this is going to be a long, sad post - i am going through my first arduino project and i am having a hell of a time trying to get the electronics going - a design student here with no experience but loads of enthusiasm so will appreciate any advice you might help with.
The project is simple I think: I want to install 6 LED flexible strips on the ceiling of a corridor. A webcam tracks the motion of passers by and lights up the LEDS so that the motion triggers a light wave of a sort. I am using flash for the programming part and have managed to program the desired behavior, but could only test it on single leds connected directly to the arduino board - I am really really bad on electronics and that's where I need help - how do i connect the led strips and the arduino in a circuit?
SO far my plan is: a single 5V transformer to power up all the components, a transistor and a resistor turn these 5V to 12V to power up each of the lamps. The arduino controls the transistors and consequenty - the amoung of light emitted from each lamp. I think this should be OK as a plan, but he details are a real pain:
electric transformer: I will have 6 strips x 7.2 W each running on 12V. This means I need 3.6A. Because my arduino BT is running on 5.5V max, I will need a transformer on 5.5V, 3.5A. I did manage to find one that is on 3A only, though - do you think this will work? Are my calculations correct?
transistors: I tried to apply as much as I got from Transistor Circuits and decided that the TIP 122 will work for me (specs here: http://www.learn-c.com/tip120.pdf). Do you think that's OK? A bit worried it says 100V, but thought this is the max value and it should work for lower volts too..?
resistors: got totally lost here.. really no idea how to calculate what kind of a resistor I need... any direction will be truly appreciated.
Hope someone pick up on this - if not solutions I will appreciate at least direction to resources that can help me get this done. A BIG BIG thanks in advance!
SO far my plan is: a single 5V transformer to power up all the components, a transistor and a resistor turn these 5V to 12V to power up each of the lamps. The arduino controls the transistors and consequenty - the amoung of light emitted from each lamp. I think this should be OK as a plan, but he details are a real pain
Hmm, perhaps I do not understand your intent, but you cannot turn 5V into 12V with a transistor, you need a transformer to do that, and only if it's AC. But since you are probably using DC, you would need a DC-DC converter. Such a converter is complicated and expensive, and is probably not what you really want. I think that you should plan on starting with a 12V supply. Are you sure you need 12V for the LEDs? What do your strips consist of, are they a series of LEDs in series (making the 12V requirement)? Do you have a spec sheet for them?
As for the 100V on the transistor, you are correct, it can handle up to 100V, you could get away with the 60V version.
As for the resistors, you need to know what current your LEDs need, back to the spec sheet. Likely this is somewhere around 20ma. You also need to know the forward voltage of those LEDs which it sounds like it might be 12V which might mean that you should actually start with a slightly larger supply, perhaps 14V? If you have a 14V supply and the Vf (forward voltage of the LEDs) is 12V, then you will drop 2V across a current limiting resistor that you should put in series with the LEDs. If you want 20ma through this resistor (and thus through the LEDs), V=IR -> V/I=R -> 2V/.020A = 100 ohms.
If they are strip LEDs it is likely that they already contain the current limiting resistors in them and you don't need any more.
Incidentally I assume you mean 101 to indicate it is simple. In the UK 101 indicates something terrifying, as in Room 101 from George Orwell's novel "1984".
electric transformer: I will have 6 strips x 7.2 W each running on 12V. This means I need 3.6A. Because my arduino BT is running on 5.5V max, I will need a transformer on 5.5V, 3.5A. I did manage to find one that is on 3A only, though - do you think this will work? Are my calculations correct?
No.
Wattage is volts times amps. To get 3.6 amps at 12 volts, you need 8.4 amps at 5 volts. If you can simply increase the voltage and keep the current, you'd have a source of free energy. Also, you can't use a transistor and resistor or any easy way to step up 5VDC to 12VDC.
It's also not that simple to calculate the power needs, you have losses when you try and regulate 12V DC out of an AC transformer, other losses in your circuit, and often the rating on a transformer is the point where it pours out the magic smoke, so you want to build in some extra room. If you need 3.6 amps at 12 volts, you probably want something rated for a regulated DC supply of 8 amps at 12 volts.
I'm also not clear if you mean the electronic component "transformer", or a AC to DC power supply that is often called a "transformer", but in this day and age might not even contain a transformer.
If you are just using the arduino to turn on the switching transistors (which in turn pass higher current 12V to the lights), the currents in those transistors (e.g. the 5V control current) will be minimal. You can also use that 12V source to power the Arduino's built-in regulator (after changing the power jumper to EXT). Take a look at this example.
-j
PS, Grumpy_mike:
Incidentally I assume you mean 101 to indicate it is simple. In the UK 101 indicates something terrifying, as in Room 101 from George Orwell's novel "1984".
In US universities, courses are frequently numbered with the hundreds place indicating the year (freshman/first year = 100 level) and less significant digits indicating term, so 101 is an entry level course. Entry level, introductory material is frequently referred to as "subject X 101" even when not a college course.