board (code SC 06111891). The inside sections of the clip pillars on the case bottom must be filed down as shown here. This is to allow the PCB to sit directly on the mounting pillars. 28 SILICON CHIP Work can now begin on the coils. Fig.5 shows the winding details for Ll-14 while Fig.6 shows the winding details for the balun (L5). 11 , 12 and L3 are all wound on the 5mm coil formers using 100mm lengths of 0.8mm enamelled copper wire (ECW). To begin each winding, strip 3-4mm of enamel from one end The two catches on the case lid (one on either side) are filed away so that the clip pillars can slide past the PCB when the lid is attrached. ( ROUND Off CORNERS J gJ TINPLATE SHIELD Fig.7: here are the dimensions for the tinplate shield. It is supported by two PC stakes between L1 & L4. of the wire and solder this end to the start pad on the PCB (see Figs.4 & 5). This done, wind on the correct number of turns for the particular coil as shown in Fig.5, starting at the bottom and moving up the former . Finally, insert the free end of the coil into the finish pad, strip back the enamel and solder. You can easily identify the start and finish of each coil by comparing Figs.4 & 5. 14 is made by winding six turns of 0.8mm ECW onto a 5mm former (eg, a 13/64-inch drill bit). It should be pushed all the way down onto the PCB and soldered in position as shown in Fig.4. 15 is wound on a Neosid balun transformer core using two 100mm lengths of 0.8mm ECW. Wind the balun exactly as shown in Fig.6 (one wire is shown black, the other in colour), then connect two of the leads together to form the centre tap. Strip back the enamel from the All the wiring to switch S1 and to the battery is run directly to the copper side of the PCB. Note that many of the tracks are close together so be careful to avoid solder bridges. The pen points to the .01uF capacitor beneath IC3. centre tap and the other two leads before soldering them to the PCB. The PCB assembly can now be completed by installing the small metal shield adjacent to 11. Fig.7 The leads from the coiled cord are terminated in a 6.5mm stereo jack plug for the microphones & a mono plug for the speakers. Use heatshrink tubing to sheath the leads to the plugs. shows the shield dimensions. It can be cut out from a piece of tinplate. Wiring Fig.4 shows the wiring details. The 4-way battery holder must be modified by soldering a wire link across one of the battery positions. Be careful - too much heat can melt the plastic around the terminals. OCT0BER1989 29 Begin by installing the LED on the copper side of the PCB. It should be positioned so that it protrudes through the hole in the front panel when the board is mounted in the case. The wiring to switch S1 and to the battery should also be run to the copper side of the PCB. The pot can be wired to the top of the PCB using short lengths of hookup wire. Note the link between one of the pot terminals and the pot case. The next step is to fit the Scotchcal labels to the front panel and to the thumbwheel. Fit the front panel label first, then temporarily fit the thumbwheel to the tuning capacitor and mount the PCB in the case. Now rotate the thumbwheel fully clockwise and use a pencil to mark the rim where it aligns with the line on the front panel. This done, remove the thumbwheel and fit the dial label so that the 88MHz marking aligns with the : pencil mark. Note that the dial faces toward the PCB when the thumbwheel is fitted to the tuning capacitor. Be careful here - it's all too easy to affix the dial to the wrong side of the thumbwheel. The coiled cord and antenna lead can now be installed. These both pass through a hole in the front panel and are clamped by a cord grip grommet. Fig.4 shows the wiring details. Note that the wire colours shown on Fig.4 are for the specified Tandy cord. This cord has four colour coded wires (white, red, blue & black) and the white lead is shielded (ie, there are five conductors in all). We've wired the cable in an unconventional way which gives the best overall performance from the unit. Note in particular that the white wire is used for the microphone ground connection and its shield is used for one of the speaker connections. The leads at the other end of the coiled cord are connected to a 6.5mm stereo jack plug for the microphones and to a 6.5mm mono jack plug for the loudspeakers (see Fig.8). Connect the white lead to the common earth on the microphone jack and the red and blue wires to the left and right jack terminals. 30 SILICON CHIP 6.5mm STEREO LINE SOCKET □- LOUDSPEAKERS - □ MOTORCYCLE INTERCOM - - HELMET Fig.8: here's how to mount the microphones and loudspeakers inside the helmet. Note that only one of the microphones is positioned directly in front of the user's mouth. The loudspeakers are fixed in position behind the helmet lining. RADIO INTERCOM • • • • • • • • ·- VOLUME TUNING 8 TRANSMIT • ' OFF FM INTERCOM Here is an actual size artwork for the front panel. The black lead and the shield go to the terminals of the mono jack plug (the polarity is unimportant). To make the wiring neat, use heatshrink tubing to sheath the leads to the plugs at the end of the cord and to sheath the "Y" intersection. Finally, the 4-way battery holder must be modified so that it will take only three cells. The fourth cell position should have a wire strap soldered between its positive and negative terminals. Be quick about it when soldering in this wire strap as too much heat will melt the plastic around the terminals. That's all we have space for this month. Next month we'll continue with testing and alignment. 1§:;i