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LINE G BER FOR TELEPHONES Don't put up with interruptions from extension phones. When you pick up the telephone, this simple circuit cuts the extension phone(s) dead. H ZHO~ ©3VAO£Z O!tl" By JOHN CLARKE & GREG SWAIN We all know what it's like to be in the middle of a phone call and have someone pick up an extension phone and start dialling. Or do you have young children in the household who like to mischievously listen in? Or maybe you operate a modem in parallel with your existing phone. Any interruption from an extension phone during transmission will result in garbled data, which is annoying to say the least. This simple circuit solves those problems. We've called it a "Line Grabber" because that's exactly what it does. When you pick up the phone it "grabs" the line and cuts the extension phones out. It makes it impossible for someone else to interrupt from an extension phone. 18 SILICON CHIP Any extension phone picked up while you have your phone off-hook will be completely dead and will remain so until your call is finished. If another phone is then picked up, that phone will then "grab" the line and all the other phones will be dead. As far as incoming phone calls are concerned, the same thing applies. The phones ring normally and if you pick up the phone first, you've got the call. The outside caller does not perceive any difference in the way the phone works though and nor will you, apart from the complete freedom from interruptions. The benefits are obvious: no more eavesdropping, no more unwanted dial tones or clicks on top of your conversation, and no more in- The single version is designed for installation inside the telephone, hut can also be built into a plastic case fitted with a telephone plug and socket. terruptions to data transmissions. With the Line Grabber, the phone line is exclusively yours until you hang up. Of course, if you want to transfer an incoming call to another extension that can still be done. Just hang up the phone you first took the call on and then pick up the extension - the incoming call will be there. A separate Line Grabber circuit must be built for each extension phone. That sounds messy but the circuit only uses a few cheap parts so the overall cost will not be high. We've produced two different versions so that you can choose the one that best suits your particular application. The first version consists of a single circuit built onto a small PCB. This version could be installed inside the telephone itself (one for each phone) and is used where the phones are plugged into different sockets. Alternatively, you could build this single Line Grabber into a zippy box, combined with a phone plug and socket. Either way, you will need two Line Grabber circuits for two telephones, three for three telephones, and so on. The second version has two Line Grabber circuits on a single PCB. This is housed in a small plastic case which is fitted with a telephone plug and two sockets. It is the logical choice where a telephone and a modem are operated in parallel from the same socket. Additional single Line Grabber circuits can then be installed inside other extension phones as required. Circuit details Fig.1 shows the circuit details for a two-telephone system. Each sec- The dual version has two Line Grabber circuits on a single printed circuit hoard. This is the version to go for where two telephones (or a telephone and a modem) are operated in parallel from the same socket. tion uses a bridge rectifier and an associated SCR (silicon controlled rectifier) to provide a DC and an AC path for the phone. Diodes Dl to D4 form the bridge rectifier for phone 1 while D6 to D9 form the rectifier for phone 2. Normally, when the phones are onhook, there is no DC path through either phone and therefore the two bridges and their associated SCRs are effectively out of circuit. Now let's take phone 1 off-hook. When this happens, a DC path is created through the phone. Current then flows via the bridge rectifier and SCRl and through the phone. This allows you to either answer an incoming call or dial for an outgoing call. Let's just look at that sequence in a little more detail. Normally, when a phone is on-hook, the voltage across the phone lines will be about 50V DC. When the phone is taken off-hook, DC voltage is applied via the bridge rectifier and LED 1 to the anode of SCRl. At the same time, current flows via the 100kQ resistor and zener diode D5 into the gate of SCRl. This gate current immediately causes SCRl to conduct and allows current to flow through the phone. So as far A 4x1N43R4 2 A/BN WHT 2 2 WHT WHT LINE IN 0.1 8250VACI PHONE 2 PHONE 1 GOBLUE mrn K AG PHONE LINE GRABBER KA G SC12·1·488 Fig.1: each section of the circuit uses a bridge rectifier and an SCR to provide a DC and an AC path for the phone. This diagram shows the details for a two-telephone system. MARCH 1988 19 -.~<.:..-~;,;-\,) PHONE ') , , C, '~ , ·,.co c ·; Fig. 2: parts layout for the single version of the Line Grabber. You will have to build one of these for each extension phone. The O.lµF capacitor is fitted to the first board only and is omitted from the rest. · 20 SILICON CHIP ··.•.,(.',\(. ) ;·,0:1 " ' ' c :.> 250VAC , · c, PHONE 2 Fig.3: follow this wiring diagram if you wish to build the dual version. It has two identical sections on the one PCB. The PCB for the dual version clips into the integral supports inside the case. The leads from the sockets and plug terminate on the back of the PCB. as phone 1 and the phone lines are concerned, everything is normal. There is an extra voltage drop across the bridge rectifier, LED 1 and SCRl of about 3.5V but that does not affect the operation of the phone. Now consider what happens when phone 2 is lifted? Normally, without the Line Grabber, someone would be able to listen in to your conversation on phone 1. However, with phone 1 off the hook, the voltage across the line is only about 10 to 12V. This means that no gate current can flow via 18V zener diode DlO to the gate of SCRZ and so there is no DC path to phone 2. C' As far as phone 2 is concerned, the line is dead. Phone 2 cannot be used until phone 1 is hung up. Similarly, if phone 2 is used first, it kills the line to phone 1. The LED associated with each SCR is illuminated when the associated phone grabs the line (because of current flow via the SCR). That sums up the operation of the Line Grabber as far as conventional telephones are concerned but there are still a few wrinkles to consider. What about electronic phones which always consume several microamps of DC when they are on-hook? The Line Grabber Fig.4: the Line Grabber circuit can also be built on Veroboard. Use an oversize drill to make the cuts in the copper tracks. caters for this situation by providing a DC path through the bridge rectifer, the l00kQ resistor, the zener diode and the 2.2kQ gate resistor of the SCR. This current is not sufficient to trigger the SCR but is does allow the circuitry in pushbutton phones to · function normally. The only remaining component to be explained is the 0.lµF 250VAC capacitor across the incoming line. This is intended to suppress transient voltages which could falsely trigger the SCRs. For example, if phone 1 is in use, a spike on the line could falsely trigger SCR2 if phone 2 happened to be off-hook. This would then place both phones on the line, which would defeat the purpose of the Line Grabber. While it won't be obvious from our circuit description above, an attractive feature of the Line Grabber is that it causes no loading of the phone lines when the phones are on-hook. This is good because it means you can build as many Line Grabbers as you want, without worrying about line loading. Building it First, you must decide which version you are going to build. If you build the dual version you will need the large printed circuit board which measures 60 x 46mm. The smaller version measures 38 x 46mm. Whichever board you use though, you will need the same case. We used a plastic jiffy box from Altronics r- PARTS LIST 1 PCB, code SC12-1 -488-1, 60 x 46mm (double version) 1 PCB, code SC12-1 -488-2, 38 x 46mm (single version) 1 plastic case, 83 x 54 x 28mm (Altronics Cat No H-0105) 1 telephone line plug 1 telephone line socket Semiconductors Fig.5: above are actual-size etching patterns for the two PCBs. =.-I~.{1{1 j.' Capacitors and Resistors f,/:J· PHONE LINE GRABBER PHONE 1 e e PHONE 2 Fig.6: full-size front panel artwork for the dual version. measuring 83 x 54 x 28mm (Cat No H-0105}. You can mount the phone sockets on the base of this box while the phone plug is wired to a flying lead. The PCB is wedged into the integral supports inside the case, as shown in the photo. The SCR is laid on its side as shown in the photos. The LED pokes out through a hole in the side of the case. You can also make the Line Grabber on Veroboard. We have shown a wiring layout for a single version, to fit in the same case. If you want the double version just build two on a wider section of Veroboard. Assembling the boards should present no problem at all. Just make sure that you observe correct 1 C106D silicon controlled rectifier 1 red light emitting diode 1 18V 400mW zener diode 4 1 N4004 silicon diodes 1 0.1 µ,F 250VAC metallised polycarbonate capacitor 1 1 OOkO 0.25W resistor 1 2.2k0 0.25W resistor 1 1000 0.25W resis tor Note: the above parts list is . for the single version only, unless noted. Grabbers you build you only need one 0.1µ,F 250VAC capacitor. When you're finished the board or boards, they need to be wired to the sockets and phone plug. The wiring is shown in the circuit diagram (Fig.1}. Checking it This is what the Veroboard version looks like. Take care - it's easy to make a mistake with Veroboard. polarity for the diodes, SCRs and LEDs. You can leave the LED[s) out if you wish. Just install a link in place of the 1000 resistor(s). Regardless of how many Line What About the Off-Hook Indicator? You may be wondering about the pros and cons of the Line Grabber as compared to the Off-Hook Indicator circuit published in the N~vemb~r 1987 issue of SILICON CHIP . The Line Grabber has advantages 1n that 1t is a simpler circuit and does not cause any loading of the phone lines . It also stops eavesdropping and you can build as many as you like. By comparison, the Off-Hook Indicator allows you to have more than one extension in use at a time, when you want to have a 3-way con~ers~tion . It also tells you when any extension is off the hook which 1s important. It is possible to check the circuit before you connect it to your phones. You'll need an adjustable DC power supply with an output of 20 volts or more. Connect the Line Grabber to the power supply in series with a limiting resistor of several hundred ohms. Polarity is not important because of the bridge rectifier. Initially, no current should flow. Now short the connections between pin 2 and pin 6 on the Line Grabber phone socket (for phone 1, if you've built a dual version). Wind up the supply voltage and observe that the LED is illuminated once the supply voltage rises above 18 volts. If the LED illuminates at quite low supply voltages it is possible that you have reversed the polarity of D5 (or DlO}, so that it is not working correctly as a zener diode. If current is flowing (you can continued on page 93 M A RCH 1988 21 Line Grabber for Phones sweep mode is also available. The GOS-522 has a genuine 20MHz bandwidth and has a 20 nanosecond/division sweep range to make timing measurements at high frequencies much easier. For further information on the GOS-522 contact your GW Instruments stockist or the Australian distributor, Emona Instruments, 86 Parramatta Road, Camperdown, NSW 2000. Phone (02) 519 3933. check this by measuring the voltage across the limiting resistor) but the LED is not illuminated, it is likely that the LED is installed the wrong way around. If you can't get the LED to illuminate for supply voltages above 20 volts, try shorting out the SCR. This will indicate whether the SCR and its associated components are faulty or not. Repeat these tests for your other Line Grabbers. You should be sure that they are working correctly before you connect them to the phone lines. A further wrinlcle There is another variation of the Line Grabber you could use if you Amateur Radio tronics Pty Ltd, 5 7 Vore Street, Silverwater, NSW 2141. Phone (02) 648 3488. continued from page 21 wanted to be clever. Say you had a phone which you use a lot and you don't want to fit the Line Grabber to it. That's OK. All you do is fit Line Grabbers to all the other extensions but not to your phone. This will allow your phone to grab the line at any time but if another extension is picked up before yours, you can still listen in. That could be useful in situations when another extension answers a call intended for you. ~ Acknowledgement: we thank Arista Electronics Pty Ltd for giving us the idea for this project. They will have a commercial version available shortly. Antennas physically long. Suitable only for UHF due to physical instability. Next month, we'll describe a few practical antennas that you can build yourself. Corrections In Table 1 on p.77, January 1988, the location of the VK4RAT transmitter should have been listed as Townsville (not Brisbane). Also, the vision input signal should read 426.25MHz (not 444.25MHz). Would readers also please note that the address of the Sydney ATV continued from page 71 Group is now 24 Larra St, Guildford, 2161, NSW. The repeater operates from 6.30-9pm on Mondays, Tuesdays and Thursdays, and from 12-5pm on Saturdays and Sundays. We thank the two readers who contacted us with the above information. Finally, the author would like to acknowledge the following amateurs who provided information on amateur TV for the January issue: VK2BTV, VK2ZZO, VK2AAK, VK3PC, VK3BFG, VK5AWA and VK5KG ~ High, Low, Sink & Source Strobe warning light This self-contained Xenon strobe light can be used as a warning beacon on boats or cars, as an attention-getter for shop displays, as a Christmas or party decoration, or as an external indicator for a domestic burglar alarm. It runs from 12V DC, battery or mains plugpack, and draws about 150mA. Flash rate is about two per second. The screw-on lens cap is available in red, orange or blue. The unit is weatherproof and has a screw mounting base. It retails for $35 from Arista outlets. source up to about 18 milliamps but depending on the output voltage it can sink only about one milliamp. Some logic circuits can sink a lot more current than they can source. The prime examples of this aTe TTL (transistor-transistor logic) devices which can typically sink about 25mA, or a lot more in the case of Schottky devices, when their outputs are low (ie, close to 0V) but can source virtually no current when their outputs are high (5V). These examples of devices which have unequal source and sink current capability invariably have output stages which are essentially non-symmetrical. In some cases, they may have open-collector out- continued from page 65 puts which means that they can sink quite a lot of current but can source no current at all unless they have an external "pull-up" resistor to pull their outputs high. Finally, before we leave this discussion, there is another definition of high and low which is relevant to comparators and logic circuitry. A signal is said to be high if it is high enough to cause a comparator or logic gate to change state. In this definition, high means above the positive threshold of the device's input. For example, in a logic circuit running at 15V, high may be any voltage above + 7.5V. Similarly, low many be any voltage below + 7.5V. ~ MARCH 1988 93