Transistor switching on another voltage supply

[Juzzac]

Hi, Can someone please help point me in the right direction, I am wanting to switch on a separate 12v circuit with a transistor or similar, the circuit i wish to switch it on from is about 0.2 volts when off and about 1 Volt when on, I would use a solid state relay but can't find one that will switch on with just 1 volt. The circuits don't and can't share the same earth, what would be the best solution ?

Thanks Juzza

 

[Colin Mitchell]

Add a transistor to drive an optocoupler and the other side of the optocoupler switches the 12v circuit.

 

[cjdelphi]

Or better still, use an NPN power transistor voltage of 1v is too low for a mosfet , 12v + side to your device to power, eg 12v bulb and the other side to the collector and the emitter tied to ground

that leaves say a 300ohm resistor from the 1v to base of the npn.

 

[cjdelphi]

Opto's are over kill for this imho....

 

[Martaine2005]

12v + side to your device to power, eg 12v bulb

Very clever.
Using the bulb as an opto isolator.

Opto's are over kill for this imho....

And bloody pricey..
Martin

 

[cjdelphi]

Alright fine 2am never read it properly regarding it can't share grounds...

But why?

 

[Martaine2005]

Alright fine 2am never read it properly regarding it can't share grounds...

But why?

I think you read me wrong..
I meant VERY clever. They used to use light bulbs to isolate circuits.
Martin

 

[cjdelphi]

How?

Then it occurred to me, there's only 1v which is not enough for the optoisolator to light the LED, you'd need the transistor to share grounds with at least a 3vish source to fire an opto at which point why bother and tie grounds with the 12v source ...

 

[cjdelphi]

https://en.m.wikipedia.org/wiki/Resistive_opto-isolator

That?

 

[Martaine2005]

The whole point of isolating circuits is to completely isolate them.. No 'common' either.
It would be an independant circuit controlled by the first low voltage circuit.
For safety reasons no voltage/current can be fed back to the controlling circuit.
Martin

Ps, I will try and find something I read years ago about the light bulb isolator.

 

[Martaine2005]

https://en.m.wikipedia.org/wiki/Resistive_opto-isolator

That?

No, it was when I was at college sitting OR failing my part 2 electronics.
Martin

 

[Martaine2005]

No, it was when I was at college sitting OR failing my part 2 electronics.
Martin

Sorry can't find any of my old city and guilds books yet.
But possibly davenn, Bobk or Colin will be able to shed more light on this..Oh and of course my favourite encyclopedia Howard.

Martin

 

[Minder]

Set a LM311 to switch over at 1v operate into a 4n25 (25¢), if you need to condition the output for a little more power a 2n7000 will do it up to 200ma.
Total ~ $1.00
Max.

 

[BobK]

Sorry can't find any of my old city and guilds books yet.
But possibly davenn, Bobk or Colin will be able to shed more light on this..Oh and of course my favourite encyclopedia Howard.

Martin

Colin gave the answer in post #2. It is the same one I would have given.

Bob