Fullscreen HTML5Med Res (??MB)HTML4

Slow ADSL speeds? It could be your phone line polarity! Phone Line Polarity Checker Do you suspect your broadband speed (DSL) is slower than it should be? It could be as simple as incorrect polarity in your phone wall socket. Build this very cheap, very simple device to find out whether you need to change your wiring! I t might not seem that telephone line polarity is important, since the ring and voice signals sent over telephone lines are AC. However the lines are actually biased to 48V DC (less when in use) and so the polarity can matter. The main problem with incorrect polarity is that some DSL (Digital Subscriber Line) modems and routers can perform poorly in this circumstance. Since telephone wires are colour-coded, it should be possible to simply check that the sockets are wired correctly. Unfortunately, there are multiple wiring colour schemes and they have changed over time. Believe it or not, the old colour scheme (from around 15 years ago) is identical to the current colour scheme except that the polarity of both lines is reversed! This is why so many homes have this problem and yours may well be one of them. as their power requirements are far in excess of what the telephone line can deliver. (As an aside, that is the reason it is important to keep a line-powered telephone in your home so you can still make and receive calls if the mains power goes out. Telephone exchanges can usually supply power from their backup batteries for up to some days, even if they are blacked out). Usually, telephone lines are run with 4-core cable. This allows up to two lines on the one cable. The first line is on the inner pair (pins 2 and 3) and the second line, if present, is on the outer pair (pins 1 and 4). Modern telephones use modular plugs, specifically RJ11 (6P2C, one line), RJ14 (6P4C, one or two lines) or RJ25 (6P6C, 1-3 lines). By the way, 6P4C stands for “six pins, four connectors”. Incidentally, “RJ12” connectors are physically compatible – and commonly available – so that is what we have used in this project. Because modern phones rectify the DC voltage from the Telephone line basics telephone lines before regulating it and because the ring The common telephone line is simply a copper pair, ie, and voice signals are AC, for voice communications the two wires. As mentioned, there is usually a 48V DC bias polarity doesn’t really matter. For a number of reasons presumably due to the details of across the pair which drops to around 8V when a telephone is “off-hook”. The ring voltage (around 90V AC) and the the DSL chipset or line interface implementation, certain audio signal voltage (also AC) are overlaid on this DC bias. ADSL modems work much better when the polarity is corThe DC power is “rectified” by each telephone on that line rect. Measurements comparing correct and incorrect polarity connections to the same modem show to run its own circuitry. Note, though, that this does not include cordless Design by David Drane striking differences in performance. In one case, with the incorrect polarity phones which usually use a plugpack, 86  Silicon Chip siliconchip.com.au RJ12 CONNECTOR 6 5 4 3 2 1 YELLOW 12k A LED2 GREEN BLACK LED4 RED 12k  K LED1 K A K A  LED3 LINE POLARITY CHECKER  K A SC TELEPHONE 2011  K A LEDS Fig.1: the circuit diagram – with just two resistors and four LEDs, it could hardly be simpler. amount of current under typical conditions. With all the telephones on that line on-hook, we expect 48V DC and so around 4mA flows through each forwardbiased LED, lighting it up quite nicely. If the LED lights dimly, this either means that one or more handset is off the hook or else that the line voltage is low (usually due to a fault). Building it (as the house was originally wired), the modem reported a very poor signal-to-noise ratio of 6dB and a line attenuation of 78dB, resulting in a downlink speed of just 800kbit/s. Correcting the polarity of the line resulted in the download speed going up to the full 1500kbit/s (limited at the exchange), with the signal-to-noise ratio improving to 34dB and the attenuation down to 45dB. This is just one example; this solution has been successfully tested many times in the field. The speed, signal-to-noise ratio and attenuation figures can be read off most modem/routers by accessing their web interface (see the documentation provided with your modem/router for more details). If your speed is well below what it should be, it’s possible that it is due to swapped line polarity. Obviously there can be many other reasons but this simple check at least makes sure your line is not the culprit! Of course, you can check the line polarity using a multimeter but it is a very fiddly process as it’s hard to make contact with the modular connector pins using standard probes. It’s also hard to figure out which polarity is correct. This project solves that entirely. Simply plug this unit in, using a standard telephone cable and the LEDs immediately indicate the polarity of the one or two telephone lines. How it works The circuit, shown in Fig.1, is very simple. The polarity is correct if pin 2 is at a positive voltage relative to pin 5 and pin 4 is at a positive voltage relative to pin 3. To check this, for each line we connect two LEDs of different colours (green and red) in inverse parallel, ie, anode to cathode. They are then connected to the lines, with a 12kΩ current-limiting resistor, so that the green LED will be forward biased (and the red one reverse-biased) if the line polarity is correct and vice versa if it is swapped. This results in the green LED for each line lighting if its polarity is correct and the red if it isn’t. The 12kΩ resistor value is chosen to give a reasonable siliconchip.com.au Before building the board, decide how you are going to house it. We mounted ours in a small, translucent UB5 jiffy box. In this case, the LEDs do not need to protrude through holes in the case as they can be seen through the lid. If you use an opaque UB5 jiffy box you will need to mount the LEDs higher, so they can protrude through appropriately positioned holes. Other arrangements are possible but we will leave the details up to you. Check the copper side of the PCB for defects and repair if necessary. If you are going to mount the board in a jiffy box and the corner cut-outs have not already been made, cut and file the board to the shape shown so that it will clear the pillars in the box. Refer now to Fig.2, the overlay diagram. Solder the two resistors in place and trim the leads. The resistors should have a colour code of brown black orange brown (4-band code) or brown black black red brown (5band code). Then install the modular connector. Position its pins over their mounting holes and then push it down fully onto the board, so that the plastic posts lock. If they will not fit then you may need to slightly enlarge the holes that they go in. Once it is flush with the board, solder the six pins. Now install the LEDs. Be careful to ensure that the anodes (longer leads) go into the holes marked with “A” on the overlay diagram. If you are using a translucent jiffy box, the Parts list – Phone Line Polarity Checker 1 PC board, coded 12105111, 56.5 x 48.5mm 1 6P6C RJ-12 PCB-mount modular socket (Jaycar PS1474, Altronics P1425) 2 red 3mm LEDs (LED2, LED4) 2 green 3mm LEDs (LED1, LED3) 1 UB5 jiffy box (preferably clear/translucent) 1 spacer, 21mm long (see text) 1 label (optional but recommended) 2 12k 0.25W resistors (any tolerance OK) May 2011  87 RJ12 CONNECTOR Fig.2 (left) same-size component overlay, along with a matching photo at right. With so few INNER PAIR OUTER PAIR components you could 12k hardly go wrong, unless LED4 LED3 LED2 LED1 5 you get the LEDs back to A A A A 2 front or around the wrong way, or bend a pin on the 12k RJ12 connector, or make a PHONE REK CE HLINE C YTIPOLARITY RAL OP E NCHECKER IL E N O HP dry joint, or . . . 12105111 11150121 © 1 12011 02 © SC top of each LED should be flush with the top of the modular connector. Otherwise, the flat section at the base of each LED lens should be flush with the top of that connector. Once you have trimmed the leads, you can still check that the LED orientation is correct by looking for the flat side of the lens. In each case, this should face away from the “A” on the overlay diagram as it indicates the cathode side. Housing it If you are using a UB5 jiffy box, the completed PCB should snap into the indentations moulded into the inner columns of the case. You will then need to cut a notch in the end for the modular socket to protrude through and this is most easily done using a nibbling tool. Make the hole slightly too small then use a needle file to finish it off. Because the board is shorter than the box, you will need to prevent it from moving when a modular plug is inserted. We achieved this by making a spacer which fits exactly between the end of the board and the box, then used hot melt glue to hold it in place. The spacer can be made from practically any rigid material – we cut a small length off some surplus plastic DIP tubing which cannot be seen once the translucent lid is in place. Our spacer measured 21mm long. You could also simply glue the board into the box although this may eventually fail due to the stress from repeated connector insertions and removals. If your box is opaque then you will need to drill four Here you can see the cutout for the RJ-12 socket (front of pic) along with the scrap of plastic (actually a piece of DIP IC rail) we glued into the bottom of the case to stop the PC board moving within the case. 3mm holes in the lid to accommodate the LEDs as it is screwed in place. A photocopy or print-out of the PCB pattern can be used as a drilling template if carefully positioned on the lid to match where the board is mounted. It’s probably a good idea to glue a label to the lid so that you can remember which LEDs mean what. You can use a photocopy of Fig.3 or print it out from the SILICON CHIP website. These days, we use our trusty hot laminator to make our labels virtually indestructible; certainly impervious to fingerprints and junk-box dirt. Of course, you’ll need to punch some holes through the label (even if using a translucent jiffy box) so the LEDs can be seen glowing underneath. We recently purchased a set of ten leather punches on eBay for the princely sum of $5.00 – and they’re perfect for punching holes in laminated paper labels. Using it It doesn’t get much easier than this. Simply plug the telephone line in question into the unit and observe the colour of the LEDs. If possible, use the same cable used to connect the equipment to the line in case the cable itself is incorrect (unlikely but not impossible). One of the “inner pair” LEDs should light and if there is a second line present, one of the “outer pair” LEDs will light too. Green means the polarity is correct and red means that it is swapped. If the green and red LEDs in a pair are rapidly flickering, PHONE LINE POLARITY CHECKER     LINE INNER OUTER SILICON CHIP www.siliconchip.com.au Fig.3: as well as being a label in its own right, you can photocopy (or download) this front panel label to use as a drilling template if you aren’t using a translucent case. You’ll also need to punch out the four circles so you can see the LEDs, regardless of which type of case you use. 88  Silicon Chip siliconchip.com.au Here’s how the PCB “snaps” into place in the slots on the case moulding . . . plug the phone back in because that indicates someone is ringing you! If the LEDs are dim, the line may be in use. If it is not then the line is probably faulty, resulting in a low line voltage. You can confirm by measuring with a multimeter; if the voltage is below 40VDC then you have a line problem. This is often caused by water getting into the junction pit which is generally outside the premises. In some rare cases, it’s possible that the line voltage is excessively high and this will be indicated by the LEDs being brighter than usual. The 12kΩ current-limiting resistors are high enough that even if the line voltage is double what it should be, no damage will occur. siliconchip.com.au And here’s how it looks complete with the front panel glued on, with cutouts so you can see the LEDs underneath. When the polarity is wrong, it’s simply a matter of rewiring the connector to swap the offending pair or pairs. Modular wall sockets usually use “punchdown” type termination at the rear and in this case, it’s simply a matter of pulling the wires out of their slots, rearranging them to be correct and then using a punchdown tool to push them back in. This tool strips the insulation and “punches” the wire into the socket to make the electrical connection. (Dare we say it? We also bought a brand new punchdown tool on eBay for $2.00 including postage!) If your socket has a different termination style (eg, screw terminals), the principle is the same. Swap the offending pair(s) and then check that the polarity is correct. It should be plain sailing after that. SC May 2011  89