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I have three terminals. terminal 1 supply's + 12v terminal 2 supply's -12v terminal 3 switches from 0v to +12v I want to make a simple circuit so that when terminal 3 is at 0v the 12v led is on when terminal 3 is at +12v the LED is off Any ideas please?

What is the purpose of the -12V supply at terminal 2?

From your description, it is redundant.

This would work:

Sorry I am new hear and should have been clearer. Terminal 1 & 2 are 12v pos & neg supply. Thanks for your help, I will give that a try.

Hi @rontronic,

Welcome to CL.

If the voltage difference across terminals 1 and 2 = 12V

and terminal 1 = +12V

then you would normally describe terminal 2 as ground or 0V.

In CL, the little triangular symbol = ground

I have just built this circuit but unfortunately It didn't do what I needed it to :-( The led switches on when the base of the transistor is grounded.

I need the LED off when 12v is on at terminal 3. LED on when terminal 3 is off

Any more ideas please?

Sorry but the circuit does exactly what you asked for:

"I want to make a simple circuit so that when terminal 3 is at 0v the 12v led is on when terminal 3 is at +12v the LED is off "

Things to learn from this.

i) No matter how simple it is:

Think about what you want to do.

Write down a design specification.

Read it.

Think about what you want to do.

Update your spec if what you read does not describe what you want.

ii) You didn't have to build it to discover that it didn't do what you expected.

That's what simulation is for.

:)

Hi Signality Yes I realise I did word it all wrong and once I had looked at your circuit I thought that you had done what I mistakenly asked for. I suppose what I should have said was I have 2 terminals which supply the plus and ground 12 volts. I have another terminal which switches on 12 volts then off. When the switched terminal is off I want an LED to light, when the terminal is on
I need the led to be off. Hope I have worded this correct this time and thanks for your help.

It's still not clear enough:

"When the switched terminal is off I want an LED to light, when the terminal is on I need the led to be off. "

Define off.

Is that:

off = goes open circuit or high impedance;

as in an on/off switch being opened?

Or is that:

off = goes to 0V?

This is one way to do what you actually wanted:

It will cope with either definition of off.

Run the simulation to see how the current through the LED behaves with the input voltage. CL - like most circuit simulators - does not show light output power but the current through the LED is an almost exactly equivalent indication.

There are others but this uses about the minimum number of lowest cost components.

See the section on "A design process" in:

https://www.circuitlab.com/forums/support/topic/58b454g6/op-amp-output-different-than-real-world/#comment_2195

Also have a read of:

https://www.circuitlab.com/forums/basic-electronics/topic/6n4hgm38/pnp-transistor-as-a-switch/#comment_3258

It is off as in an on/off switch being opened?

I will have a look at your links in my lunch hour. Thanks