Use arduino to turn backlit 12v photographs on and off

Projects General Guidance
1

I am a visual artist and make back lit photographs which are lit from behind using variable quantities of cut-able stick down LED strips. I want to use an Arduino to allow me to control a group of these images so they turn on and off in a determined sequence. Ideally I would use a motion detector to initiate the ARDUINO sequence, but I would also need an "all-on" and an "all off" button. I plan on wall mounting everything, with the ARDUINO and all wiring and buttons exposed. My questions:

  1. Best ARDUINO for this project - UNO?
  2. (More confusing to me) type of relay/switch to place in line between arduino and each image: MOSFET? or RELAY? Is there anything that would also allow dimming in the relay?
  3. Any general guidance for the pre-empting of arduino functions with the motion detection, "all-on", "all off" switching?
2

Hi cerebral,

We need more numbers. How many pictures? How much current will be needed for each one?

Relays are no good for dimming. FETs or bipolar transistor would be more suitable, depending on the current.

Only around 6 outputs on an Uno can normally be used for dimming, using a technique called PWM. But there is a library that allows more to be dimmed using software.

Paul

3

Since this is a body of work for me, the numbers vary from three back lit photos to 20. I will have to look at a couple and count the LEDs to get some averages for current.

4

Well, with 20 pictures and a few buttons, you will run out of input/outputs on an UNO. Certainly no room for expansion. So a Mega may make life easier. Even then, you only have 15 PWM outputs, so you may still need to use the library to achieve PWM in software for some outputs. The library is called SoftPWM.

You could still go for an UNO, or some other smaller Arduino like a Nano or Micro, but you would need extra chips to give you more outputs.

It is the maximum number of LEDS that's important, not the average, in selecting some suitable transistors. For up to 500mA per channel, you could use 3 x ULN2003. An alternative would be 20 x NPN bipolar transistors like BC337 for up to 800mA per channel. If you need more current, select a logic-level FET such as IRL520 for each channel.

5

You may wish to consider programmable LED strips. Some of them have addressable pixels, so you can control practically-unlimited numbers of them with just an I2C port or similar. It's going to cost more than plain LED strips, but will be programmatically and electronically far simpler for you I think.

6

polyglot:
You may wish to consider programmable LED strips.

Yes, might be worth giving those a trial, but just buy enough to try out on one picture at first. You may have to tweak the colour balance to your liking. You may never be able to get the exact same quality of illumination you get from your white LEDS. Only one way to find out...

7

Thank you both very much. That is an awesome start for me. I just really needed to narrow the field on the relay/FET question to get started. Is there going to be a decisive advantage somewhere for logic level FET vs. transistors, or is it just matching maximum current?
The SMD3528 single chip LED strip I use is .08w per diode and typically I will rarely use more than 5 meters of it, or 300 diodes per picture (often well less, and occasionally I could have as few as 6 diodes). For the 24 watts or less per 5 meter strip, I buy the 3 amp 12V transformer . I may ultimately use a multi terminal, heavier duty transformer to run each "picture" to, but for now, they each have an individual 110/24V transformer. Thanks again for your help.

8

cerebral:
Is there going to be a decisive advantage somewhere for logic level FET vs. transistors, or is it just matching maximum current?

Just current really. Bipolar NPNs are cheaper but can only handle so much current. FETs cost several times as much each, but still reasonable cost, are physically larger and may require heatsinks for very high currents. Make sure you get logic-level FETs, which also have a low "on" resistance to minimise the heat that they will generate. There are "better" ones around these days than the IRL520 I mentioned before.

cerebral:
110/24V transformer

How can you use that with your 12V strips? EDIT: you put two strips in series, obviously! Should not be a problem, most FETs can handle 24V easily. HOWEVER, the Arduino's power input can't handle 24V, it would be damaged. A DC-DC converter would be better, just a small one to deliver 5V direct to the Arduino's 5V input.

9

there are LED panels on the market now. a 5cm x 5 cm panel on e-bay is about $7.00

not sure how large your project space is.

a mini costs about $5.00 and one mini, and as many panels as needed for each frame.

you might even connect wireless and have one supervisor instruct which panels to light, which to dim, sequences, even dimming rates.

just make sure anything you get will be listed as dimmable.

10

cerebral:
Thank you both very much. That is an awesome start for me. I just really needed to narrow the field on the relay/FET question to get started. Is there going to be a decisive advantage somewhere for logic level FET vs. transistors, or is it just matching maximum current?

Relays are mechanical things that go click, they're fairly easy to use and very robust. FETs are smaller, can switch very fast and therefore do PWM for brightness control, but have a habit of blowing up if maltreated slightly. BJTs are robust but have a big (~0.6V) voltage drop, so they tend to waste more power (get hotter) than a FET carrying the same current. Really, either will work fine for your application but FETs will make more sense if you need to do dimming and control more than about 3A. Large FETs and BJTs will both still require some support circuitry in order to provide enough current to their input (base/gate) - the BJT needs lots of current continuously just to turn on, and a FET needs a short but large spike of current in order to switch quickly and minimise self-heating while it's half-on.

cerebral:
The SMD3528 single chip LED strip I use is .08w per diode and typically I will rarely use more than 5 meters of it, or 300 diodes per picture (often well less, and occasionally I could have as few as 6 diodes). For the 24 watts or less per 5 meter strip, I buy the 3 amp 12V transformer . I may ultimately use a multi terminal, heavier duty transformer to run each "picture" to, but for now, they each have an individual 110/24V transformer. Thanks again for your help.

Don't buy parallel LED strips/panels, get series ones. You can buy LED power supplies from eBay for $5-$10 that have current regulation and will support any output voltage in some large range, e.g. 15-40V or 27-60V, see items #400687848188 or #400687964254 for example. That means you're dealing with much smaller currents, but you can't really do PWM because it will interfere with the current regulation in the power supply.

If you want to do PWM, you need a fixed-voltage (not fixed-current) supply. Again, you can happily get 40/60/72V supplies that will run really long chains of LEDs in series with not much current. You just need a transistor or FET that can handle the higher voltage (easy), but at far lower current than if you did them in parallel.

Beware of PWM on long chains of LEDs though, you're building a big antenna and driving it with a high power square wave. That means lots of EM interference out to many MHz unless you get the suppression right.

11

Thank you all very much for your responses - very helpful. Eventually I will provide link to photos of my finished work. Thank you again!