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Illuminated switch problem.

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Once again, I turn to the Common Room of Sideways U. to seek enlightenment!

I have an illuminated switch.   It has three terminals, marked '+', A (for accessory?) and an Earth sign.    It works, in that it will switch on and off between the + and A terminals.

As I understand it, the Earth sign terminal should be earthed, and will then allow the internal LED to light up.   Which it does.

BUT, only if the 'A' wire is disconnected from the teminal.   When the accessory is connected, only that works, the LED stays unlit.

So when the switch is on, I can have either the accessory, OR the switch LED on, not both.   Since I want the LED to warn me that it's turned on, that's no good!

Please advise?

It's a switch IN a gearknob, that I've carved to take the switch, so I'd rather not source a different one.  The switches seem easy to find, but I'd like to know why it behaves like this, in case another does the same.   Thanks!


Edited by JohnD
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What are you trying to switch are you using the switch to connect the accessory to Earth or supply 12V to it thru the switch?  Do you have a cct diagram. 

I expect the LED is being powered thru the accessory. Is this like this?
https://www.carbuilder.com/uk/mini-rocker-switch-illuminated-red if so i have one in the garage I can play with to help work out whats happening!



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Mike, the switch is almost exactly the same as the one on Car Builder, except it has a large LED.

I don't have an internal wiring diagram for the switch, but that's what I thought it would look like, and what I assume it is.

I'm testing the switch in situ and on the bench, using this circuit:


"Above the symbols is the switch, with presumed connections, below my test cirucit with a test bulb.     The "switch2 under the Earh symbol represents my touching the wires from there and from the A terminal together.    That, if the switch itself is 'ON' flips the test bulb off and the LED on, and back again if I untouch the wires.    Hope you can explain this!

Would it be different if the test bulb were 'down stream' of the switch?  Between A and the battery negative terminal??  Like this:


Yes, it is different - the LED doesn' t come on at all!  But the test bulb works on and off by the switch.

I'm even more confused!


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Hang ON!

What is the Earth connection went straight back to the battery?  like this:


BINGO!    LED & Lamp on and off together!    That's it!   

Thank you, Mike, for making me THINK!   That is the power of the Common Room Effect!


Edited by JohnD
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