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Here's a hands-free telephone that anyone can afford. With the Speakerphone you can have relaxing conversations without the need to hold the phone to your ear. And if you are put on "hold" you can continue with your work while you wait. By LEO SIMPSON & JOHN CLARKE How would you like to be able to talk on the telephone with your hands completely free? Well, now you can. If you are taking down details over the phone there is no need to "crook your neck" to hold the phone on your shoulder while you write. And if you are talking to friends or relatives, other people with you can listen in or join in the conversation - it's like listening to the radio, except that you can talk back! Imagine the freedom of being able to talk on the phone while you move around the room, do your exercise routines, lie in bed, or carry on virtually any activity, all without touching the phone itself. And if you 14 SILICON CHIP are put on "hold" it is great to be able to carry on with other activities without being glued to the phone. For long distance calls to your relatives or friends, the Speakerphone can be a real money-saver since all members of your household can listen in or participate in the conversation instead of each family member waiting for his or her "turn". Well it sounds marvellous, doesn't it? And it is. We predict that in a few years' time, virtually every household will have one, such is the ease and convenience of its use. Of course, you can buy a phone with a loudspeaking facility right now but they're not cheap, at around $250 or more. Features The Speakerphone does not replace your existing telephone but operates in parallel with it. To make a phone call, you dial from your existing Telecom phone in the normal way. Having established contact, you then switch over to the Speakerphone and "hang up" the Telecom phone. To answer an incoming call you can pick up your Telecom phone and speak and then switch over to the Speakerphone or you can switch on the Speakerphone directly and answer immediately. If you don't tell the person at the other end, it is likely that they will not know you are using it - it's that good. For the hard of hearing Should you wish to have a more private conversation but still have your hands free, you can use a standard pair of stereo headphones to ELECTRET MICROPHONE MICROPHONE AMPLIFIER MC34018 BUFFER 9 TRANSMIT ATTENUATOR 4 TRANSMIT LEVEL DETECTOR . SIGNAUNOISE DETECTOR TRANSMIT/ RECEIVE COMPARATOR ATTENUATOR CONTROL SIDETONE NETWORK RECEIVE LEVEL DETECTOR PEAK LIMITER 15 SPEAKER TRANSMIT OUTPUT RECEIVE ATTENUATOR 27 RECEIVE INPUT SPEAKER AMPUAER REGULATOR HOOK ~ 16 DC INPUT TELEPHONE LINE 24 18 ':' -!- 0 RECEIVE VOLUME CONTROL Fig.1: block diagram of the MC34018 speakerphone IC. It contains all the amplifiers, attenuators and control circuits necessary to produce a high-quality hands-free telephone. listen to the other party. If you or someone in your household is partially deaf, the Speakerphone can be a real boon. With the headphones plugged in it acts as a very effective and comfortable telephone amplifier. For elderly people too it could be very convenient, especially if they have arthritis and cannot easily hold the phone. In fact, the more we used the prototype Speakerphone, the more enthusiastic we became. It is hard to understand why commercial versions of this product have not been marketed years ago. The Speakerphone is housed in a small plastic case containing a loudspeaker and sensitive electret microphone. It has two switches, one to connect or disconnect it from the phone lines and one to mute the microphone. There is also a volume control for the loudspeaker and a stereo headphone socket. No batteries or mains power connection is required, since the Speakerphone is powered directly from the phone lines. To connect the Speakerphone you need an approved Telecom parallel adaptor socket. No other modifications are required. Motorola's MC34018 Check to see whether or not your microphone is sealed at the back. If it isn't, sleeve the microphone body with a 15mm length of heatshrink tubing, then seal the back with a doh of plasticine. What makes the Speakerphone possible is Motorola's MC34018, called, funnily enough, a voiceswitched speakerphone circuit. The MC34018 chip contains all the amplifiers, attenuators and control functions necessary to produce a high quality hands-free telephone (see Fig .1 ). It includes a microphone prea-mplifier, a small power amplifier for the loudspeaker, transmit and receive attenuators, a background noise monitoring system and automatic gain controls for the transmit and receive sound levels. The intended applications for the MC34018 are for domestic speakerphones, intercoms and automotive telephones. It can be used in conjunction with other telephone integrated circuits which incorporate SEPTEMBER 1988 15 PARTS LIST 1 PCB, code SC12-1-0988-1, 122 x 126mm 1 plastic instrument case, 155 x 1 59 x 65mm (Jaycar HB-5913 or equivalent) 1 front panel label, 146 x 59mm 1 Arlec 45035 or Ferguson MT-620 6000 1: 1 isolating transformer 1 1 20mm loudspeaker (see text) 1 6.5mm stereo headphone panel socket 1 20k0 linear potentiometer 2 SPOT toggle switches 1 electret microphone insert 1 knob 1 small grommet 2 metres of two core telephone wire 1 telephone plug 1 telephone double adaptor (Altronics P-0995 or equivalent) Capacitors 1 1 OOOµF 1 6VW PC electrolytic 4 4 7 µF 1 6VW PC electrolytic 1 1 OµF 16VW PC electrolytic 3 4. 7 µF 1 6VW PC electrolytic 3 1µF 1 6VW PC electrolytic 4 O. 1µF metallised polyester 4 0.068µF metallised polyester 1 0 .047µF 250VAC 2 0. 04 7µF metallised polyester 4 0.01 µF metallised polyester 2 O.004 7 µF metallised polyester 1 0.001 µF metallised polyester Resistors (0.25W, 5%) 2 x 2.2MO, 1 x 1 MO, 2 x 470k0, 1 X 220k0, 1 X 200k0, 1 X 100k0, 1 x 91k0, 1 X 68k0, 3 X 47k0, 1 X 33k0, 1 x 30k0, 1 X 27k0, 2 x 22k0, 1 X 18k0, 1 X 15k0, 1 X 10k0, 1 x8.2k0, 1 x 6.8k0, 3 x 4 . 7k0, 1 x 4.3k0, 1 x 3.3k0, 2 X 2 .2k0, 1 X 4700, 1 X 2700, 1 X 270, 1 X 220 Semiconductors 1 4 7 1 MC34018P speakerphone IC BC548 NPN transistors 1N4007 1A diodes 7.5V 400mW zener diode dialling functions or it can be used on its own, as in the circuit published here. Fig.1 is a block diagram showing the functions performed by the MC34018 integrated circuit, together with the additional functions needed for a complete Speakerphone. When all the external componentry is taken into account it adds up to a fairly complex circuit, although most of the extra parts are low in cost. Before we discuss the complete circuit, the concept of conventional phones and the Speakerphone should be described. Simplex and duplex In a conventional telephone system the transmit (microphone) and receive (earpiece) signals are carried over two wires and then converted to a four-wire system inside the phone itself by a hybrid transformer (also known as an "induction coil"). So inside the phone there are two wires for the 16 SILICON CHIP Miscellaneous Rainbow cable, solder, dob of plasticine or blue tack, PC stakes, self tapping screws. microphone circuit and two for the earpiece. As well as separating the microphone and earpiece circuits, the hybrid transformer also provides a degree of "sidetone". Sidetone is the process whereby some of the microphone signal is deliberately fed into the earpiece; it gives the person speaking an idea of how loud he (or she) should speak. In this way, sidetone compensates for the varying losses of long or short lines. Without sidetone, the line will sound "dead". Apart from the ins and outs of sidetone, the conventional telephone has the advantage of being a true "duplex" system, meaning that both parties can talk at the same time! Most radio communications systems, by contrast, are simplex, so only one party can talk at a time. The Speakerphone too, is ·a simplex system. In normal conversation, with polite persons using the Speakerphone, only one person speaks at a time. If you're rude though, and talk very loud, you can override the person at the other end. The MC34018 operates on the principle of comparing the transmit and receive signals to determine which is stronger and then switching into that mode. It also contains circuitry to discriminate between noise and speech in both the transmit and receive modes so that it is not triggered by noise. How it works Let's now have a look at the complete circuit diagram of Fig.2. Rather than describe each and every function of the 28-pin MC34018, we'll assume that you will refer to the data on the IC elsewhere in this issue. At the lefthand side of the circuit is an electret microphone. This is biased from the VB line ( + 2.9V at pin 21) using a 2.2kO resistor. The microphone signal is then coupled via a 0.lµF capacitor to pin 9, the microphone amplifier input. This has a gain of 34dB (50 times) and feeds the Signal/Noise Detector and Transmit Attenuator. The Signal/Noise Detector (see Fig.1) discriminates between speech signals and those due to ordinary background noise (which may originate from computer fans, air-conditioning, etc). If the signal from the microphone is speech, a logic signal is fed to the attenuator control which says "pass the signal through the Transmit Attenuator" to the Transmit Output, at pin 4. But another logic function is involved. The Transmit Level Detector, pin 5, monitors the Transmit Output at pin 4 and its output signal is fed to the Transmit/Receive comparator. This controls whether the circuit is switched into the transmit or receive mode. So when the Transmit Attenuator is at maximum gain (ie, mic signals passing through), the Receive Attenuator is at maximum attenuation (ie, fully off). They are never both on or both off. If the signal at the transmit end is due to background noise, the Speakerphone will normally be in receive mode. Switching times between receive HOOK FERGUSON MT620, ARLEC 45035 T1 >--oA(2) 06 .047 250VAC .01 07, 220k 68k B (6) 01 BC548 15k .01 .068 .01 .,. 08 1N4007 1M .., 470k .,. .001 .,. .068 4.7k 0.1 ___ .06.,8_......, m 5 1 4.7k .068 3.3k 10 27 13 XD1 TXI 0.1 RU V 16 ,.__ _ _ _ _ _ ____.ITXO HEADPHONES +2.9V - .,. VOLUME VR1 20k ~ 21 VB 1 16VW 47 + 16VWI _+ 17 6C IC1 MC3401B 24 VLC SK 15 + 2 , 1:ff 47 / - 16VW 22k SPEAKER RS .,. RTX 2 91k *Rl+RS RR 1 30k = 25rl 18k B EO VIEWED FROM BELOW ... SPEAKERPHONE SC12·1 ·0988·1 Fig.2: the two key components in the circuit are the isolation transformer and the MC34018 speakerphone IC. The circuit is powered from the telephone lines via the isolation transformer (ie, no external supply is required). and transmit modes are very short although there is seldom any evidence of clicks. The transmit and receive attenuators also take care of differences in voice levels so tHat people who shout do not overload the system while there is plenty of gain for people who speak softly. The Transmit Output signal at pin 4 is fed to filter and buffer stages consisting of Ql, Q2 and Q3. Ql is a high-pass filter with a gain of unity. Ql 's output signal is fed to the Transmit Level Detector at pin 5, as already mentioned, and also to the emitter of Q2 which operates as a grounded base stage. Q2's collector output couples directly to the base of Q3 which has little gain but operates as a phase splitter. SEPTEMBER 1988 17 The PCB assembly is secured to integral pillars on the base of the case using four self-tapping screws. We soldered the MC34018 directly to the board but you can use a 28-pin IC socket if you wish. r- ~~~ ~=•D'-~:-7 !o :r,~~ -. -. . . 0 0 !J 18 SILICON CHIP Fig.3 (left): save money by using this full-size pattern to etch your own printed circuit board. Q3's collector output drives the line transformer Tl while its emitter output provides a sidetone signal via the .068µ,F capacitor. The receive signal from the phone line is coupled into the high pass filter consisting of Q4 via the 15k0 resistor. Its emitter output couples into the receive input, pin 27, and the receive level detector, pin 7, on the MC34018. Note that the sidetone signal from Q3 couples into the input of Q4, the receive input buffer. This signal is there to cancel the transmit signal which would otherwise be coupled into the receive input. As such, it is not really a sidetone circuit, is it? After passing through the internal attenuators, the receive signal is fed to the Speaker Amplifier (see Fig.1) which has a fixed gain of 34dB. The signal level is set by the DC volume control input, at pin 4. The amplifier is capable of delivering up to 100 milliwatts on an intermittent basis, into a 250 speaker. Because 250 loudspeakers are Special Note While the Speakerphone is designed to conform with Australian Telecom standards, any unit built from the design featured here will not be a legal installation as far as Telecom is concerned. To be legally connected, the Speakerphone or any other phone device must be tested and approved by Telecom and must carry a Telecom permit sticker. not readily obtainable, our circuit uses a low-profile but efficient 40 car radio speaker and so it is fed· via a 220 resistor. The isolating transformer The function of transformer Tl is not immediately clear from the circuit since it does not provide complete isolation of the circuit from the phone lines. It has two isolating functions. First, it allows DC from the phone lines to be fed into the circuit via the primary and thence via the diode bridge, Dl to D4. Thus DC is fed to the circuit without any loading of the transmit and receive signals. Second, the transformer isolates the low voltage DC rail for the circuit from the incoming phone lines. The low voltage DC for the circuit is clamped to 7.5V by zener diode D5 and then fed to pin 16 of the IC. An internal regulator then produces a fixed + 5.4V for the other internal circuit functions. Diodes D6, D7 and D8 plus the .OlµF capacitor across the 4.7k0 collector load resistor for Q3 pre. vent excessive transmit signals. The .047µF capacitor across the phone lines suppresses large transient spikes. There ate two switches in the circuit. St connects the circuit to the phone line. S2 mutes the electret microphone by shorting its output signal, when you want to talk without the party at the other end, hearing anything. It's handy if you want to blow your nose while the other person is speaking. A set of stereo headphones may be connected. When the headphones are plugged in, the speaker 0 - 6 B , HEADPHONES SPEAKER r'\ ELECTRET MICROPHONE Fig.4: install the parts on the board as shown here but don't install the IC until after the initial circuit checks detailed in the text have been made. The telephone cable enters through a grommeted hole in the rear panel and loops through two holes in the PCB. is disconnected. The series 2700 resistor limits the signal fed to the headphones. Construction The Speakerphone is housed in a standard plastic instrument case measuring 155mm wide, 159mm deep and 66mm high (Jaycar Cat. No. HB5913). Most of the circuitry inside it is mounted on a PCB measuring 122 x 126mm and coded SC12-1-0988-1. Assembly is a straightforward matter of installing the components and soldering them into the board, assembling the hardware into the case and then completing the internal wiring. Fig.4 shows the wiring details. Start by installing all the small components on the board. Note that a number of the capacitors adjacent to the IC may need to be laid over on their sides, to clear the loudspeaker magnet. These capacitors can be seen in the internal photograph of the unit. When all the small components including the four transistors have been soldered into place, solder in the transformer. The recommended types are the Ferguson MT-620 and the Arlec 45035. These are 6000 1: 1 isolating transformers normally used in Telecom approved modems. Do not install the IC yet. Initial circuit checks Connect a power supply, capable of delivering between 25 and 50 volts DC to the circuit via a 5600 5 watt resistor to the board inputs marked 1 and 3, adjacent to the SEPTEMBER 1988 19 The Speakerphone is simply plugged into a standard double adaptor plug in parallel with your existing telephone. You can buy the double adaptor plug from parts retailers and hardware stores. transformer (see Fig.3}. Check that the voltage across the zener diode D5 is close to + 7.5 volts. The same voltage should appear at the collectors of Ql and Q4. When that check has been performed, the MC34018 can be inserted and carefully soldered into place. Use a 28-pin socket if you wish. We did not bother. The rest of the circuitry should now be completed. You can use PC stakes to make the wiring easier or you can just solder short lengths of hook-up wire directly to the board and then solder the other ends to the switches and other off-board parts. The PCB is secured to integral pillars on the base of the case using four self-tapping screws. Installing the microphone The electret microphone may require special care to install, depending on whether it is fully sealed at the back or not. Ours wasn't so there was a problem with sound from the speaker inside the case getting into the back of the microphone and partially muting the circuit. To cure this problem we first sleeved the microphone with a 15mm length of heatshrink tubing and then pushed the whole assembly irito a close-fitting hole in the plastic front panel. After soldering on the two wires, we filled the sleeving at the back of the microphone with plasticine. The loudspeaker should be as large as possible while still being capable of being fitted into the case. We used a 40 speaker intended for use with car radios. It has a nominal diameter of 120mm and a front-to-back dimension of 35mm. With all the wiring complete, reconnect the DC power supply as before, via a 5600 5W resistor. Check the voltages again. The voltage at pin 16 of the IC should be close to + 6V while at pin 21, + 2.9V should be present. If these checks are positive, the unit is ready to be connected in parallel with your existing phone. You will need a double adaptor phone socket and a phone plug. Connect the wires from the Speakerphone to pins 2 and 6. Plug the unit into the phone lines in parallel with your phone. With S1 in the off position, no DC voltage should be present across D5. You should be able to use your existing phone in the normal way. With S1 in the On position and the standard phone "on hook" you should hear dial tone from the speaker. Check that the loudness can be varied with the volume control. Now switch off the Speakerphone and make a phone call in the normal way. When you're ready, switch over to the Speakerphone and hang up the other phone. You can now sit back and enjoy a "hands-free" conversation. ~ 7 SPEAKERPHONE OFF MUTE 0 0 0 0 ON VOLUME L 20 SILICON CHIP 0 HEADPHONES _J Fig.5: this is a full-size reproduction of the front panel artwork.