RasPi and PWM controlled LED strip

I honestly in my opinion would use a 3.3v or 5v relay. But im not sure im only 14

I honestly in my opinion would use a 3.3v or 5v relay. But im not sure im only 14

Keep up the thinking , a very valid observation.

Here is why it is mostly not done in this scenario (It is a valid approach but read on)

If you use a relay (3V3 or 5V) you will still need a driver which will still probably be a ULN2003 or equivalent circuit (As there so cheap) as most controllers do not have the drive to switch a relay directly (Not without overloading the pin). As the load in this case is low voltage (Just high current) it is not necessary to add the addition of a relay as the driver already has the ability to control the LEDs.

If the load was to be say mains voltage (~110V or ~220V depending on location) then it would be very prudent to provide Isolation between the load and your circuit. firstly to prevent  accidentally blowing up your board or other attached devices to the PI and also to protect you when touching the board or connected stuff. it also prevents mains interference and spikes (Board Killers) from getting to your stuff.

So as a general rule if your load low voltage and don't need the isolation, a driver circuit like the ULN2003 is perfect for the job, you can always add a power MOSFET for very high currents. If you need to isolate the load for various reasons, then use a relay (Solid State or Mechanical). If your driving AC loads then a relay makes it easy. If you don't know what your going to control, a relay is a safe bet to have max flexibility

Hope this clarifies and helps you to decide for future projects

Hi Hayden age is not an issue at all, knowledge doesn't know about ages, old guys as me just had more time to learn.

The problem with a relay for this setup is they are going to be good just for the on/off strip and timing.

The PWM strip can't use relays as relays won't see the pulses as light dimming and just will see low voltages on low duty cycles and will turn on on high duty cycles.

Relays are also quite slow for lights, when you want to do fancy light effects.

They are also more expensive than darlington arrays, the ULN2003 is just a handful of cents, and you can drive many nice stuff with them like stepper motors and other things.

They are also safer to use as they are not inductive and produce no flyback when the electric field in the inductive load collapses.

They are quite fun devices to play with. I always have some in my components box, you can use them for a lot of things.

If you don't understand any of those terms just ask. I will leave here some links just in case ( I don't mean you don't know them, I put them just in case as I said ).

Pulse-width modulation - Wikipedia, the free encyclopedia

Flyback diode - Wikipedia, the free encyclopedia

Darlington transistor - Wikipedia, the free encyclopedia

Inductance - Wikipedia, the free encyclopedia

Great point Peter.

Sometimes I forgot this is the Raspberry forum.

You won't drive a 5V relay directly from the Pi GPIOs.

Relays are not really suitable for this kind of applications because although they can certainly switch the voltage/current they can only realistically operate sub 10 times a second and at this rate they would wear out in a very shot time.

10 times a second would not give any perception of a dimming controller just a flashing light and very unpleasant .

Most dimmers operate on the 50/60hz of the mains and in practice it's much nicer if you switch at much higher frequencies than this !!

If i lead anyone to think I was talking about using them for the PWM channel... Sorry but no

I was only referring to the simpler on off channels. John is absolutely right in his statement, relays are not fast enough or suitable for dimming

the ULN2003 of course will work for all the above required situations, PWM and ON/Off alike

Peter