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Simple Cat 5 Network Tester

This circuit came from a need for a "quick and dirty" network tester that could be operated by one person. All the commercial units I tried required a person at the other end to check the remote LEDs, as the transmitters could not be made to cycle through the test continuously to allow one person to check both ends. It must be noted that this unit will only check for pair continuity, pair shorts, crossed wires, and shorts to other pairs. It will not test bandwidth, etc. Operation is fairly basic.


Half of the 4011 quad 2-input NAND gate is an RS flip-flop (IC1a, IC1b) which controls the other half, IC1c & IC1d, operating as a clock oscillator. You can either start and stop the oscillator running by pressing the Start and Stop switches or by virtue of diode D1 connected to pins 12 & 13, use the Stop switch to allow manual clocking of the 4017 counter. The 4017 drives one of eight LEDs and the lines to the RJ45 socket. An output "High" on the 4017 decides which line is under test, and if the circuit is complete, the test LEDs current is "sunk" by the 4017 and the LED will light.

If the corresponding test LED on the remote fails to light, then there is a short of that pair in the cable under test. If more than one LED lights, it indicates a short with another pair. A dark test LED on the transmitter indicates that pair is open circuit. "Start" starts the circuit cycling at a rate determined by the 470nF capacitor and 220kO resistor and "Stop/Step" stops cycling, steps through the lines, and when stepped so that no channel LEDs are alight, effectively switches the unit off with a standby drain current of less than a microamp.