Any railway modeller will be familiar with the problem: you have a locomotive turntable or switch-yard and you are never sure which track is actually selected, unless you go and have a close look.

Or you could have the same problem with a traverser which selects rolling stock storage tracks. With a locomotive turntable you may well be sure that a track has been correctly “indexed” but you still don’t know which one has been selected. So the solution to that problem is a reed switch associated with each output track and a magnet on the turntable to activate each reed.

The bank of nine (or less) switches is wired effectively as a single-pole rotary switch and then can be coupled to this single-digit display. From there, the concept can be applied to any situation where a rotary switch is used, with one or two provisos which we will come to later.

Normally open switches

More specifically, this Switch Indicator is designed to operate with normally-open switches, such as reed switches. With no switches closed, the single-digit display will show zero (0). With a switch closed, the display will show the number of the switch.

This brings us to another important point – the circuit is designed to operate correctly only if one switch is closed at the one time.

The arrangement of the reed switches and magnets should be such that as one switch opens the next switch closes. In other words there should not be a period when two reed switches are closed.

If two or more switches are closed, the display will show a blank or an incorrect value which may be quite unrelated to the switches that are closed. For example, a closed 4 and 6 switch will show a 6, a closed 1 and 2 switch will show a 3 while a closed 8 and 3 switch will show a blanked display.