PNP 12v boost using 5v input

Hi here is the basic question,

I am trying to drive a 12v load from 5v input.
Both 12v and 5v are complete different individual sources.

When the ground of 5v is connected to PNP base i need to drive +12v supply to my load.
I already tried using BC558 to drive 12v relay.

1. Base - 0v(Ground of 5v source) - 10k
2. Emitter - +12v - No resistor
3. Collector - +ive terminal of 12v relay (-ve permonently connected to ground)
Output - Nothing happens.
But it turned ON relay only when i connected +5v supply along with +12v supply(Emitter).

I dont want to mix both the sources since it may affect my 5v system(uc).
Can someone suggest me how to achieve this??

Your 5V signal that's switching the transistor really needs to get close to +12V to switch it off (12V - Vbe), but it's only getting to 5V. Think of that part as referenced to the +12V line. You should have the 5V signal control an NPN transistor, with the collector of that connected to the base of your NPN transistor, and with a pullup to +12V. Emitter would connect to ground.

Hope I explained that properly.

Still I am not pretty clear, can you give me some picture of it??

Check my tutorials.

for total isolation what about an opti couplier driving the relay.

@ Nigel,

Your tutorial is wonderful.
Another doubt is, the circuit below which active when microcontroller output is high.
But i want to source 12v for a relay when uc output is low.
So what changes should i make?

Add another inverting transistor before the first one - but it's probably easier to alter the software?.

Then its fine.
And for a practical application how about opto isolator for sourcing/sinking 12v, by simply giving if high to anode or low to cathode to make it work??

Does opto isolator also needs transistor boosted 5v or direct microcontroller output itself enough to activate??

An opto-isolator can source or sink, depending how you connect it.