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Circuit of line continuity tester for Ethernet and serial cables with RJ45 and RJ11 connectors. Contain master unit with line signal generator (left side) and remote receiver (vertical LED row with 3 diodes at right side). Sequentialy lit LEDs in line order 1,2,3,4,5,6,7,8,G (G for shielded cables) in direction upside down. Has normal and slow speed modes.
Circuit has 555 timer IC based astable mutivibrator as clock generator, 4017 digital counter as sequential line signal activator and transmitter line LEDs with few reverse current diodes for most used lines in master unit. Receiver unit has another row of LEDs with reverse current diodes. Line LEDs are lit because of voltage difference between high logic level voltage in active line and low logic level voltage at remaining lines through reverse current diodes. Which eliminate need for separate ground connection with master unit at receiver side. Speed modes are set by choosing one of lower two resistors between 555 IC pins 7 and 2,6 with a switch. Particular 4017 counter in this circuit is CD4017xx that can work directly from 9V battery and has power voltage (9V here) as logic high.
Due to EveryCircuit shortcomings this circuit doesn't include 4017 counter reset on power cycle, without it real life tester will begin counting from random line. To avoid that, use upper circuit from https://everycircuit.com/circuit/5179569178673152 where junction between capacitor and 100 kOm resistor is connected to 4017 pin 15 (RESET). Also 4017 pin 11 is connected to pin 15 (reset on end of line countdown) and pin 13 is connected to ground. 100 Ohm resistors in signal lines are optional - usually voltage drop in a cable is large enough to eliminate need of separate resistors.
This same circuit with few modifications is used in gazillion of cheap RJ45 cable testers worldwide. Some use even simpler clock generators like a pair of NAND gates or discrete components. This particular circuit was created after fried LogiLink WZ0010 tester rebuilding where original digital parts are made as COB and buried under epoxy with speed mode switch resistors in parallel.
NB: 4017 circuit is borrowed from https://everycircuit.com/circuit/5427772071346176
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