I'm trying to convert a signal coming from an electronic controller. the controller has 2 internal switches (relays) and resistors I cannot acces.
Depending on which relay is active I would like to light one or the other LED.
Unfortunately I can't figure it out myself. I tried to build something from transistors and comperators, but it did't work

All help or ideas are welcome .

 

Sounds like schoolwork.

 

Sounds like schoolwork.

hahah left school 20 year ago. I guess I should have paid more attention in class then.

 

If you can measure the current in each status of relays (maybe with a shunt resistor), then you can specify which relay(s) is ON.

 

Is it possible to interchange the two switches?

 

there is no possibility to physically acces the relays, nor to interchange the two switches

 

It is an boring problem if you just brute force it, more interesting if you try to come up with a simple, clever solution.

Can you come up with a set of comparisons that will tell you which of the four states it is in? After you do that it is simple.

A brute force solution would have 4 outputs from the comparators, only one of which is ever high. Can you do that much?

 

hi,
Is the Green arrowed line normally to 0V or some other point?
E

 

Only if the output from the two resistors has no other connections:
An opamp can be used as a comparator to light a white LED when the input current is 6mA (from the 2k resistor) or light a green LED when the current is 12mA (from the1k resistor).

 

hi,
Is the Green arrowed line normally to 0V or some other point?
E

It is clear from the diagram. It is floating is neither switch is on.

 

Hi,
If the Relays are in a box, powered from +12V where is the return line.??
How can it be 'floating'
E

 

A small MCU with an ADC could do this pretty easily—unless I am missing something, which is possible.

 

A small MCU with an ADC could do this pretty easily—unless I am missing something, which is possible.

I had assumed that TS wanted a solution using comparators and logic gates, not MCU.

 

I had assumed that TS wanted a solution using comparators and logic gates, not MCU.

Well, he said he failed doing it that way but I didn’t read it as a specification. The ATTiny85 would do it, it has 4 10-bit DACs and the parts count would certainly be very low.

 

hi,
Is the Green arrowed line normally to 0V or some other point?
E

it was originally connected to 1 led, that had 2 brightness levels depending on the which switch is active. As the brightness difference was hard to see, I decided to use 2 leds instead

 

I had assumed that TS wanted a solution using comparators and logic gates, not MCU.

yes I would like to do it with logic components. I found a solution using a comperator, but this requires a reference voltage from the source. My goal is to avoid running an extra wire from the battery, as it not easy to run an extra wire to this location.

The goal is to only use the available chassis ground and the output wire from the red box.

 

yes I would like to do it with logic components. I found a solution using a comperator, but this requires a reference voltage from the source. My goal is to avoid running an extra wire from the battery, as it not easy to run an extra wire to this location.

The goal is to only use the available chassis ground and the output wire from the red box.

Of course, there must be a return wire to the battery.

 

i don't think you have any options at all including any logic, analog or a micro without having the positive voltage available.

 

How much current do each of the LEDs need?

When SW2 is closed (and SW1 is open), you would only get 6 mA if the output of the box were shorted to ground. With a White LED, you are looking at only about 4 mA. Unless your LED is able to be sufficiently visible at something like 1 mA, you aren't going to have much to work with.

If you LEDs are able to work at the 1 mA level, then I think there is a solution that is pretty simple.

 

The goal is to only use the available chassis ground and the output wire from the red box.

Then you should have said that in the fist post. In your diagram the 12V line, as well as the ground is outside the box, so I assumed those two were available.