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Baby room monitor & FM transmitter This Baby Room Monitor allows you to listen to your baby from a remote location using a conventional FM receiver. It runs from a single 1.5V AA battery & includes a muting facility so that it only transmits sounds above a certain level. By JOHN CLARKE The SILICON CHIP Baby Room Monitor is an ideal aid for busy parents wanting to keep a check on their baby at all times. It's basically a simple FM transmitter fitted with a microphone. In use, it's positioned close to the baby and broadcasts to a standard FM receiver while you get on with the gardening or washing the car. This project will also be useful if you have a large house and the baby's room is some distance away from the main bedroom or the living room. The useful operating range is about 30 metres, so it will have no trouble broadcasting from one end of the house to the other. As can be seen from the photos, the project is housed in a small plastic case. There's just one control - a small slider switch on the side to turn the power on or off. The remaining parts are mounted on a small PC board and include a trimpot to adjust the sensitivity of the unit and two coils to adjust the transmitting frequency. The receiver can be any FM radio ranging from a personal portable to a mantel or "ghetto blaster" style receiver, or even a stereo hifi tuner. You simply preset the monitor to transmit on an unused frequency of the FM band and then tune the receiver to match. Block diagram Fig. l shows the block diagram of the Baby Room Monitor. The circuit is based on an FM stereo transmitter IC, the BA1404 made by the Rohm Corporation of Japan. This IC includes left and right channel amplifiers, a multiplexer which can be locked to a crystal, a mixer, an RF oscillator and an RF amplifier which drives the antenna circuit. These circuit blocks can be connected to produce a stereo FM transmitter, as described in SILICON CHIP, October 1988. In this application, however, the left and right channel amplifiers are effectively connected in cascade so that the BA 1404 only transmits a mono signal. This is done for two reasons: (1) to provide sufficient gain for the microphone signal; and (2) because no-one really wants to listen to a baby wailing in stereo. The signal from the electret microphone is applied to the input of the right channel amplifier which operates with a nominal gain of 30. Because its output is connected internally to the multiplexer switch and is not available on any pin of the IC, we have derived the amplified signal from the right channel balance output. This output is a high impedance and so is buffered using Ql to prevent loading by the gain control (VRl). From VRl, the signal is applied to the left channel amplifier which also has a nominal gain of 30. The balance JANUARY 1992 53 output of this amplifier is connected to a voice operated switch (VOX) which controls the internal multiplexer switch via the oscillator input at pin 5. When the signal level from the microphone is low, the signal from the left channel balance output and thus from the VOX is also low. Thus, the multiplexer diverts the right channel output through to the following internal buffer stage. Because the right channel output provides only the first stage of amplification, the signal level from the multiplexer buffer is thus effectively muted. Conversely, when the signal level from the microphone goes high, the output of the vox also goes high and the multiplexer switches the left channel output through to the following buffer stage. Because the signal has now been through two stages of amplification, it is at quite a high level. This signal is AC-coupled via C3 into the RF mixer oscillator stage. 11 and Cl are used to tune the mixer output, while the two 15pF capacitors on pins 9 & 10 provide positive feedback to ensure that the mixer oscillates. Its output drives an RF amplifier stage which in turn drives the antenna via C4. 12 & C2 tune the RF amplifier output and effectively filter out any sidebands generated by the mixing process. Circuit details Fig.2 shows the final circuit for the Baby Room Monitor. Supply de- This close-up view of the assembled PC board clearly shows the winding details for Ll & L2. Note also the orientation of multi-turn trimpot VRl. The PC board clips into guides running down each side of the plastic case. coupling for the electret microphone is provided by a 1kn resistor and lOOµF capacitor, with the microphone then fed via a second lkQ resistor. Its output is coupled via a 10µF capacitor into the right channel amplifier input at pin 1. The right channel balance output is buffered using transistor Q1. This is wired as an emitter follower and provides a gain of close to 1. The output at the emitter is tapped using VR1 to provide gain control and the signal then applied via a lOµF capacitor to the left channel input. The left channel balance output appears at pin 17 and is applied to the base of Q2 via a O. lµF capacitor and lOkQ resistor. Q2 & Q3 form the vox. When the signal from the left balance output is low, Q2 is biased on and so Q3 is off. Thus, the vox output at Q3's collector is low and so the output of the right channel amplifier inside ICl is switched through to the internal buffer, as described previously. Conversely, when the signal from the left balance output swings high (ie, when the baby cries), Q2 turns off and Q3 turns on. Q3's collector is now high and thus the output of the left channel is switched through to the buffer. The lOOµF capacitor on Q3's collector filters the vox output +1.5V C1 BUFFER 15pF .p 15P.F C2 10 RF MICROPHONE ELECTRET ] ANTENNA BUFFER AMPLIFIER ,__.......,.~ ]l_.,.1 _,..._c,• Rf MIXER OSCILLATOR OSCILLATOR IN 17 5 IC1 OUT BA1 404 13 14 12 C3 vox Q2,Q3 54 SILICON CHIP Fig.1: block diagram of the Baby Monitor. The circuit is based on the BA1404FM stereo transmitter IC which has been adapted here for mono operation. 1k 100. +1.5V + - 16VW+ C3 10 .01I + 14 C1 47pF _NPO .,. 16VW 12 vc OSC'-"O.___ _ 1k 15pF NPD GAIN VRk1.l!l--------t 1 10 osc--15pF NPOl .,. 16VW - IC1 BA1404 + 1 RIGHT IN BAl L 7 ELECTRET MICROPHONE RF 8 GN RF OUT AUDIO BIAS 2 + 10 16VW:r ANTENNA · ~ · .. :f ----NPO .0011 0.1 LOW• RIGHT CHANNE AT MPX OUT HIGH • LEFT+ RIGHT CHANNELS AT MPXOUT 10k .,. C2 47pF NPO 02 BC558 E VIEWED FROM BELOW 100k L1, L2: 1.5T0.62mm ECW DN 5mm FORMER WITH F29 CORE 100 ROOM MONITOR + 16vwr- .,. Fig.2: the circuit uses Ql to buffer the right channel balance output while Q2 & Q3 form a VOX circuit. When the signal level from the microphone goes high, the output of the VOX also goes high and the multiplexer inside ICl switches the high gain left channel output through to a following buffer stage. This signal is then ACcoupled via C3 into an RF mixer stage & thence to an RF amplifier which is tuned by C2 & L2. to prevent rapid cycling of the multiplexer switch. Strictly speaking, Q2 & Q3 do not form a vox circuit at all. Instead, this part of the circuit is really a threshold switch which is used to select between two levels of gain. However, its effect is exactly the same as a true VOX circuit, in that it effectively "kills" CAPACITOR CODES 0 0 0 0 0 0 0 0 Value IEC Code EIA Code 0.1µF .033µF .01µF .001µF 47pF 15pF 4.7pF 100n 33n 10n 1n 47p 15p 4p7 104 333 103 102 47 15 4.7 the output under no-signal conditions. Power for the circuit is derived from a single 1.5V battery and is switched by Sl. The remaining components depicted on the circuit (except for the lOµF audio bias and .OlµF decoupling capacitors) are exactly as depicted on the block diagram (Fig.1). Construction Fig.3 shows the construction details. All the parts except for the switch arid the electret microphone are mounted on a small PC board coded SC06210911 and measuring 62 x 46mm. This is housed in a standard plastic case which is fitted with & self-adhesive dress label to indicate the switch positions. Start construction by checking that the PC board fits neatly into the case and that it is supported on the stops of the PC guides. These stops enable the PC board to be mounted horizon- tally but they must be lowered by about 4mm so that there is sufficient clearance for the battery. You can achieve this by carefully cutting away the guides with a pair of side cutters. If the PC board is too wide, it can be filed down to size until it fits neatly into the case. This done, check that the two coil formers can be fitted.into their respective mounting holes. Enlarge the holes if necessary (using a tapered reamer), then install the two coil formers from the copper side of the board and glue them into position using 5-minute epoxy. The IC can now be installed on the board, followed by the three transistors. Note that Ql is an NPN type while Q2 & Q3 are both PNPs, so don't get them mixed up. The next step is to wind the coils on each former. Each_coil is wound by · first stripping the enamel from one end of the enamel copper wire and JAN UA RY 1992 55 SINGLE CELL HOLDER (SEE TEXT} PARTS LIST Fig.3: here's how to.mount the parts on the PC board. Keep all leads as short as possible & note that most of the resistors are mounted end-on to save space. The coil formers for Ll & L2 are installed from the underside of the board. 1 plastic case, 82 x 54 x 30mm 1 PC board, code SC06210911, 62 x 46mm 1 electret microphone 1 miniature double-throw slider switch 1 AA cell holder 2 5mm screw core formers 2 F29 ferrite screw cores 1 100mm length of 0.62mm enamelled copper wire 1 750mm length of medium duty hookup wire 1 1kn 25-turn top adjust trimpot, Bourns 3296W (VR1) Semiconductors 1 BA 1404 stereo FM transmitter (IC1) 1 BC548 NPN transistor (01) 2 BC558 PNP transistors (02, 03) The microphone is secured to the case by carefully reaming its mounting hole to size until it is a tight fit. Note that the switch terminals must be splayed outwards as shown here to provide clearance for the PC board. soldering it to the board. You then wind on 1.5 turns and solder the free end into the remaining pad. The remaining parts can now be installed as shown in Fig.3. Note that all but one of the resistors are mounte.d end-on to save board space. Make sure that the five electrolytic capacitors are all correctly oriented. If you can't obtain a single AA-cell holder, you can make one by cutting a dual cell holder in half. The spring at the negative end is soldered directly to the board, while the positive termi- nal is connected to the board using a stout piece of tinned copper wire. Once the PC board is complete, the case can be drilled to accept the microphone and on/off switch. These parts are both mounted on one end of the case. Drill a small pilot hole for the microphone to begin with, then carefully ream it to size until the microphone is a tight fit. The square cutout for the switch can be made by first drilling a series of small holes and then filing to get the correct shape. The switch is then Capacitors 2 100µF 16VW PC electrolytic 4 10µF 16VW PC electrolytic 1 0.1 µF monolithic 1 .033µF metallised polyester 3 .01 µF ceramic 1 .001 µF metallised polyester 2 47pF NP0 ceramic 2 1SpF NP0 ceramic 1 4.7pF NP0 ceramic Resistors (0.6W, 1%) 1 100kQ 2 1kQ 310kQ secured to the side of the case using two small self-tapping screws. It will be necessary to splay the switch terminals to clear the edge of the PC b'oard (see photo). A small (2mm) hole can now be drilled in the other end of the case for the antenna wire. This done, connect the microphone and switch to the PC board using light-duty hookup wire and connect a 750mm length of medium-duty hookup wire for the an- RESISTOR COLOUR CODES 56 0 No. Value q 1 0 0 3 100kQ 10kQ 1kQ SILICON CHIP 4-Band Code . brown black yellow gold brown black orange gold brown black red gold 5-Band Code brown black black orange brown brown black black red brown brown black black brown brown C06210911 1IJl!!!f BABY ROOM MONITOR ON OFF Fig.4: actual size artwork for the front panel. Fig.5: actual size PC artwork. tenna. Tie a k:hot in the antenna wire before passing it through the case to prevent it from coming adrift. Finally, push the PC board into the case and attach the adhesive dress label to the base (which now becomes the front panel). Be sure to attach the label so that the switch markings are adjacent to the on/off switch. Check your work carefully for assembly errors, then install the battery, switch on and check the supply voltage to the IC. You should get a reading of 1.5V between pins 15 & 3. Tuning adjustment The unit can now be adjusted for correct operation using an FM receiver set to an unused frequency near the centre of the band (around 100MHz). Set VRl to minimum gain (ie, rotate the adjusting screw clockwise until you hear a click), then screw the slugs into the coils. Initially, set Ll so that its slug is level with the top of the former and L2 so that its slug is about 2mm above the top of the former. Now adjust Ll until the transmitter frequency matches the receiver. This will be indicated by the level meter on the receiver (if it has one) and by a sudden drop in the hiss level. When the correct point has been found, wind The completed PC board fits inside a small plastic zippy case to give a really compact assembly. Oscillator tuning coil L1 at top right is used to tune the unit to the desired frequency, while L2 tunes the RF amplifier for maximum range. VRl slowly anticlockwise until you hear a signal from the radio (make sure that the microphone is picking up sound) and adjust L2 for maximum signal. To set the transmitter to a lower frequency, wind the slugs further into the coil formers. Conversely, to set Specifications Operating voltage ............................................ 1.5V Quiescent current ... .. ....................................... Typically 3mA Frequency range ............................................. 88-108MHz Range .............................................................. 30 metres (approx.) Dimensions ...................................................... 82 x 54 x 31 mm the frequency to the high end of the band, wind the slugs further out of the formers. Note that both slugs should be set to similar positions in the formers to get the best result. The coils are also slightly interactive which means that adjusting one will affect the other. Always use a plastic alignment tool to adjust the coil slugs. Don't use a metal screwdriver as this will damage the slugs and give misleading results. Finally, adjust the gain control so that the unit transmits only those sounds that reach the required threshold. Don't advance the gain control too far though, otherwise you'll get instability which will give a highpitched whistle from the receiver. SC JANUARY 1992 57