"random" blinking LEDs: purpose-built circuit, or Arduino or =

[RCH]

I'm hoping to get some guidance on an outdoor lighting project I'm undertaking. It will involve an array of 30 LEDs (technically, UV emitters) that will be suspended in a tree on the ends of two-to-four foot-long leads, so they can sway in the breeze. I want each emitter to blink off and on in a seemingly random way, with more time being spent off, than on -- a bit like fireflies. Each emitter will be coated with a little glow-in-the-dark powder mixed with a clear medium like silicone, so the powder will be activated by the UV light and will glow after the emitter blinks-off, with the glow gradually decaying. I hope that makes sense.

Since the emitters use +/-3v, I figured I'd have a solar panel feeding a storage battery, off which the circuit would run. I would want the circuit to only operate when lighting falls below a certain threshold; there's no point having it going during the daytime.

I definitely want each emitter to appear to be independent of the others, rather than multiple ones blinking at the same time. I've seen circuits using 555 chips that will control up to 8 LEDs and give a random blinking effect for each LED, but each LED appears to spend approximately equal amounts of time on and off, and I'd like them to be more off than on. I even found one circuit that ramped-up the brightness (rather than just going on with full brightness), which I thought was great; it would look more "organic" than a hard on/off.

The main question I'm having right now is whether to try to put something together using ICs/capacitors/resistors/transistors, or instead using something semi-prefab like Arduino or Raspberry Pi. I have no experience with such things, but I can read a schematic and use a soldering iron and have put together circuit projects before. Am I correct in assuming that it would be difficult to achieve the effect I want using ICs and discrete components? Would I be able to achieve the effect using Arduino or some other microcontroller? Having some ability to tweak flash rates would be helpful, and I don't know if that's doable with just 555 timers, and their ilk.

Any advice would be appreciated.

 

[Harald Kapp]

I've seen circuits using 555 chips that will control up to 8 LEDs and give a random blinking effect for each LED, but each LED appears to spend approximately equal amounts of time on and off, and I'd like them to be more off than on

Reading the first lines of your post that was the idea that came to my mind. The ratio of on-time to total time (on+off) is called duty cycle, short DC (so we can talk on a common basis). The DC of a 555 based circuit can be adjusted within certain limits, so you can achieve the desired effect (short on pulse, long off duration).
A "breathing" effect can be achieved with a modified 555 circuit, where the voltage across the timing capacitor is used to control an LED (instead of the "real" output pin 3). A few additional components are required.
Pulse duration and ratio can be tweaked using potentiometers instead of fixed resistors.
In my view this setup has the charme that you can build it on a small PCB and easily use as many LEDs as you like. Simply add more circuits when the tree has grown

Using an arduino is a completely different venue as it requires programming. Also, an arduino has higher initial cost. The cost per LED will decrease with each additional LED as the driver (transistor) required to drive the LED is dirt cheap. Once you've used up all free port pins, however, you'll have to resort to some sort of "port extender" to control additional LEDs (these "extenders" come in different flavors).
Tweaking the DC and pulse rate requires either re-programming the arduino or adding some control (e.g. a potentiometer) which is read by the software and used to control DC and/or pulse duration.

Making the LEDs flicker in a random fashion is free with the 555 soluation, as you will not be able to tune all 555s to exactly the same frequency. Therefore the LEDs will blink at different rates, Tuning each 555 deliberately to a slightly different frequency from the one before will enhance this effect.
Using a microcontroller (arduino or whatever) requires you to use a so called pseudo random routine to achieve a similar effect. For this purpose a very simple pseudo random generator can be used so there is no whitchcraft involved here.

As your installation is going to hang out in a tree, I assume it is outdoors. A 555 circuit is relatively insensitive to disturbances (e.g. lightning) compared to a microcontroller (note neither of them will survice a direct hit by a lighning bolt, I'm talking about interference caused by lighning at a distance). Hardening the microcontroller based circut will require additional protection elements (inductors, cpacitors, zener diodes and the like).

Last not least: either of the two setups can be turned on at dusk and off at dawn using an off the shelf automatic night light controller as it is used e.g. to control the lighting of house entrances. In this case the controller would be used to turn on/off the power suply to the installation.

 

[RCH]

Thank you so much, Harald, for the very informative response!

From your description, it sounds like building the circuitry myself is the way to go, rather than trying to use Arduino or its equivalent.

How many LEDs can each 555 timer control, in this application?

I've heard that there are some variants of the 555 that are the equivalent of having 2 or 4 of them on the same chip. Would there be any advantage to using them rather than single versions? (And I should mention that cost isn't really a factor with this project.)

It's my understanding that the standard 555 requires a minimum of 4.5v to operate. The LEDs I'll be using (actually UV emitters) require at least 3v, so I had figured that I would just use a photovoltaic cell that produced 3v and run it to 3v of batteries, and have that drive the circuitry and thence, the LEDs. But it sounds like I would either have to bump that power-supply up to 4.5v, or perhaps go with lower-power 555; apparently there are ones that operate on 3v. Any suggestions there?

The housing which will contain the circuitry will be concealed in a cleft at the base of the tree, and from there a multiple-conductor cable (15 pairs twisted of 24-gauge stranded wire) will go up the trunk about 20 feet, and into a smaller housing at the main branch, with connectors for the wires to each of the LEDs, which will dangle up to 10 feet away from that housing (6 feet out the branch and 4 feet down).

That raises two questions: given that this is low voltage and amperage, will the 30 feet of copper wiring between the circuitry and the LEDs be a problem, power-wise? Also, would the capacitance of that much wire cause other troubles?

Finally (for now!), you mentioned an off-the-shelf lighting controller to turn the circuit on at dusk. All such things that I've seen have themselves required power, which complicates things for me since the only power at the site is what I can generate with the solar cell. I can imagine a PVC that operates as a relay and closes contacts when the lighting drops below a threshold, but I haven't seen such a thing, and a little Google searching didn't produce any results, only versions that required 12vDC or 110-120vAC. Any suggestions?

Thank you again!

 

[BobK]

Um, why are you using UV LEDs? You know the light they produce is not visible, right?

Bob

 

[(*steve*)]

Um, why are you using UV LEDs? You know the light they produce is not visible, right?

He's coating them with something phosphorescent (and presumably with a long decay time)

 

[BobK]

I guess I missed that

Bob

 

[RCH]

Um, why are you using UV LEDs? You know the light they produce is not visible, right?

Bob

Well, perhaps I'm doing something wrong, but they seem to be producing visible light for me . . .

 

[RCH]

He's coating them with something phosphorescent (and presumably with a long decay time)

Right. I want the effect to be less harsh than just bright LEDs flashing, and I saw a few videos where people coated UV emitters with glow-in-the-dark materials. The decay gives a more "organic" effect, like I wanted.

 

[BobK]

Well, that does say "purple UV", so it is not strictly UV.

Bob

 

[RCH]

Well, that does say "purple UV", so it is not strictly UV.

Bob

Ah, I see your distinction.

Strange -- every UV lamp I've ever run into produces at least some visible light, although I'm sure there are plenty of kinds I've not encountered.

 

[Herschel Peeler]

I'm hoping to get some guidance on an outdoor lighting project I'm undertaking. It will involve an array of 30 LEDs (technically, UV emitters) that will be suspended in a tree on the ends of two-to-four foot-long leads, so they can sway in the breeze. I want each emitter to blink off and on in a seemingly random way, with more time being spent off, than on -- a bit like fireflies. Each emitter will be coated with a little glow-in-the-dark powder mixed with a clear medium like silicone, so the powder will be activated by the UV light and will glow after the emitter blinks-off, with the glow gradually decaying. I hope that makes sense.

Since the emitters use +/-3v, I figured I'd have a solar panel feeding a storage battery, off which the circuit would run. I would want the circuit to only operate when lighting falls below a certain threshold; there's no point having it going during the daytime.

I definitely want each emitter to appear to be independent of the others, rather than multiple ones blinking at the same time. I've seen circuits using 555 chips that will control up to 8 LEDs and give a random blinking effect for each LED, but each LED appears to spend approximately equal amounts of time on and off, and I'd like them to be more off than on. I even found one circuit that ramped-up the brightness (rather than just going on with full brightness), which I thought was great; it would look more "organic" than a hard on/off.

The main question I'm having right now is whether to try to put something together using ICs/capacitors/resistors/transistors, or instead using something semi-prefab like Arduino or Raspberry Pi. I have no experience with such things, but I can read a schematic and use a soldering iron and have put together circuit projects before. Am I correct in assuming that it would be difficult to achieve the effect I want using ICs and discrete components? Would I be able to achieve the effect using Arduino or some other microcontroller? Having some ability to tweak flash rates would be helpful, and I don't know if that's doable with just 555 timers, and their ilk.

Any advice would be appreciated.

Here is an LED flasher that blinks the LED for a fraction of a second every 8 seconds or so. With 30 LEDs in the system how many did you want on at any given time? This circuit blinks on for about 200 ms. No current limiting is used on the LED so it drives all out for that 200 ms. I used a white ED to test it with. The 2% or so duty cycle falls within the safe limit of the LED at high current. Timing can be adjusted easily.

 

[Herschel Peeler]

View attachment 29439


Here is an LED flasher that blinks the LED for a fraction of a second every 8 seconds or so. With 30 LEDs in the system how many did you want on at any given time? This circuit blinks on for about 200 ms. No current limiting is used on the LED so it drives all out for that 200 ms. I used a white ED to test it with. The 2% or so duty cycle falls within the safe limit of the LED at high current. Timing can be adjusted easily.

How about 10 LEDs that each blink for about 200 ms. About 80 seconds to run a complete cycle. I use the high side current limiter of the TTL circuits to limit the LED current. I haven't tried this yet with white or UV LEDs.

 

[RCH]

View attachment 29441

How about 10 LEDs that each blink for about 200 ms. About 80 seconds to run a complete cycle. I use the high side current limiter of the TTL circuits to limit the LED current. I haven't tried this yet with white or UV LEDs.

That sounds pretty close to ideal! So, since the circuit you provided drives 10 LEDs and I want to drive 30, I guess that means that I'll need to build 3 complete, separate circuits, which will give me an overlap of 3 LEDs flashing nearly in unison. That would be fine, especially since I can determine where those LEDs will be, and arrange them so that the flashing is aesthetically "balanced", rather than all 3 of them being right next to each other.

I don't see any potentiometers in the circuit. How would I be able to control the frequency or duration of flashes?

Thanks again!

 

[Herschel Peeler]

That sounds pretty close to ideal! So, since the circuit you provided drives 10 LEDs and I want to drive 30, I guess that means that I'll need to build 3 complete, separate circuits, which will give me an overlap of 3 LEDs flashing nearly in unison. That would be fine, especially since I can determine where those LEDs will be, and arrange them so that the flashing is aesthetically "balanced", rather than all 3 of them being right next to each other.

I don't see any potentiometers in the circuit. How would I be able to control the frequency or duration of flashes?

Thanks again!

Yes to three circuits but I doubt if they would be synchronized. Or you could adjust the timing resistor a little on each circuit to be certain. You could use a pot as the timing resistor and make it variable. R2 a 1 M pot.

 

[RCH]

Yes to three circuits but I doubt if they would be synchronized. Or you could adjust the timing resistor a little on each circuit to be certain. You could use a pot as the timing resistor and make it variable. R2 a 1 M pot.

I would actually prefer if they weren't synchronized; I just want to avoid them blinking together. It sounds like they'll take care of that on their own, through the variances between their oscillation rates. And I'm big on adding pots wherever practical for that very purpose (being able to tweak things).

I'll stop by the local electronics store tomorrow to pick up the necessary components, and commence breadboarding.

Thanks again!

 

[seanspotatobusiness]

Has this projected been able to proceed? It sounds really interesting.

 

[AnalogKid]

None of those schematics have current limiting resistors for the LEDs.

Look into joule thief circuits. If you can get the flash circuit small enough, you could have one at each LED rather than a central controller. Then, the only thing connecting the LEDs is power and ground, so you can expand very easily by just hanging more lights on the power buss. Think about a wine cork or smaller, with two wires coming out the top end for suspension and power, and the LED hanging out the bottom end.

ak