I'm having trouble understanding how this instrument switch works.

Image of instrument lighting switch assembly

Here is the schematic illustrating the three switch states:

extract of schematic showing instrument lighting switch

How do I interpret these numbers?

If I were to guess, I'd say that the two relays are shown in the open position (NVG):
enter image description here

On closes the top relay:
enter image description here

Off closes the bottom relay:
enter image description here

I don't have any confidence in my guess though.

Also, I don't understand why there are two junctions not connected to anything:
enter image description here
Hopefully this is standardized, and not specific to my wiring diagram.
I couldn't find the numbers referenced anywhere on the sheet.

  • \$\begingroup\$ You mean the 1, 2, 3 etc, or the S70021? \$\endgroup\$
    – user98663
    Commented Nov 18, 2016 at 17:23
  • \$\begingroup\$ looks like an odd 2P3T switch with top position off physical but centre off in logic diagram.cwhere did you find this? \$\endgroup\$ Commented Nov 18, 2016 at 17:42
  • \$\begingroup\$ looks like a Navy design for ship landing with NV goggles. do they still use positive ground in ships like the MGB did? \$\endgroup\$ Commented Nov 18, 2016 at 17:56
  • \$\begingroup\$ @TonyStewart.EEsince'75 This is for a civilian helicopter. \$\endgroup\$ Commented Nov 18, 2016 at 18:09
  • \$\begingroup\$ ok so On and NVG are exclusive OR power with dimmer for dash lights and centre off is weird. Does this actually control relays on high side or low side? or direct power from terminal block TB70083 to load? \$\endgroup\$ Commented Nov 18, 2016 at 19:11

2 Answers 2


The most likely conclusion is:

  • The dotted line indicates that internally the two switches are mechanically linked. meaning that they switch over at roughly the same position and instant.
  • Each switch has three possible settings:
    • NVG: 2-3 and 5-4
    • ON: 2-1 and 5-6
    • OFF: 2-none and 5-none (in the photo this is the top position)

Since nothing is connected to 4, the 5-4 connection is not needed, but the designer probaby chose an off-the-shelf product.

Technically, you would call this a double-pole, triple-throw, top-off switch

  • \$\begingroup\$ or top off in photo so inverted orientation \$\endgroup\$ Commented Nov 18, 2016 at 17:53
  • \$\begingroup\$ @Tony agreed - slightly edited the answer \$\endgroup\$
    – neonzeon
    Commented Nov 18, 2016 at 18:00
  • \$\begingroup\$ does the contact symbol look like a convention for arcing during switch off? and both poles in. logic diagram move in same direction but arc depends on load whether high side switched or low side switched? yet non-applicable with no connection as in break before make sequence \$\endgroup\$ Commented Nov 18, 2016 at 18:01

When reverse-engineering one part of a schematic, it can become essential to trace connections from there to other parts or pages of the schematic.

We can't do that with the snippet shown here (and even if you could provide the whole schematic, that might become a copyright problem). This limitation is affecting my answer (I want the whole schematic!) but here is my analysis, based on the provided small diagram:

  • IMHO there aren't 2 switches or relays, as you speculate. There is one, two-pole (double-pole), three-position switch S70021. The part number starting with S is a good indication of a switch, not a relay (check the BOM / parts list for that component, for confirmation).
  • When you have the manufacturer's part number for that switch, you might be able to find out more about its internal contact arrangement and hence what happens in each of the three switch positions. Don't limit yourself to reverse-engineering the schematic, if you can also use the BOM / parts list.
  • The switch has 6 pins, labelled 1 through 6. The top pole is pin 2; the bottom pole is pin 5.

I'm not going to try to decode the likely connections in each of the three positions (I need more of the schematic to do that, and several cups of coffee) but we can make some deductions, for example:

  • Power is supplied to the top pole of the switch, from the 28VDC ESS BUS II via circuit breaker CB10063.
  • In one of the three positions, that 28V is connected via switch pin 3 to position H in terminal block TB70083.
  • From there, power goes via position J in the same terminal block to somewhere I cannot see via LF1002E16.
  • Trace that power - is it related to the NVG part of the instrument panel lighting operation? Somewhere else in the schematic, the variable control for instrument panel brightness when using NVG (seen in position "LO" on that top picture) must be shown. That will give you a good clue, about whether you have traced the connections for the switch in position "NVG" or "ON".

What about the other connections? I would use the same approach as in the example above, to trace the wider power supply connections, for example:

  • Where does the connection LF1027A16 from pin 1 go to? Does that make sense in the NVG position, or only in the ON position?
  • Follow the connections from switch pins 5 and 6, off the diagram given. In one (and likely, only one) of the switch positions, pins 5 & 6 are connected. Trace those connections to see whether that connection makes sense in switch position "NVG" or "ON".

What about the switch in position "OFF"? Since the switch has only two pins per pole for connections (pins 1 & 3 for the top pole, and pins 4 & 6 for the bottom pole) yet is a three-position switch, then it seems likely to me that in the "OFF" position, there may be no connection between the poles and either of the connections for that pole. This is common with "centre-off" switches, for example.

Due to the connection between switch pins 5 & 6 in only one switch position, and since the extra functionality (i.e. the variable light control) is probably for use in NVG mode, then that would be my starting hypothesis if I was reviewing the connections. Then try to confirm or deny that hypothesis, using the rest of the schematic. Does it make sense that those two points would be connected in NVG mode?

Hope that helps to show a general reverse-engineering approach. As I said, don't limit yourself to trying to interpret a single switch; use the rest of the schematic (and any other information sources like BOM, manufacturer's part number etc.) to help you.


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