Like all its counterparts, this network wiring tester comprises two elements, a transmitter unit, powered and fitted at the network start point, and a receiver unit, passive, which can be moved around from socket to socket. Each of these units carries eight LEDs, identically labeled 1 to 8. By operating a push-button in manual mode, or using a clock in automatic, the eight LEDs light up in sequence on the transmitter unit and obviously, they should do the same on the receiver unit. In this way, just by watching the LED lighting cycle on the receiver unit, you can immediately spot any crossed wires, as well as any open circuits (the relevant LED never lights up) or shorts (two or more LEDs light at the same time).
The transmitter unit circuit is simple. The Schmitt-input NAND gate IC1.A is wired as a multivibrator, whose speed can be adjusted using P1, while IC1.B is wired as a simple debounce circuit for button S2, used in manual mode. Switch S1 lets you apply the output of one or the other of these to the input of IC3, a decade counter IC, which here we force to count up to eight by connecting its Q8 output back to its reset input. Its outputs are not capable of driving LEDs, especially over wiring that is ‘dangerous’ for them (a short, for example), so a ULN2803 is used to drive the outputs. This integrated network of eight Darlington transistors, each capable of switching up to 500 mA, drives the eight LEDs fitted to the transmitter unit (D12–D19) and feeds its signals to the socket comprising contacts O1–O8, to which the wiring to be tested must be connected. At the other end of the cable, via the socket comprising contacts I1–I8, is the receiver unit which contains just eight LEDs (D20–D27) and their current limiting resistors. For the latter to work, there obviously needs to be a common connection between transmitter and receiver. In the case of screened network wiring, the screen can be used for this purpose. Another solution consists of using the earth wire of the electrical installation to fulfill the same function. But if neither of these solutions is feasible, then you’ll have to resign yourself to running a flying lead for this purpose.
The transmitter unit power supply is obtained from a ‘plugtop’ adapter supplying around 9 V at around 10 mA or so. The supply to IC1 and IC3 is regulated at 5 V, even though it’s not strictly necessary. For occasional short use, a 9 V battery could be used.
If the project is intended solely for testing network wiring, O1–O8 and I1–I8 will be in the form of RJ45 sockets and COM will be connected to their screening contact. Take care to stick to the same numbering for the LEDs on the transmitter and receiver units, and if the project is going to be used in automatic mode, that the LEDs are in the correct order.