# Brain tester.

Discussion in 'General Electronics Discussion' started by Shane barden, Nov 20, 2016.

1. ### Shane barden

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0
Nov 20, 2016
Hi guys
I'm new on here but need some help!
I'm currently working on a project and the electrician has rob the wrong cable to some hotel rooms.
It should be a 6 core to light up the three difrent LED's, they have basically run a two core.
Is there a way to make this work using resistors or a circuit?????

Thanks

2. ### duke37

5,364
769
Jan 9, 2011
No, not without a great deal of complexity and expence. Get her to fit the proper cable.

3. ### hevans1944Hop - AC8NS

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Jun 21, 2012
You can do a LOT of things with resistors and a circuit, but I am confused by your terminology. Does "6 core" mean six individual wires while "two core" means a single pair of wires? If so, the answer depends on how you want to "light up the three difrent LED's". Do these LEDs light up one at a time, with only one LED illuminated at a time? Or are there situations where one, two, or all three LEDs light up at the same time? Since you said "it should be a 6 core" circuit, I assume you intended to dedicate two wires for each LED and have the LEDs operate independently so that one, two, or three LEDs can be illuminated simultaneously and independently.

If that is so, you may have to resort to a complicated solution that involves coding and decoding the electrical "signal" applied to just two wires. Instead of doing that, I would first ask the electrician to come back and install two more wires for a total of four wires: one wire for common, one wire for each of the three LEDs.

If adding two more wires is impossible because of budget constraints or accessibility to install the additional wiring, you could encode eight different voltage levels on two wires using three binary-weighted switches. A simple op-amp circuit that sums the three switch closures from a constant-voltage source through binary-weighted resistors would then (for example) produce output voltages of zero, 1, 2, 3, 4, 5, 6, or 7 volts, representing switch closures combinations of 000, 001, 010, 011, 100, 101, 110, and 111. At the other end you would need to "decode" these eight voltages into three LED illumination commands. All this is way more complicated than adding two more wires, but you do what ya gotta do to make things work... even if the result is a kludge. Been there, done that, try not to do it again.

4. ### AnalogKid

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Jun 10, 2015
Power, signal discrimination, and LED drive with only two wires? Yes, certainly possible, but not without some electronics at each 3-LED location and corresponding electronics at the driving point.

What is the current through each LED?

Are the 3 LEDs a) mutually exclusive (only one at a time is on); or b) are all 7 combinations valid? If a (and the LED current is low), this can be done with one chip like an LM339 and a few resistors and small signal diodes.

Without any other information, one approach is to send out over the two wires 3 different DC levels, one for each LED. A quad comparator chip at each location detects the levels for each LED and drives it. This simple approach works only if the LEDs are mutually exclusive. If not, and you need to send 7 DC voltage levels to effect all possible LED combinations, that still is not terribly difficult, and stepped DC is way less complicated that a 3-bit serial data system. For example, assume a 12 V system. Each integer voltage from 5 V to 11 V represents one 3-LED combination. Everything below 5 is all-off, and everything above 11 is all-on. The 5 V minimum assures enough voltage to power the comparator chips and make valid determinations.

If you are up for some firmware development, a PIC at each location can implement any of the above for very low cost and low circuit complexity.

No matter what, something at the head end has to take the LED commands and turn them into the transport protocol.

How many LED locations?

What generates the LED commands?

How are the LEDs mounted?

This could shake down to about \$1 additional parts cost (plus the bare pc board cost, which could be 5x that depending on quantity) per location for the receiver/drivers.

ak

hevans1944 likes this.