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This Cable and Wiring Tester has a row of four LEDs to indicate the condition of a pair of wires: open circuit, short, reversed and good. A diode is hooked across the far end of the wire pair to assist the test which is done automatically as soon as you press the button. By LEON WILLIAMS Here’s an easy to build and simple to use tester that will prove indispensable to anyone involved in the installation or maintenance of cables or wiring systems. Small enough to carry in a pocket, the tester employs four LEDs to speedily indicate the health of a pair of wires. Tracing faults in cables, especially those in large build­ings can be very difficult if you are working on your own. If you have a partner and some form of communication, you can use a multimeter set to measure resistance at your end while you get your partner to apply a short circuit and then remove it. With the short removed the meter should show an infinite resistance, and with the short applied a low resistance. This is obviously difficult on your own, as you would have to travel between ends to place the short and remove it in bet­ween taking readings 14 with the meter. Thankfully there’s an easier way. Diode testing A technique that has been used for a long time to test cable pairs is to place a diode across the A and B wires of the pair at the remote end. When a meter is placed across the pair at the local end, a low resistance will be obtained with the meter leads connected one way and a very high (ideally infinite) re­ sistance with the leads reversed. This happens because the diode only passes current in one direction; ie, when the anode is more positive than Silicon Chip’s Electronics TestBench the cathode by about 0.7V. A big advantage of the diode test is that fault conditions such as a short or open circuit can be diagnosed quickly. If a pair has a short circuit somewhere along its length, a low re­sistance will be seen when the meter is connected either way. Conversely an open circuit will show an infinite resistance with the meter connected either way. The diode test will also show a reversed pair; ie, where the A and B wires get crossed along the way, as the low resist­ ance/high resistance results will be opposite to those for a good pair. This goes to prove that the best ideas are sometimes the simplest. Fig.1 illustrates the four common pair combinations and the results obtained. Easy to use Carrying around an expensive multimeter, continuously turn­ing it on and off and reversing the leads to test pairs is tedi­ous. With the Cable and Wiring tester all you have to do is connect the two test leads to the pair under test and press the Test button. The tester will automatically test the pair and display the result on one of four LEDs. The orange LED (O) will flash to indicate an open circuit and the yellow LED (S) will flash if the pair is short circuit. A pair that is reversed will cause the red LED (R) to flash, while a pair that is in good condition will cause the green LED (G) to flash. Of course you will need to connect the diode at the other end of the pair you are testing. Circuit description The Cable and Wiring tester works just like the manual diode testing shown in Fig.1 but it does it automatically in two phases before it displays the result. I will refer to these as phase 1 and phase 2. Fig.2 shows the circuit. An oscillator is formed with IC2c, one section of a 40106 hex Schmitt trigger inverter, a 330kΩ resistor and a 0.22µF capacitor. It produces a square wave output with a frequency of about 20Hz. IC2d, a 100kΩ resistor and a 0.1µF capacitor form a delay circuit. The output at pin 10 of IC2d is a delayed and inverted replica of the output from IC2c. The reason for the delay circuit is to separate the sample and display pulses from the unstable periods when the analog switches are swapping the Fig.1: this series of the diagrams illustrates the method of testing a cable pair with a multimeter and a diode connected to the far end. The Cable and Wiring Tester runs through these tests automatically. polarity of the line. The oscillator controls the overall operation of the tester and when its output is low, it is in phase 1, and when its output is high, it is in phase 2. In phase 1, analog switch IC1a connects wire A of the pair to pin 1 of IC2a, while IC1b connects wire B to ground. If the pair is good (ie, not reversed) and the diode is connected Fig.2: the Cable and Wiring Tester works by alternately applying DC voltage to a cable pair in one direction and then the other. The four possible conditions are indicated by the LEDs. Silicon Chip’s Electronics TestBench  15 Fig.3 (left): the component layout for the PC board. Take care to ensure that all polarised parts are correctly orientated. Fig.4 (below): this is the actual size artwork for the PC board. ent on the wires being tested, most likely in the form of static charges, each input is protected with a series 680Ω resistor and a 9.1V zener diode. A .001µF capacitor is also connected between the two inputs to shunt any RF signals that might otherwise be picked up by the wires under test. The tester operates from a standard 9V battery which should last quite a long time. Note that the Test switch is also the power switch and is connected in the negative supply lead instead of the positive supply lead as is normal practice. This was done simply because it made the PC board layout easier. Construction with its cathode to wire A, no current will flow through this circuit and pin 1 of IC2a will be pulled high by the 4.7kΩ resistor. If the pair is short circuit or the diode is connected in reverse, current will flow and pin 1 of IC2a will be pulled to ground. Assuming that all is well, pin 2 of IC2a will be low. IC2f, a 33kΩ resistor and a 0.1µF capacitor form a mono­ stable which produces a narrow negative pulse when the output of IC2d goes high, which is only within phase 1. The negative pulse from IC2f closes analog switch IC1c and charges the 0.22µF “memory” capacitor connected to pin 12 of IC1c to the voltage present at pin 2 of IC2a. When the pulse ends, the gate opens but the charge on the capacitor remains as the only discharge path is via the very high input impedance of inverter IC2b. The high output of IC2b is applied directly to the B input of the 4028 BCD-to-decimal decod­er IC3. During phase 2 the states of IC1a & IC1b are reversed and wire A is connected to ground while wire B is connected to pin 1 of IC2a. With a good pair, current will flow through 16 the circuit and pin 1 of IC2a will be pulled to ground. The output (pin 2) of IC2a is connected directly to the A input of IC3. IC2e, a 0.1µF capacitor and a 100kΩ resistor form a monostable which produces a positive-going pulse when the output of IC2d goes low, which is only during phase 2. This pulse is applied to the C input of IC3 and effectively becomes an enable input, as with this input low none of the LEDs can be selected. One of the LEDs will be turned on when the C input is high, depending on the state of the A and B inputs. Note that the D input is permanently connected to ground. With a good pair, both A and B will be high. The LEDs are only turned on for the period of the pulse from IC2e which has the added benefit that the current drain from the batteries is less than if a LED was on constantly. In summary, the result of phase 1 is stored in the memory capacitor until the result of phase 2 is available, at which point they are both applied to the decoder and the respective LED is turned on. Since high voltages could be pres- Silicon Chip’s Electronics TestBench The Cable and Wiring Tester is mounted in a small plastic case with a row of four LEDs and a pushbutton on top. At one end is a 3.5mm jack socket to enable connection to a pair of wires. Pressing the button flashes one of the four LEDs depending in the test condition: Open (Orange); Short (Yellow); Reversed (Red); and Good (green). All the components apart from the test socket are mounted on a single-sided PC board. Fig.3 shows the wiring diagram. Begin construction by soldering in the five tinned copper wire links, ensuring that they are straight and lay flat on the board. Follow this with the resistors, the zener diodes and the PC stakes. Next, solder in the capacitors, remembering that the 22µF capacitor is polarised and must be inserted the right way. The integrated circuits can be installed next, ensuring that they are in the correct way. These are CMOS types and can be destroyed by static electricity, so earth yourself and take care not to handle them too much. The LEDs are installed with the top of each LED 25mm above the PC board. They should protrude slightly from the lid of the case when it is fitted. Similarly, the pushbutton switch is installed in a vertical position by soldering its tags to two PC stakes. Again, the switch should be at the correct height with the case closed. Install the PC board in the bottom case half with four self-tapping screws. If you find it won’t sit properly, you can lightly file the edge of the board or cut out the small plastic tabs inside the edge of the case. Drill a hole in the centre of the top endplate and mount the 3.5mm test socket. Place the two The four LEDs and the pushbutton switch are stood off the board so that they protrude through the lid of the case. end plates in the slots on the bottom half of the case. The bottom half has four holes for the case mounting screws while the top half has threaded brass inserts. Solder two wires from the socket to the PC stakes on the PC board. Now solder in the battery clip and trim the length of the wires so that they sit neatly with the battery positioned as shown in the photographs. You may find it necessary to cut off some of the plastic tabs on the inside of the top half to clear the battery clip when the two halves are screwed together. Drill the four holes for the LEDs and for the test switch in the top half of the case. The positions for these can be quite easily found by firstly making measurements with a ruler and then mark­ing with a pencil before drilling. The test lead is made from a short length of figure-8 cable. The type used in the prototype was coloured red and black. I soldered the red A wire to the centre pin of the 3.5mm plug and the Parts List 1 PC board, code 04411971, 51 x 88mm 1 plastic case, 120 x 60 x 30mm 1 3.5mm mono phono socket 1 3.5mm mono phono plug 2 small black alligator clips 2 small red alligator clips 1 normally open pushbutton switch 6 PC stakes 1 9 volt battery clip 4 No. 4 x 6mm self-tapper screws 1 5mm red LED (LED1) 1 5mm yellow LED (LED2) 1 5mm orange LED (LED3) 1 5mm green LED (LED4) Semiconductors 1 4053 triple analog selector (IC1) 1 40106 or 74C14 hex Schmitt trigger (IC2) 1 4028 BCD-to-decimal decoder (IC3) 2 9.1V 1W zener diodes (ZD1,ZD2) 1 1N4004 diode (remote test diode) Resistors (0.25W, 1%) 1 330kΩ 1 4.7kΩ 2 100kΩ 6 680Ω 1 33kΩ Capacitors 1 22µF 16VW electrolytic 2 0.22µF MKT polyester 3 0.1µF MKT polyester 1 .01µF MKT polyester 1 .001µF MKT polyester Miscellaneous Tinned copper wire, hookup wire, figure-8 cable, small piece of scrap stripboard, heatshrink tubing Resistor Colour Codes ❏ No. ❏  1 ❏  2 ❏  1 ❏  1 ❏  6 Value 330kΩ 100kΩ 33kΩ 4.7kΩ 680Ω 4-Band Code (1%) orange orange yellow brown brown black yellow brown orange orange orange brown yellow violet red brown blue grey brown brown 5-Band Code (1%) orange orange black orange brown brown black black orange brown orange orange black red brown yellow violet black brown brown blue grey black black brown Silicon Chip’s Electronics TestBench  17 to­gether. Now clip the tester leads to the diode leads, with the red A wire clips connected together and the black B wire clips connected together. Press the Test button and verify that the “G” LED flashes. Now reverse the connection to the diode leads, press the Test button and check that the “R” LED flashes. Once you are happy with the testing, screw the case together with the four screws supplied, checking that the drilled holes line up with the LEDs and switch without placing stress on them. Using the tester The basic operation of the tester should be quite apparent. Simply connect the diode to the remote end with the red clip connected to the A wire, the tester to the local end with the red clip connected to the A wire, press the Test button and monitor the LEDs. Multiple wire cables Another view of the prototype Cable & Wiring Tester. Power comes from an internal 9V battery. black B wire to the ground pin. To finish the lead, solder a red alligator clip to the red wire and a black alligator clip to the black wire. The diode assembly can be made next. It simply comprises a diode soldered to a length of figure-8 cable as before. Its anode is soldered to the black wire and the cathode to the red wire. I used a scrap piece of strip board to give the assembly some mechanical strength and then covered it with heatshrink sleeving to prevent accidental shorting. The red alligator clip is sol­dered to the red wire and the black alligator clip to the black 18 wire. Finally, fit a good 9V battery into the case. Testing With the assembly complete, press the test switch briefly and check that the “O” LED flashes at about 20Hz. If it does, you can proceed with the rest of the testing. If it doesn’t work, have a good look at the assembly again and check it for construction errors. Plug the test lead into the socket and connect the two alligator clips together. Press the Test button and check that the “S” LED flashes to indicate that the wires are shorted Silicon Chip’s Electronics TestBench So far this article has referred to just testing a pair of wires, such as those in a telephone cable or Local Area Network (LAN) cabling. However, the tester can be used to test cables with multiple wires even if they are not paired. The simplest way is to select one of the wires as a common A wire and then progress through the other wires as a second B wire. If you are working on cable that has, for example three pairs, you might construct a multiple diode lead with three diodes and six leads so that you could check all the pairs at one time. Some cabling systems use a special socket to terminate a multiple pair cable. An example of this is an RJ45 socket used in modern building cabling where four pairs provide computer and telephone connections at one socket. A plug could be adapted to hold four diodes and plugged into the remote socket while the tester could be plugged into a mating socket at the local end. A switch would need to be incorporated in the tester leads to select the pair to be tested. Finally the tester can be used as a general continuity tester to test diodes, speakers, audio/video cables, etc. The tester will indicate a short circuit with about 2kΩ or less placed across the test leads but this will vary from unit to unit and is dependent mainly on the character­istics SC of the ICs used.