I'm interested in developing a passively-powered circuit that can distinguish Vcc from Vcc with a pull-up. The scenario is this: I have a 4 conductor cable, and I want to make sure it is wired correctly. The four conductors carry: Vcc (+5), two input lines and ground. The input lines are pulled-up to Vcc via >2K resistors.

I could just check resistances with a multi-meter to determine which lines are Vcc, GND and the inputs. However, I'd like to make a circuit I can plug the cable into and have it give me a visual indication of whether or not it is wired correctly. For instance, I can verify that the GND conductor is correct by placing an LED across the assumed "Vcc" and "GND" conductors, and it will light up only if GND is really GND.

So now I only have to distinguish Vcc from Vcc with a pull-up. Any ideas? Also, I'd like the solution to be powered by the cable (i.e. not require its own power.)

Update: I think I just came up with a solution... use 3 LEDS connected as follows:

A ---R--|>|--------R--- D
A ---R--|>|--- B ---R--- D
A ---R--|>|--- C ---R--- D

A, B, C and D are the conductors. R is a 1K resistor and -|>|- is an LED. When all three LEDs light up, then A is Vcc, D is GND and B, C are the signal lines, and this should be the only way all the LEDs can get lit. Also, the resistors provide short-circuit protection.

 

That would work but you might want to consider bi-colour LEDs which glow red when the current flows in one direction and green when it flows in the other direction.

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Well, it almost worked. I had to add a few diodes:

A ---R--|>|--------R--- D
A ---R--|>|--- B ---R--|>|-- D
A ---R--|>|--- C ---R--|>|-- D

Otherwise, if A if Vcc and B is GND, all three LEDs would light up.

This was an interesting project, and it would make a good 'extra-point' question on a homework set.

 

Which are the LEDs and which are the diodes?

You don't need any diodes, since LEDs can easilly hold-off a reverse voltage of 5V.

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The ones near the A terminal are the leds. I had to add the diodes between B,D and C,D because if A is Vcc and B is GND, the current would find a return path through D, i.e.:
and I want to ensure that D is at a lower potential than B. Those sneaky electrons!