Hi Guys,
I'm new to this so please excuse what is probably a very basic query! I have tried searching first but couldn't find what I'm looking for (I may not have used the best search terms so feel free to direct me to a relevant previous thread if there is one).

I want to design a circuit to provide a remote indication of the setting of a 3-position switch, ie. there will be a switch with positions 1,2 and 3 on one unit, connected by a cable to another unit some distance away which will have 3 LEDs, with only one lit at a time depending on the switch position. There is an obvious solution of simply using 4 bus lines (gnd, 1, 2 and 3) however I want to minimise the number of cores required in the cable (to two if possible), and also allow for a 'failure' mode whereby if the connecting cable is disconnected or cut, a specified one of the LEDs will illuminate, rather than them all going out. I'm imagining something along the lines of a polarity based system with current in one direction illuminating LED1, the reverse direction LED2, and no current at all LED3. In this case, LED3 would also be the 'failure' indicator. What do I need to realize this at the LED end though? Power will be batteries, and will be available at both ends of the system.

Any pointers please?

 

Following your description of the operation, you would not be able to tell the difference between 'open cable' and 'position 3 selected' but if that is acceptable to you then it is certainly possible. Alternatively, the open cable could light all three LEDs which would be an unambiguous indication.

 

Following your description of the operation, you would not be able to tell the difference between 'open cable' and 'position 3 selected' but if that is acceptable to you then it is certainly possible. Alternatively, the open cable could light all three LEDs which would be an unambiguous indication.

The ideal would actually be for the 'open cable' to be indistinguishable from 'position 3 selected'. The purpose of the circuit is as a training aid for a piece of railway signalling equipment, to simulate realistic operation. The real equipment is designed to 'fail safe' ie. if the cable is cut, the instrument gives the 'danger' signal automatically (which is what 'position 3' would be). The real equipment achieves this using a complicated setup of relays and also interlocking with other equipment. I want to replicate just the operation, but this needs to include replicating a 'fault' situation realistically too.

 

In your arrangement is the three position switch only providing the signals on the two wires or is it switching something else as well?

The end with the LEDs will need a power source (battery or mains adaptor) but the switch end could be done with no power at the expense of the LED end being a bit more complicated. Which do you fancy?

Does it need to use positive and negative current or could it use, say, 1V for position 1, 2V for position 2, and 0v for position 3?

 

In your arrangement is the three position switch only providing the signals on the two wires or is it switching something else as well?

The end with the LEDs will need a power source (battery or mains adaptor) but the switch end could be done with no power at the expense of the LED end being a bit more complicated. Which do you fancy?

Does it need to use positive and negative current or could it use, say, 1V for position 1, 2V for position 2, and 0v for position 3?

In my case, the switch only sends the signal to light the LEDs on the other unit, nothing else. There's no reason why battery power couldn't be provided at both ends. There's also no reason why it couldn't be different positive voltages for the various indications. I just thought I was simplifying matters by having the two polarities as this would mean the LEDs for position 1 and 2 could simply be wired across the control lines, with one lighting for positive polarity and the other for reverse. All that would be needed is a 'no voltage detector' circuit to light the third LED when required.

 

The circuit below should do the job. For the switch end you can use a 1 pole switch with two battery packs or a two pole switch with one battery pack. The LEDs can be any normal red, orange, yellow, or green but not white or blue (there won't be enough voltage tor them). The bridge can be an actual bridge rectifier of four small signal diodes, e.g. 1N4148. The transistor can be any small signal NPN transistor.

The switch end is switched off in position 3, but you will want an on-off switch at the LED end as it always draws current.

 

The circuit below should do the job. For the switch end you can use a 1 pole switch with two battery packs or a two pole switch with one battery pack. The LEDs can be any normal red, orange, yellow, or green but not white or blue (there won't be enough voltage tor them). The bridge can be an actual bridge rectifier of four small signal diodes, e.g. 1N4148. The transistor can be any small signal NPN transistor.

The switch end is switched off in position 3, but you will want an on-off switch at the LED end as it always draws current.
View attachment 137415

That's really helpful thank you.