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Do you spend long periods on the phone? Would you like to have your hands free for taking notes, using a computer or other tasks? Well now you can, by building this headset adaptor which can be used with most phones which use RJ11 modular plugs and sockets. Telephone Headset Adaptor By JOHN CLARKE 10  Silicon Chip www.siliconchip.com.au S ome people have “prehensile” necks and can quite easily hold a phone handset between their chin and shoulder so they can talk “hands free” and take notes, etc. Other people are normal and can’t do it. But we’ll give you a tip: even if you can do it, it isn’t good for your neck anyway. Ask your local chiropractor how many business people they see with crook necks – and most are caused by holding a phone handset without hands. Instead, do what they do in call centres: get yourself kitted up with a phone headset and build this little adaptor to connect it to your phone. It makes life so much easier if you have to spend long periods on the phone. What the headset adaptor does is connect between your telephone and the handset. To use the phone, the handset is lifted off the rest (“offhook”) to answer or make a call. A switch on the adaptor then allows you to select the handset or the headset. To hang up, the handset is placed back on the phone as normal. The adaptor is housed in a small plastic case with two RJ11 4P/4C modular sockets at each end. These US-style modular sockets are used on just about all phones these days, so the handset can just be plugged into the adaptor’s output socket. The input socket on the adaptor connects to the telephone using another curly handset lead. The headset is then plugged into a 3.5mm stereo socket on the adaptor. Essentially the adaptor works by connecting either the normal handset or the headset to the telephone. The handset has a small loudspeaker in the earpiece and a microphone and these are connected to the telephone via four leads and the RJ11 socket. The headset also has a small loudspeaker for the earpiece and a microphone on a flexible support which you can bend to suit yourself. The headset has a figure-8 shielded cable running to a 3.5mm stereo jack. One wire carries the microphone signal while the second wire carries the loudspeaker drive. The shield wires are commoned to the sleeve connection on the jack. Therein lies a problem. While the ground (shield) wires for the headset microphone and earpiece speaker are commoned, a phone handset has completely separate wiring to the microphone and loudspeaker. Unwww.siliconchip.com.au fortunately, the two ground wires on the handset cannot be simply shorted together to form a common connection suitable for directly connecting to the headset. This is because most telephones drive the loudspeaker with a push-pull output, with both lines swinging above and below the microphone ground level. Thus shorting one loudspeaker output to ground will short out one side of the loudspeaker amplifier in the telephone. Another problem with connecting the headset to the telephone is that its loudspeaker impedance is a nominal 32Ω while typical telephone handsets have a nominal 128Ω impedance. Connecting a 32Ω loudspeaker could cause the amplifier within the telephone to be damaged and at the very least, the mismatch will result in greatly reduced sound level. Fortunately, both of the above problems can be solved by using a transformer. The transformer isolates the drive to the headset loudspeaker so that it can be connected to the mi- crophone common lead without shorting the telephone amplifier. And the transformer can be wound to provide the impedance transformation from 128Ω down to 32Ω. So as far as the telephone is concerned, it sees a 128Ω load when the transformer is driving a 32Ω loudspeaker. One further difference between the handset and headset is that some handsets use a dynamic microphone while others use an electret type. The headset uses an electret microphone which requires a DC supply and so the headset adaptor includes a 1.5V cell. This will not be required for use with telephone handsets which have an electret microphone; they can directly power the headset microphone. Confused? Have a look at the circuit and all will be made clear. Circuit diagram Fig.1 shows the circuit of the telephone headset adaptor. It shows the four lines that connect from the RJ11 socket on the phone to the socket (CON1) in the adaptor and these have The complete adaptor, shown here with its microphone and earphone headset. The white curly cord goes off to the telephone “handset” socket while the handset itself plugs into the socket at the bottom of the unit. July 2002  11 TO TELEPHONE HANDSET SOCKET S1a 2 MIC’ LS’ S1b RJ11 PLUG 22F R2 10k MIC CON1 RJ11 SOCKET TELEPHONE HANDSET LK1 1 MIC MIC’ MIC LS’ 1 LS CON2 RJ11 SOCKET 2 LS SPEAKER 150 100T 22F BP 200T T1 R1 S1c 1k 1 1.5V 3mm STEREO PLUG S 2 R TIP SLEEVE T CON3 3mm STEREO JACK SC 2002 TELEPHONE HEADSET ADAPTOR SPEAKER RING MIC HEADSET Fig.1: the circuit of adaptor: it is used in conjunction with the existing handset to give hands-free operation. Not all phones are suitable – the text explains how to check if yours can be used. been labelled MIC, MIC’, LS and LS’. These refer to the microphone and loudspeaker lines. The MIC line is the common ground for the microphone and the loudspeaker in the headset. Switch S1 selects between the headset and handset. When it is in position 1, it connects the handset (CON2). When S1 is in position 2, the headset (CON3) is selected. The MIC line is normally connected to the headset microphone ground, while the MIC’ connects to the microphone positive side via link LK1 or the 22µF capacitor. The capacitor removes the DC voltage from the microphone if it is supplied with current via the 1kΩ resistor and the 1.5V cell. The 10kΩ resistor holds the MIC’ DC voltage at ground. As we mentioned before, this electret supply is only required if the telephone itself does not provide power (ie, when the handset microphone is a dynamic type). If the handset uses an electret, then the headset will be powered directly and the 10kΩ and 1kΩ resistors, the 1.5V cell and the 22µF capacitor can be omitted. In this case connection of the positive side of the microphone to the telephone is made using link LK1. The headset loudspeaker is driven via transformer T1. The LS and LS’ lines from the telephone drive the primary side of T1 via a 22µF bipolar capacitor and 150Ω resistor. At 12  Silicon Chip frequencies above about 180Hz, the 22µF capacitor can be considered close to a short circuit and the transformer then directly drives the loudspeaker connected to the secondary. The transformer is wound with 200 turns on the primary and 100 turns on the secondary. This provides a 4:1 im- pedance transformation so that the LS drive from the telephone “sees” 128Ω while driving the 32Ω loudspeaker. The 22µF capacitor is included to protect the telephone’s audio amplifier from driving a very low impedance at low frequencies. This would happen since the primary winding of T1 is This version of the adaptor is for electret mics which require power (hence the 1.5V battery). If your phone already has an electret, this should not be needed. www.siliconchip.com.au Telephone testing Not all phones are suitable for this adaptor circuit. This is because there is no amplification to compensate for any differences in sensitivity between the loudspeaker or microphone. So if you have trouble hearing using the handset of your telephone you probably will have more difficulty hearing with the headset. There is a small reduction in volume level when changing to the headset because of losses in the transformer and possible lower sensitivity of the loudspeaker. Microphone sensitivity is less of a problem between headset and handset but this may also vary with different telephones. Before you can fully assemble the Telephone Headset Adaptor, some tests will need to be made to the telephone, to find out which connections are for the microphone and which are for the loudspeaker. This is done by partially assembling the adaptor PC board and then using a multimeter to measure some voltages and resistances. These measurements need to be made since it seems that 22F S1 LS' LS' LS BP (-) MIC CIM SL 'CIM 'SL CON1 RJ11 4P/4C SOCKET (TO HANDSET OUTLET OF PHONE) MIC' MIC' ROW1 ROW2 ROW3 ROW4 150 about 1.7Ω at DC, only rising to above 128Ω at beyond 180Hz. Thus the 22µF capacitor introduces impedance at these lower frequencies. The 150Ω resistor allows DC current to flow if this is required for correct operation of the telephone amplifier. 'SL 'CIM CON2 RJ11 4P/4C SOCKET (TO PHONE HANDSET) + TESDAEH ENOHPELET 12070121 CON3: 3.5mm STEREO SOCKET TO HEADSET Fig.2: initial assembly of the PC board, ready for connection to the ’phone and checking which links need to be inserted in which holes. each telephone is different; there is no standard for the pinouts for the handset RJ11 socket. Construction The Telephone Headset Adaptor is constructed on a small PC board coded 12107021 and measuring 79 x 49mm. All the components including the sockets and switch mount directly onto this PC board. It is housed in a small utility case measuring 82 x 54 x 31mm. Begin construction by checking the PC board for possible shorts and breaks in the copper tracks. The four corners of the PC board need to be cut to shape to clear the integral pillars in the case. You can do this by drilling out the centre hole with a 6mm drill, breaking off the unwanted corner portion and then filing to the contour shown on the copper side of the PC board. You may also need to drill holes for the integral mounting pins on the RJ11 sockets so that they clip in correctly to the PC board. The Altronics socket differs slightly to the one sold by Jaycar and so we have provided both hole positions for the mounting pins. Also check that there are suitable (1.5mm) sized holes required for the pins on the switch and 3.5mm stereo socket. The toroidal transformer is secured with cable ties through the holes as shown in Fig.3. Check that these holes Here’s how we cut the slots in the box and filed out the guides to acccommodate the PC board, with that accommodated board shown at right! It’s a tight fit so you have to be careful when cutting the slots. Don’t throw the waste out from the end slots: with careful cutting and filing you can make a tiny piece to take up the slot in the side (above the 3.5mm socket). www.siliconchip.com.au July 2002  13 150 (-) LS' LS' MIC CIM SL 'CIM 'SL MIC' LS MIC' 22F CON1 TO HANDSET OUTLET OF PHONE BP MIC LS' MIC' LS 'SL 'CIM S1 CON2 TO PHONE HANDSET + PRIMARY 200T CABLE TIE T1 CABLE TIE LK1 TESDAEH ENOHPELET 12070121 SECONDARY 100T CON3: TO HEADSET 150 (-) LS' LS' MIC' 22F CIM SL 'CIM 'SL Testing the telephone MIC' LS CON1 TO HANDSET OUTLET OF PHONE MIC LS MIC LS' MIC' BP Fig.3: fully completed PC board for a phone with an electret microphone. These links suit a Telstra Touchfone 400. 'SL 'CIM S1 CON2 TO PHONE HANDSET + PRIMARY 200T CABLE TIE T1 CABLE TIE LK1 TESDAEH ENOHPELET 12070121 SECONDARY 100T CON3: TO HEADSET Fig.4: similar to above, but this one suits a Sharp F0165 fax machine. Each phone must be individually checked and the links installed as appropriate. – + T1 LS' LS' MIC' CON2 TO PHONE HANDSET + + R1 1k PRIMARY 200T CABLE TIE 'SL 'CIM S1 CABLE TIE 10k R2 MIC' 22F CIM SL 'CIM 'SL (-) – 150 LS CON1 TO HANDSET OUTLET OF PHONE BP LS' MIC LS MIC' MIC 1.5V AA CELL HOLDER TESDAEH ENOHPELET 12070121 + SECONDARY 100T 22F CON3: TO HEADSET Fig.5: the typical wiring for a phone which uses a dynamic microphone, requiring bias voltage for the electret mic used in the headset. This happens to suit an NEC telephone attached to a PABX system. 14  Silicon Chip are of the correct size. The plastic case has integral side clips which will need to be removed so that the PC board will slide into the case. Remove these with a sharp chisel or utility knife (ie, Stanley) and check that the PC board fits into the case without fouling. The initial assembly of the PC board is shown in Fig.2. Install all the links as shown. The RJ11 sockets can be installed now along with the 3.5mm stereo socket. The switch may need its pins crimped together slightly with pliers to allow its eyelet terminals to be inserted into the holes. You can also install the 150Ω resistor and 22µF bipolar capacitor now. At this stage, the telephone connections will require testing (before the final components are fitted) to determine the linking required. The curly lead from your telephone handset back to the phone itself usually has an RJ11 connector plugging into a socket on the phone (usually on the back or side but often underneath). This must be unplugged. You do this by squeezing the release tab attached to the RJ11 connector towards its lead and gently pulling on the lead. Plug the now-free RJ11 connector into the handset socket (CON2) on the adaptor. Then use another RJ11 to RJ11 phone cord to make the connection between the phone socket (CON1) on the adaptor and the now-vacated socket on the phone. Now check that your phone still works. You should hear dial tone in the earpiece and you should hear your voice in the earpiece loudspeaker when speaking loudly into its microphone. If it does not work, check the connecting cords or indeed your soldering). In this test, the handset is connected straight through the adaptor so it should work normally. Now set your digital multimeter to read AC millivolts, lift the handset and check which two links on rows 1 to 4 have AC voltage on them. We measured up to 46mVAC with a Telstra Touchfone 400 connected and 23mV with a Sharp FO165 facsimile telephone. This is the dial tone signal across the earpiece loudspeaker. Disconnect the lead between the adaptor and the telephone and set your multimeter to read “ohms”. Now conwww.siliconchip.com.au nect the multimeter to the two links that showed the ACmV reading. There should be a scratching noise heard in the earpiece of the handset when the multimeter probes are connected and disconnected to these links. If this is so, these two links are the loudspeaker connections. Check the DC resistance across the loudspeaker, which will probably be between 100Ω to 150Ω. If it is lower than this, make a note of its value for later. Label one of the rows connecting to these links as LS and the other row as LS’. The other two rows are the microphone connections. Set your multi-meter to read DC Volts and connect the lead back into the telephone. Lift the handset and measure the voltage across the microphone links. If there is a DC voltage of around 1V to 6V, then the microphone is almost certainly an electret. Label the row with the positive voltage as MIC’ and the other row as MIC. If there is no DC voltage or very little voltage then the microphone is a dynamic type. Disconnect the lead to the telephone and measure the resistance across the microphone links. The resistance will probably be around 100Ω to 1000Ω. This indicates the microphone impedance. Check if there is any resistance between one of the microphone links and one of the loudspeaker links. If there is a low resistance between two of them, label this row of microphone links as MIC. Label the other row of microphone links as MIC’. If there is a high resistance, then simply label one link as MIC and the other as MIC’. These may need to be changed later if the microphone in the headset does not work. Once you know the connections, the links can be installed. The four PC connections to the left of S1 labelled MIC, LS, MIC’ and LS’ connect to the rows labelled the same. So MIC connects to the row marked MIC, LS connects to the row marked LS and so on. The links above S1 will need altering so that only the MIC and LS links are connected. Link connections to the right of S1 connect LS’ to the row marked LS’ and MIC’ to the row marked MIC’. We show three examples of how we assembled the PC board for three different telephones and these are shown in Figs.3, 4 & 5. We labelled www.siliconchip.com.au Parts List – Telephone Headset Adaptor 1 PC board coded 12107021, 79 x 49mm 1 plastic utility case, 82 x 54 x 31mm 1 front panel label, 79 x 50mm 1 monophonic hands-free headset with single 3.5mm stereo plug lead; Jaycar AA-2018 or equivalent 1 3-pole double-throw toggle switch (S1) 2 4P/4C RJ11 PC-mounting modular sockets; Jaycar PS-1470 or equivalent 1 3.5mm stereo switched PC-mounting socket 1 18 x 10 x 6mm ferrite toroidal core (Jaycar LO-1230 or equivalent) (T1) 1 3m 4P/4C telephone handset curly cord 1 22µF 50VW bipolar electrolytic capacitor OR 1 150Ω 0.25W 1% resistor 1 500mm length of 0.8mm tinned copper wire 1 8m length of 0.25mm enamelled copper wire 1 50mm cable tie Extra parts for powering electret microphone 1 AA cell holder 1 AA cell (1.5V) 1 22µF 16VW PC electrolytic capacitor 1 10kΩ 0.25W 1% resistor 1 1kΩ 0.25W 1% resistor 2 PC stakes 1 100mm length of red hookup wire 1 50mm length of black hookup wire each row with the MIC and LS designations after the measurements were made and the links were placed as shown. Note that Fig.3 and Fig.4 show connections for telephones that had electret microphones in the handset while Fig.5 shows a telephone which had a dynamic microphone. The 1.5V cell provides the power for the head- OR set electret when S1 is switched for headset use. Note that R2 is set at 10kΩ. This may need reducing in value if the signal from the electret headset microphone is found to be too high. This may become apparent during subsequent testing if your voice is too loud to the person being called. (You can reduce A close-up of the completed adaptor with front panel fitted. While the plastic screw-hole covers mar the appearance of the panel a little, you will need to gain access to the inside if you have a 1.5V battery. If you don’t need the battery, the panel could be glued over the screw holes and the covers left out – once everything is checked and working, of course! July 2002  15 R2 down to the a value as small as the DC resistance measured for the dynamic handset microphone). Winding the transformer The transformer is wound using 0.25mm enamelled copper wire on the toroidal core, with 200 turns for the primary and 100 turns for the secondary. The frequency response of the resulting transformer is quite good, in fact more than adequate for telephone work. Our prototype was reasonably flat from around 100Hz to above 20kHz. 12107021 TELEPHONE HEADSET MIC’ LS’ LS’ + MIC’ LS MIC (-) Full-size artwork for the PC board (above) and the front panel (below). These can also be downloaded from www.siliconchip.com.au PHONE TELEPHONE HEADSET ADAPTOR Wind the primary and secondary turns evenly distributed over the entire core (it doesn’t matter which order and they don’t have to be neat, side-by-side turns) and terminate into the holes provided on the PC board. The polarity of the windings also does not matter. The wire ends will require the insulation to be stripped off them before soldering. Some coatings can be removed with a hot soldering iron. Alternatively, use some fine grit sand paper to remove the coating. The transformer assembly is then secured with a cable tie. Place the PC board assembly in position over the case and mark out the cutout positions for the sockets. We cut the box with a fine-toothed hacksaw and broke out the pieces with pliers. The cutouts were then filed to shape. Only cut the holes to the depth of the sockets plus 2mm. Also, a slot is required in the side of the case for the 3.5mm socket. Test the PC board for fit into the case and adjust any of the cutout sides accordingly. We made up a rectangular piece of plastic salvaged from the socket cutouts to fill the slot above the 3.5mm socket once it is inserted in the box. The lid will require a hole for the toggle switch. You can use the front panel label as a guide to its positioning. Glue the front panel label to the case and cut out the holes with a sharp knife. Finally, you can test the headset adaptor on the telephone. Check that the volume is satisfactory and that the listener on the other telephone can hear. Switch between handset and headset to check that the levels are similar. If your telephone uses a dynamic microphone, the MIC and MIC’ links This is the Jaycar headset (AA2018) on which the project is based. Other headsets may be suitable but make sure the wiring to the 3.5mm stereo plug is the same. may need to be swapped so that the electret in the headset will work. Variations If the telephone and headset use a dynamic microphone, you do not need R1, R2, the 22µF polarised electrolytic capacitor and the 1.5V cell. Install link LK1. If the telephone uses an electret microphone in the handset and the headset is dynamic, you do not need R1, R2 and the 1.5V cell. However, the 22µF polarised capacitor will be required and it will need to be inserted with the opposite polarity to that shown on the PC board component SC overlay. Catering for different headset speaker impedances HEADSET 6 SILICON CHIP www.siliconchip.com.au HANDSET 16  Silicon Chip As presented, the toroidal speaker matching transformer for this project has 200 turns for the primary and 100 turns for the secondary. This provides a nominal 4:1 impedance transformation from the nominal 128Ω speaker in a typical telephone handset, down to the 32Ω speaker found in typical headsets. However, if the loudspeakers in your handset and headset have greatly different impedances to this, you can tailor the turns ratio to suit. To calculate the turns ratio required, divide the handset loudspeaker impedance by the headset loudspeaker impedance and take the square root of this value. For example, if the telephone handset loudspeaker impedance is 150Ω and the headset loudspeaker microphone is 16Ω, the turns ratio required will be the square root of 150/16 or about 3:1. For this ratio, we would suggest 240 turns for the primary and 80 turns for the secondary. www.siliconchip.com.au