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Phone patch for radio amateurs This easy-to-build circuit will take the hassles out of phone patch operation. It features a VOX circuit for automatic transmit/receive switching and can be matched to virtually any transceiver that features an effective squelch control. By JOHN CLARKE & GREG SWAIN OK, so what's "phone patch'"? Well, the concept is really very simple. Basically, it's a technique that involves connecting a 2-way radio circuit to a telephone line to extend communications to a third party. This can be particularly useful in emergency situations when normal communications are disrupted or where it is not possible to establish a full radio link. To establish a phone patch 26 SILICON CHIP operation, a 2-way radio circuit is first established between two transceiver stations. One of these transceivers is linked (or patched) to the telephone lines via a suitable switching circuit. Once the radio circuit is established, it's simply a matter of dialling the required number and then switching over to link the remote transmitter to the person on the telephone. In effect, the transmitter connected to the telephone lines serves as a relay station. In one direction, it receives signals from the remote transmitter and couples these into the phone lines via its speaker output socket. In the other direction, it picks up the signal coming down the phone lines via its microphone socket and transmits to the remote transceiver. One way of setting up a phone patch circuit is to employ manual switching but that's clumsy and requires some skill on the part of the operator. What's needed is a circuit that will automatically switch the transceiver between receive and transmit in response to the voice signal coming down the line. The SILICON CHIP Phone Patch circuit provides this function. It is completely automatic in operation and provides the necessary interface between the transmitter and the phone lines. In use, the Phone Patch is connected between the microphone ,--------------------------------------, I ~-------------------. I ,-------------------. I MC34018 I -----. j TRANSMIT BUFFER - . . : : . i - - - - - - - - - - . 1 TRANSMIT I I I I OUTPUT I ATTENUATOR I .---- I I --- --13 j I I ----. ---- -....L.- ,-----___,'--....--.;,...., SIDETONE NETWORK ATTENUATOR __-+--I TRANSMIT/ RECEIVE CONTROL COMPARATOR SIGNAL/NOISE DETECTOR Jj j j RECEIVE BUFFER I --- INPUT 1----...n.--1 15 1 I I I LINE ISOLATION UNIT LrJtr~R I MONITOR j LOUOSPEAKER '- - ---51---, 16 REGULATOR - ~'-+-BV....,_-I REGULATOR SPEAKER AMPLIFIER I I I 19 I 23 24 I I ~~~rr~: ~------ ------7 VR2 MICROPHONE LEVEL VR1 I I ~-----.J I I PHONE PATCH CIRCUITRY I I 1-----------------------~ 1 I 0-012VAC POWER I PLUG I I 26 RECEIVE IDLE CONTROL I I RECTIFIER L__ _J-----{? PACK ~mp~iLJE I I TRANSCEIVER S2bl C======:::::::!-~-...1 I MICROPHONE SOCKET L_ - - - - I ,...__ _ _ 1 ---1---e - - __ J Fig.1: most of the work in the Phone Patch circuit is performed by an MC34018 speakerphone IC. The VOX circuit triggers in response to speech coming down the phone line and simulates the PTT switch on the microphone. socket and the loudspeaker socket of the transceiver. It is then coupled to the phone lines via a line isolation unit (LIU). There are just four front panel controls: a Monitor control, a Microphone Gain control, a Mic/Patch switch and a Power switch. Also on the front panel is a microphone socket. This accepts the transceiver's PTT switch and microphone while an output lead from the Phone Patch now plugs into the transceiver's microphone socket. The Mic/Patch switch connects either the Phone Patch circuit or the hand-held microphone to the transceiver. This is a particularly useful feature because it allows the operator to transfer between phone patch operation and 2-way radio operation at the flick of a switch. It eliminates the need to unplug the phone patch circuit and then plug in the microphone, or vice versa, in order to switch from one mode to the other. The Monitor control is used to adjust the volume of an internal monitor loudspeaker. This feature allows the patch operator to monitor the 2-way conversation so that he knows when to terminate the call. It also allows him to keep track of the conversation so that he can talk to either party if necessary. Line isolation unit Because the Phone Patch is powered from the mains (via a 12V AC plugpack supply), it must be connected to the telephone lines via a Telecom-approved line isolation unit (LIU). A suitable LIU was described in the February 1988 issue of SILICON CHIP but you can also now purchase a commercial unit from Altronics (see photograph). On no account should you at- tempt to couple the Phone Patch circuit directly to the telephone lines. That would be dangerous and illegal. The LIU is there to isolate the phone lines from dangerous voltages and is a necessary part of phone patch operation. It's easy to connect up the LIU it's simply plugged into a telephone double adaptor socket in parallel with the existing phone. The connections to the Phone Patch circuit are then made via banana plugs and sockets (SILICON CHIP version only). After the call has been made, the LIU is switched into circuit and holds the line in the looped condition, thus allowing the telephone to be hung up. Speakerphone IC The SILICON CHIP phone patch circuit is based on the Motorola MC34018 speakerphone IC. We first used this chip in the Speakerphone project described in the September 1988 issue. Its use in this latest role was suggested by Ron Kilgour, VK3BDM, who built a prototype and submitted the circuit JANUARY 1990 27 The Phone Patch circuit is built into a standard plastic instrument case. The Mic/Patch switch connects either the Phone Patch circuit or the external microphone to the transceiver input. to us. We subsequently developed our own version based on his suggestion but with a number of modifications to make the unit easier to build. In fact, the MC34018 is ideal for phone patch operation. Because it is a voice-switched simplex system, it eliminates the need for a hybrid transformer. And whereas the latter is so critical to adjust that it frequently proves ineffective, the MC34018 suffers no such problems. So a major phone patch problem is overcome. Inside the MC34018 are all the necessary amplifiers, attenuators and control functions necessary to produce a high quality hands-free telephone. It includes a microphone preamplifier (not used here), 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. In operation, the MC34018 compares the transmit and receive signals to determine which is stronger and then switches into that mode. Normally, it operates in the receive mode but quickly switches to transmit mode when it receives a strong enough speech signal. Block diagram Fig.1 is a block diagram showing the functions performed by the 28 SILICON CHIP MC34018 IC, together with the additional functions needed for a complete phone patch circuit. At the bottom left of Fig.1 is the transceiver. This is connected into circuit between the Transmit Attenuator and Receive Attenuator blocks in the MC34018. The telephone, on the righthand side of the circuit, is connected to the phone patch circuit via the line isolation unit. Switch S2 is the Mic/Patch switch referred to earlier. In the Patch position, it switches the output of the Receive Attenuator (pin 26) and a VOX [voice operated switch) circuit to the microphone socket of the transmitter. When speech signals are received from the telephone line, the VOX circuit triggers and closes a relay. This simulates the action of the PTT (press-to-talk) switch on the microphone, and thus places the transceiver in transmit mode. When the party on the telephone stops talking, the VOX switches off and the transceiver goes into receive mode. Any signals picked up by the transceiver are then fed into the transmit attenuator (pin 3) of the MC34018. The circuit Fig.2 shows the complete circuit details of the Phone Patch. Let's start with the signal from the transceiver's loudspeaker socket. This is coupled to the transmit at- tenuator (pin 3, TXI) and also to the Signal/Noise Detector (pin 13, XDI). The Signal/Noise Detector (see Fig .1) discriminates between speech signals and background noise. If the signal on the transceiver's output is speech, a logic signal is fed to the Transmit Attenuator which then passes the speech signal to the transmit output at pin 4 (TXO). At this point, another logic signal becomes involved. The Transmit Level Detector (pin 5, TLI) monitors the Transmit Output at pin 4 (via Ql) and its output signal is fed to the Transmit/Receive Comparator. This controls whether the circuit is switched into the transmit or receive mode. When the Transmit Attenuator is at maximum gain (ie, when speech is passing through), the Receive Attenuator is at maximum attenuation (ie, fully off). Conversely, if only noise is present at the transceiver's output, the MC34018 will be in the receive mode. The Transmit and Receive Attenuators also take care of differences in voice levels. They provide plenty of gain for people who speak softly but prevent people who shout from overloading the system. 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 unity gain. Its 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 operates as a phase splitter. Q3's collector output drives the telephone line via the LIU while its emitter output provides a sidetone signal via a .068µF capacitor. In the other direction, signals from the telephone line are coupled to high pass filter stage Q4 via a Fig.2 (right): input signals from the transceiver are fed to the TXI (Transmit Attenuator) input of the MC34018, while signals coming down the telephone line are fed to the RXI (Receive Attenuator) input via buffer stage Q4. IC2c, IC2b, IC2a & Q5 form the VOX circuit. ► +BV TO LIU 220k POWER ,-----O~S1 .,_--+---<t-~>-'O~U-',f T 7808 t'I_N__..,__-+---, .01 GND +8V + 100 .01 470~ 1M +8V MONITOR VR2 10k LOG 18k .068 4.7 12 CP2 28 RAX 5 RTX TLO ALO 4.7k 30k 91 k 16 V+ RR RU TU SK0 IC1 MC34018 15 +4 47 ..__ _ _ _ _ _ _4'fTXO 80 SPEAKER RX0 26 TXI XDI 13 XDC 23 AGC 17 CPI 11 ACF 25 4.7 100k 0.1 vcc 20 VLC 24 01 + VB 21 + 0.1 4.3k RECEIVE 2.7k 0.1 +5.4V 2.7k 4.7k INPUT FROM TRANSCEIVER SPEAKER OUTPUT i 0.1 +2 .9V 47k 10k 47J- MICROPHONE INPUT 200k 200 k ~ 10J- 47+ 06 BC547 VB 470k MIC LEVEL VR1 20k 0 1i D7 1N4148 .,. 10k 14 .047 01+ PATCH * 01! .,. ,...._ __.,_ ..,_ +sv 0.1 RLA1 TO TRANSCEIVER MICROPHONE 01 1 SOCKET * * 01 + * 01! B 100k EOc A~K VIEWED FROM BELOW 2. 7k VR3 100k +BV 1t * CERAMIC •~oo, GN0 PHONE PATCH POWER A LED1 K A JANUARY1990 29 Background to Phone Patch Operation - By Philip Watson, VK2ZPW For amateur radio operators in Australia, phone patch operation has only been legal for the last two years This legal breakthrough was made as a result of efforts by the Wireless Institute of Australia and, specifically, by amateurs Sam Voron, VK2BVS; Jim Linton, VK3PC; Jack O'Shanassy, VK3SP; and Geoff Donnelly, VK2EGD. Geoff Donnelly, in particular, was responsible for designing a line isolation unit (LIU) which, properly cons'tructed, would meet Telecom approval if submitted for their inspection. Constructional details were published in "Amateur Radio" magazine for September and November 1987, and by Garry Cratt, VK2YBX, in SILICON CHIP for February 1988. Readers are referred to these articles. But this was only a first step. The line isolation unit merely provides the legal link between the amateur's equipment and the telephone line. It does not provide the phone patch facilities. This requires a separate unit. The basic phone patch problem is that we are trying to marry two different communication concepts: the duplex concept as represented by the telephone line, and the simplex concept as represented by the radio link. The term duplex means that the circuit can carry two speech channels, one in each direction, at the same time. While your friend is talking down the line to you, you can talk back and interrupt him if you feel so inclined - and he will hear you . Conversely, the simplex concept is a one-way-at-a-time arrangement. One party puts his transmitter on the air, says his piece, switches off his transmitter, switches on his receiver, and listens for the other party, who then goes through the same procedure. Clumsy though it sounds, this can provide rapid and accurate communication in the hands of experienced operators. In practice, of course, all the switching functions are normally performed by a single press-to-talk (PTT) switch on the microphone. Because part of the link is simplex (ie, via 2-way radio), this means that phone patch operation is also simplex. There's nothing that can be done about that - it just means that the person on the telephone must also adopt 2-way radio procedure. So how do we connect the two systems together? The most elementary approach is manual switching via a suitable line isolation unit. The telephone line is connected to a switch which directs it to either the transmitter (microphone) input or the receiver (speaker) output. The patch operator selects the setting, according to which party is due to talk, and also activates the transmitter as required. In spite of its elementary nature, this technique has been used extensively in the past, particularly by US amateurs when more elaborate systems were unable to cope. It's main disadvantage is that it requires some skill on the part of the Most of the parts are mounted on a single PCB to make construction really easy. The board is fastened to integral pillars inside the case using self-tapping screws. 15k0 resistor. Q4's emitter output couples into the Receive Attenuator (pin 27, RXI) and into the Receive Level Detector (pin 7, RLI) 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 provide cancellation of the transmit signal which would otherwise be coupled into the Receive Input. As such, it is not really a sidetone circuit. Q3's collector output and the signals coming down the telephone line are also used to drive Monitor control VRZ via a 56k0 resistor. The signal from VRZ is then fed to the Speaker Amplifier input (pin 19, 30 SILICON CHIP operator. The next step, to make the system fully automatic, has two two basic requirements: (1 .) The transmitter must be equipped with a VOX (voice operated switch) system in order to sense speech from the telephone circuit and put itself on the air. Some commercial transmitters are already so equipped but if not, an external system must be added. (2.) Some means must be provided to isolate the receiver output from the transmitter input where they are both connected to the telephone line. If this is not done, a signal from the receiver that's intended for the telephone will also appear at the transmitter input. This would then trigger the VOX, put the transmitter on the air and shut down the receiver, thus circircumventing the intended operation . And this, as they say in the classics, "is the hard part" . Just how can the system be made to discriminate between the wanted telephone signal and the unwanted receiver signal? In fact, there is a circuit which SKI). The output from the amplifier appears at pin 15 (SKO) and drives the monitor speaker via a 47µF capacitor and 220 resistor. VOX circuit The output signal from the Receive Attenuator appears at pin 26 (RXO) and is fed to the microphone socket on the transceiver via S2a. This signal is also used to drive the VOX circuit via a 0. lµF capacitor. Op amps IC2c, IC2b and IC2a, together with Q5 and RLA 1, make u_µ,the VOX circuit. IC2c functions as a non-inverting amplifier with gain set by sensitivity control VR3. Its output is coupled to Schmitt trigger stage IC2b via a 3.3kfl resistor. Positive feedback around IC2b is provided by a 120kfl resistor which, in conjuction with the 3.3kfl resistor, sets the hysteresis. IC2b squares up the output from will do this, at least in theory. It is one developed by telephone engineers in the early days of trunk line systems, to enable a (valve) amplifier to amplify in both directions in a single telephone pair. It is called a hybrid circuit. Space does not permit a detailed description , but it is a balanced circuit, based on the Wheatstone bridge principle. And assuming a good balance, it can couple two separate signals into a third circuit, with minimum coupling between the two signals. Un fortunately , "assuming a good balance" is the operative phrase. This is relatively easy if all the circuits are purely resistive but practical telephone circuits also contain inductance and capacitance . And to make it harder, these values vary according to the length and nature of the telephone line. As a result , amateur systems based on a hybrid circuit often fail in practice; hence the previous reference to manual switching. This problem is overcome in this project by using the MC340 1 8 speakerphone chip which is a simplex system . IC2c and couples the resulting square wave signal to a charge pump circuit consisting of a 0. lµF capacitor, diodes D8 and D9, a lµF capacitor and a 100kfl resistor. When a speech signal is received, the lµF capacitor is rapidly charged towards the + 8V supply rail. Following the charge pump circuit is another Schmitt trigger stage based on IC2a. This stage compares the voltage across the lµF capacitor with the voltage at its inverting input as set by the VOX delay control VR4. When the voltage on pin 3 exceeds the voltage on pin 2, IC2a's (pin 1) output switches high. Bias for IC2a, 2b & 2c is derived from the VB output (pin 21) of the MC34018. This output will be at about + 2.9V. The 100kfl resistor on D9's anode provides a discharge path for the lµF capacitor to ensure that the VOX drops out when speech signals cease. IC2a's output drives transistor Q5. When the output is high (ie, when speech signals are received from the telephone line), Q5 turns on and activates the relay. This closes the relay contacts and activates the transmitter via S2b (ie, the relay contacts simulate the action of the PTT switch). Receive idle circuit Because the MC34018 compares the transmit and receive signals to determine which is stronger, a problem can arise if the telephone line is noisy. If this is the case, the MC34018 will favour (or even lock into) the receive mode and it will be difficult to get the chip to switch to transmit. This problem is solved here by adding a "receive idle" circuit consisting of Q6, Q7, IC2d and D7. Fig.1 shows how the receive idle circuit is connected to the MC34018. Q6 and Q7 are emitter followers and are connected between the Signal/ Noise Detector output (pin 23, XDC) and the Attenuator Control input (pin 24, VLC). IC2d and D7 form a peak detector circuit. Let's see how it works. The receive signal applied to RXI (pin 27) also drives the Signal/Noise Detector (pin 13, XDI) via a 0.lµF capacitor and 2.7kfl resistor. When voice signals in the receive mode exceed the background noise by 4.6dB, pin 23 switches high and turns Q6 on and Q7 off. This means that the voltage on the Attenuator Control input (pin 24) is set solely by VR1 (the Mic Gain pot). This in turn sets the signal level fed into the microphone socket from pin 26. When voice signals are no longer received from the party on the telephone, pin 23 decays to about 1.45V (½ VB). Emitter followers Q6 and Q7 are now both on, which means that the voltage on thA Attenuator Control input (pin 24) is now determined by the voltage on pin 23 (the Signal/Noise Detector output). This equalises the gains of the Transmit and Receive Attenuators and makes it easier for the circuit to switch from one mode to the other. The peak detector circuit formed by IC2d and D7 is required to JANUARY 1990 31 TO 12VAC PLUG PACK CORD GRIP GROMMET (:;. 0 MIC LEVEL TO TRANSCEIVER MICROPHONE SOCKET\ r7, CORD GRIP l GROMMET~ / FROM TRANSCEIVER SPEAKER OUTPUT Fig.3: here's how to assemble the PCB and install the external wiring. Take care to ensure correct orientation of polarised components and don't substitute for the 1 % resistors. CAPACITORS □ □ □ □ □ □ If you want to save money, you can build your own line isolation unit. This LIU was described in the February 1988 issue of SILICON CHIP. 32 SILICON CHIP No 9 3 2 9 2 1 Value 0 .1µF .068µF .047µF .01µF .0047µF .001 µF IEC EIA 100n 104 68n 683 47n 473 10n 103 4n7 472 1n0 102 disable the receive idle mode when constant level signals (eg, tones) are intentionally applied to the receive channel. Pin 14 of comparator IC2d is normally low but switches high when the incoming receive signal exceeds the comparator threshold PARTS LIST This line isolation unit is available for $99.50 from Altronics Pty Ltd. To use it with the Phone Patch, you must modify the plug wiring by moving the white lead from pin 4 to pin 2. A double-pole on/off switch should also be fitted in series with the leads to the telephone plug so that the unit can be easily switched out of circuit. (about 20mV). This charges the lOµF capacitor on pin 23 (XDC) via D7, thereby turning Q7 off and switching the MC34018 to the receive mode with gain set by VRl as before. Power for the circuit is derived from an external 12V AC plugpack transformer. Diodes Dl-D4 rectify the AC supply to produce a DC voltage which is then filtered by a 470µF capacitor and fed to a 7808 3-terminal regulator. The regulator output provides a + 8V rail and this is used to power the Phone Patch circuit. LED 1 and its associated lkO current limiting resistor provide power on/off indication. 1 PCB, code SC 12112891, 129 x 143mm 1 front panel artwork, 1 91 x 59mm 1 plastic instrument case, 205 x 159 x 69mm 1 57mm 80 loudspeaker 1 12V relay, SPOT (DSE Cat. S7120) 2 knobs 2 banana plugs 1 SPOT toggle switch 1 DPDT toggle switch 3 cord grip grommets 2 plastic P-clips 1 4-pin chassis mount microphone plug 1 4-pin microphone line plug 1 3 .5mm mono jack plug 1 1 2VAC 500mA plugpack transformer 26 PC stakes 1 piece of tinplate for shield, 82 x 73mm 2 metres of figure-8 cable 1 metre of 3-core or 4-core cable Semiconductors 1 MC34018 speakerphone IC (IC1) 1 LM324 quad op amp (IC2) 5 BC548 NPN transistors (01-04,06) 1 BC338 NPN transistor (05) 1 BC558 PNP transistor (07) 1 3mm red LED (LED 1) 6 1 N4002 1 A diodes (01 -06) 3 1 N4148 or 1 N914 diodes (07-09) 1 7808 3-terminal 8V regulator Capacitors 2 470µF 25VW PC electrolytic 1 5 4 2 4 9 3 2 4 5 2 1 1 OOµF 16VW PC electrolytic 4 7 µF 1 6VW PC electrolytic 1 OµF 16VW PC electrolytic 4. 7 µF 16VW PC electrolytic 1µF 16VW PC electrolytic 0.1 µF monolithic ceramic .068µF metallised polyester .04 7 µF metallised polyester .01 µF metallised polyester .01 µF ceramic .004 7 µF metallised polyester .001 µF metallised polyester Potentiometers 1 20k0 linear potentiometer 1 1 OkO log potentiometer 1 1 OOkO miniature vertical trimpot 1 1 OkO miniature vertical trimpot Resistors {0.25W, 5%) 1 2 2 4 1 2 1 3 2 1 1 2 5 1 1 1 1 10MO 2.2MO 1MO 470k0 220k0 200k0 1 % 120k0 100k0 91k0 1% 68k0 56k0 51 kO 1% 47k0 33k0 30k0 1% 27k0 24k0 1% 2 1 1 4 1 1 3 1 2 3 1 1 1 1 1 1 22k0 18k0 15k0 10k0 8 .2k0 6.8k0 4.7k0 4.3k0 1 % 3 .3k0 2.7k0 2.2k0 2k0 1% 1.2k0 1 kO 4700 220 Miscellaneous 200mm of hookup wire, tinned copper wire , solder, 6 selftapping screws to secure PCB. Construction Most of the circuit for the Phone Patch is mounted on a PCB measuring 129 x 143mm and coded SC 12112891. This is housed in a standard plastic instrument case measuring 205mm wide, 159mm deep and 68mm high. Before commencing assembly, check the PCB pattern carefully for possible defects. Any shorts between adjacent tracks or IC pads due to incomplete etching, or breaks in the copper pattern, are best corrected at this stage. Fig.3 shows the wiring details. Start by installing PC stakes at all external wiring points and to provide support for the metal shield (see photos). You will need 26 PC stakes in all (five for the shield). When all the PC stakes are in position, you can install the wire links and then move on to the rest of the components. Do not install ICl at this stage, however - that step comes later. It's a good idea to check all resistors with a digital multimeter before installing them on the PCB, as some of the colours can be difficult to judge. The accompanying table shows the codes used for the capacitor values. Make sure that all polarised components are correctly oriented when installing them on the PCB. These parts include the diodes, transistors, ICs, electrolytic capacitors and the 3-terminal regulator. The latter is installed with its metal tab nearest the edge of the PCB (ie, away from the relay). That's all we have space for this month. Next month, we'll complete construction and give a brief troubleshooting procedure. ~ JANUARY 1990 33