Hi all

Just start off with saying I know pretty much nothing about designing circuits, but can solder and develop a circuit quite easily from a diagram. So looking for a little bit of help with my very simple home cinema controller

In order to control devices from my computer/remote
I use a matrix orbital LCD display which has 6 general purpose outputs
These output at 5v 20mA

Now this isnt enough to drive a relay.

The two relays I have are these http://www.maplin.co.uk/Module.aspx?ModuleNo=2605&OrderCode=JH12N

My original thought is that these would have operated the 12v relays which switch 240v

But as the 20mA from the LCD isnt powerful enough, I need to develop a circuit which will maybe power the 12v relay

I have this circuit diagram.

Now few questions, what would be the best transistor to use on this?
What resistor would I require?

I have a 12v power source inside to work with, if I can't power the 12v relays directly (these are over 3m worth of cable) I will need to power the 5v relay initially, so the 12v will need to be dropped down to 5v somehow

Hope this makes sense, just don't want to wiresomething up and have it blow my LCD screen!

Thanks

 

Use a 2N2222, 2N3904 or similar "small signal transistor" for the relay.

Resistor around 1kOhm to 10kOhm.

Protection diode: 1N4148 or 1N4001 to 1N4007 would all be suitable.

 

I have also been recomended to use 2N7000

Will this circuit function correctly if there are two seperate grounds?

 

2N7000 is a N-channel MOSFET. MOSFETs are transistors too, but are completely voltage controlled instead of voltage-current controlled like BJTs.

The advantages of MOSFETs: virtually NO current flows into the gate terminal, meaning that your LCD display is completely protected HOWEVER this protection is provided by the base resistor of your circuit so is not really a concern. Also they are more tolerant of voltage overloads (but not generally current overloads.) Voltage overloads would be >60 volts (for 2N7000), probably not an issue for your circuit... Also they are capable of operating at higher frequencies; again not an issue because you are using a relay and you won't be able to get your relay to switch faster than about 100 Hz.

The problems with MOSFETs: typically, you will require at least 4 volts to switch a MOSFET on (such as your 2N7000); some MOSFETs require as high as 10 volts! Also they are often more expensive for higher current models; for example the 2N2222 is around 12p (in high quantity) and it handles 1A max., while the 2N7000 is slightly more expensive but only handles 200mA max... I guess price isn't a critical concern of your project, but it is something to think of if you are designing a price critical product.

All considered, it is easier to use a NPN small signal transistor (or BJT/bipolar junction transistor). Note the transistor must be NPN (not PNP) or you will get reverse switching action, e.g. your relay coil switches ON when there is no voltage. PNP BJT transistors are generally more expensive and handle lower currents, anyway.

If the relay's ground (e.g. your AC line) is separate to your circuit ground (e.g. your NPN emitter), that is absolutely fine, because the relay is electrically isolating the two circuits, to around 2000+ volts A.C. (which is more than you should ever need.) Remember the coil needs to be connected to your circuit ground, and nothing must come from the AC line to the circuit electronics except through optocouplers or transformers, to achieve proper isolation.

Tom