That relay is not suitable. It can't handle mains voltage. This one would be much better;
http://www.maplin.co.uk/Module.aspx?...elay&doy=17m11

I realise that it says 6v, but I have just 'clicked' a 12v relay with 4 AA batteries.

On your second point. If the relay is a DPDT ( Double pole double throw ) type, then you can switch both live and neutral. If it's a SPST ( Single pole single throw ) then you can only switch the live. However, that's what most 'mains' switches on appliances do.

On the subject of diode. I'd use an 1N4001. All it's doing is stopping the back EMF ( Elecromotive force ) that occurs when the relay de-energises from possibly damaging the transistor. As an experiment, when you get the relay, try it on 4 AA batteries and look at the sparks you get when you disconnect it.

I'm not sure what you mean by 'Which line would you break to control the switching? +12 or -12? or both?'

Originally Posted by House0Fwax

That relay is not suitable. It can't handle mains voltage. This one would be much better;
http://www.maplin.co.uk/Module.aspx?...elay&doy=17m11

I realise that it says 6v, but I have just 'clicked' a 12v relay with 4 AA batteries.

On your second point. If the relay is a DPDT ( Double pole double throw ) type, then you can switch both live and neutral. If it's a SPST ( Single pole single throw ) then you can only switch the live. However, that's what most 'mains' switches on appliances do.

On the subject of diode. I'd use an 1N4001. All it's doing is stopping the back EMF ( Elecromotive force ) that occurs when the relay de-energises from possibly damaging the transistor. As an experiment, when you get the relay, try it on 4 AA batteries and look at the sparks you get when you disconnect it.

I'm not sure what you mean by 'Which line would you break to control the switching? +12 or -12? or both?'

Hi, I'm not trying to switch mains voltage. I am trying to build a circuit that will turn on (and off) a 12v 2.0A (max output 24W) device using a 5v control line. (very first post)

I think the conversation through the thread has talked about switching mains at times though.

I was going to switch the mains for the whole unit, but the adapter for the Ext HD outputs 12v so I decided just to switch that. This means I dont have to use a bulky UK 3 pin socket.

Although now you pointed out that relay I may look at doing that again.

So I guess thats where the +12v -12v came in. But you already ansered that with your great "DPDT ( Double pole double throw )" description.

I am struggling to find the transistor needed though ... any ideas?

I generally use a BC109C, probably because I have loads kicking around.
Any general NPN type should do, you are only switching a relay. Although you should check that it can handle about 800ma upwards to be on the safe side.
I have jsut tried out the 'Picaxe' circuit on a breadboard using a BC109C and a 12v relay, and a 6v supply. Works fine.
And as for switching the voltage line. Always switch the positive (+) or live depending on if it's ac or dc

Indeed, the 5v relay will be ok. Sorry, I can't help you with the current though, I have never been good with maths and stuff like that. mind you, it's not going to be very much at all.

Any of the following transistors will be fine;
Maplin code
QF28F BFY51
QR23A 2N3053
QB38R BC141-10
VR06G BC639
QB68Y BC337-16
UM82D TIP31C <--- That one is overdoing it a bit, it's a power transistor, but if your branch of Maplin is anthing like mine, then they vary rarely have what you want. So you'd best have alternatives.

Originally Posted by audioguru

Isn't a BC109 in an old metal case and from about 40 years ago? I still have a few.
Just about any little NPN transistor can turn on a relay in that circuit.

thank you.

another question, what voltage led will work on that circuit? e.g what does adding the 1.5k resistor do?

Originally Posted by House0Fwax

Indeed, the 5v relay will be ok. Sorry, I can't help you with the current though, I have never been good with maths and stuff like that. mind you, it's not going to be very much at all.

Any of the following transistors will be fine;
Maplin code
QF28F BFY51
QR23A 2N3053
QB38R BC141-10
VR06G BC639
QB68Y BC337-16
UM82D TIP31C <--- That one is overdoing it a bit, it's a power transistor, but if your branch of Maplin is anthing like mine, then they vary rarely have what you want. So you'd best have alternatives.

woooohoooo my shopping list is almost complete ... thanks very much guys for the help. Mucho mucho appreciated.

Now all I need is an LED (see above post)

thanks

Uncle $crooge: It is indeed in a metal can. I bought a bag of about 50 many years ago and still use them. I used some about a week ago pic multiplexing project. Old and new components on one board. Teriffic.

Any standard led will work, choose your favourite colour. The resistor is there to limit the current.
Some have built in resistors.
Yesterday I bought some nice blue ones from a £1 shop that you mount in your car. ( not that I have one ) However, 3 5mm blue high brightness leds for a £1, yes please.

Uncle $crooge: Oh yes. Totally. However, I have never gotten to grips with all the maths stuff ( as I posted earlier ) though through the years, I have learned what works and what dies a horrible, yet short life.
Where leds are concerned, I without doubt start with a 1K resistor and work from there... Though I never make anything that gets further than a prototype board, ( well,,, very rarely ), I just play with things, adapt stuff, think 'see what I did there ?' Then move onto something else.

From what I have learned of the ULN2803 (never heard of it before today), which is the chip on the Velleman board. It can only 'sink' current ( ie. ground it ), throughout this thread, I have assumed that the board could 'source' ( ie. supply it. ) My fault entirely. Mind you, not to worry. From what I can gather;
Whatever is present on one of the 8 screw terminals on the velleman board will be connected to the 'clamp' terminal when you click the mouse on the appropriate bit of software.
As I see it, you have the two red horizontal ( nearly ) lines the wrong way around.

Believe me, If my local Maplin branch had one of these things in stock, I'd have bought it about 3 days ago and made it, tested it and sent you exact details, diagrams, pictures, video, video in 3-D...etc

Don't worry, you wont melt your PC.

Though, thoroughly check that your relay board works first.