I've been trying to make this cable tester work, the whole 555 side works perfectly. But then the 4017 just doesn't do anything, there is continuity with both VCC and GND and there are no short circuits, but the leds don't work and I don't understand why

 

Without a plug in the RJ45 socket there is no conduction path for the LEDs to ground or Vcc. Your schematic should show the plug and associated circuit for completeness.

 

Your circuit is incomplete. What is connected to the other side of the network cable?

 

Your circuit is incomplete. What is connected to the other side of the network cable?

 

You do not say what the schematic in post #4 is. My GUESS would be that it is the remote end of the tester. If this is so then as it is drawn it will not work. I can understand how the design should work. I think if you connect all the cathodes of the 1N4148s together it should work. The diodes will provide the return current path for the LEDs.

Les.

 

What I meant was is the other side of the cable have all their terminals connected to Gnd (0V)
Otherwise the LEDs have no complete path for the current to flow through them.

 

Hi absf, Yesterday I said if the cathodes of all the 1N4148 in the remote unit are connected together the the device should work. Today when I started to write the explanation I realised that in the remote unit all the LEDs and diodes are shown with the polarity reversed. So if they were connected with the correct polarity (Now with all the anodes connected together.) the selected LED would get its negative return via the diodes for all the non selected LEDs in series with the diodes with the non selected LEDs in the main unit. This is because all of the non active outputs of the 4017 are at ground potential.
I think it is quite a clever design.

Les.