Light Dependent Slow Blinking LED's

Thread Starter

Fishy_safc

Joined Aug 12, 2019
12
Hi Everyone,

I did electronics in high school and thats about it. So i'm a complete novice. And any help is greatly appreciated.

I'm a memorial mason by trade but am looking in to new ideas for memorials. My main one being the use of light dependent slow blinking LED's (around 10-15 in number) which turn on at sundown to replicate stars, powered by a set of small solar panels attached to the top of the memorial. I'll drill straight through the granite to place the LED's in, and the PCB and wires I plan to run in to a watertight junction box out of sight at the back of the memorial. The issue I'm having is that I have very little knowledge of all the important stuff to get this working such as power required, correct solar panels, are there pre-built PCB's out there that I can use, I have seen some but they only have 1 led on them, could I possibly take that out and power them all from that 1 connection point. Would a light dependent resistor work and where would that go etc..

Any pointers on what I need to be looking for and/or calculations I need to do prior to buying the bits would be very much appreciated.

Thanks.
 

AlbertHall

Joined Jun 4, 2014
9,022
The solar panels would need to charge a battery during the day so they can provide power when it is dark. You could use the lack of voltage from the solar panels as a signal to turn on the LEDs so a light dependent resistor is not needed.

How long do you want the LEDs to be on?
 

Thread Starter

Fishy_safc

Joined Aug 12, 2019
12
Would not need to be all night as its very unlikely there will be people in a cemetery at 3am to see them. I'd say 4/5 hours at low light.
 

MrChips

Joined Oct 2, 2009
19,923
You have a very interesting idea that is also very doable. My guess is that anything that is already available off-the-shelf can be be improved. With a custom design you can have any amount of changing patterns. I already have a demo circuit with 16 LEDs and I will post a video when I get a chance. The physical size can be very small, about 1 x 1 inch. I can ship you the PCB and chip and you can do the assembly or I can do partial assembly. Here are some options to think about:

1) up to 16 LEDs
2) different colours, red, green, yellow, blue, white in some fixed pattern
3) fade in-out is doable
4) any number of patterns in any sequence

As for power requirements, all it takes is to do a power analysis and then conduct a real test.
 

Thread Starter

Fishy_safc

Joined Aug 12, 2019
12
That sounds like a good starting point and would be very interested in moving forward with. Any idea of what voltage solar panel I should be looking at? And will I 100% need batteries? the reason I ask is that its quite common to find solar powered garden LED lights in front of graves that come on at night, I take it these will have batteries somewhere in them and the panel also acts as the switch when it turns dark?
 

MrChips

Joined Oct 2, 2009
19,923
We can work out the power requirements and electrical design.
First you need to specify the LED arrangement and light sequence that you desire.
 

AnalogKid

Joined Aug 1, 2013
8,252
its quite common to find solar powered garden LED lights in front of graves that come on at night, I take it these will have batteries somewhere in them and the panel also acts as the switch when it turns dark?
Completely correct. And, depending on the power analysis, "re-purposing" a garden light might be the best way to power a prototype at low cost.

ak
 

Thread Starter

Fishy_safc

Joined Aug 12, 2019
12
I think a series of 10 white LED's. Ideally random like stars so all fading in and out at completely random times if doable, if not I suppose just making up a sequence so they all come on once then repeating it. Not a bad idea at all, I'll look in to buying one and stripping its innards, should give me some idea of how this is going to work too.
 

MrChips

Joined Oct 2, 2009
19,923
You can purchase garden lights in quantity when on sale for less that $1 or €1.
You can either re-purpose the light as suggested by AK and simply add your own electronics or salvage the useful components such as the solar panel out of it.
 

MrChips

Joined Oct 2, 2009
19,923
I think a series of 10 white LED's. Ideally random like stars so all fading in and out at completely random times if doable, if not I suppose just making up a sequence so they all come on once then repeating it. Not a bad idea at all, I'll look in to buying one and stripping its innards, should give me some idea of how this is going to work too.
It is the other way around. You dictate how you want it to work.
As you have already requested, it is activated on sunset and runs for about 4 hours.
The garden lights I have seen have cheap NiCds cells which do not last for more than a year. I intend to use supercaps which have much longer lifespan.
 

Thread Starter

Fishy_safc

Joined Aug 12, 2019
12
Yeah, the longer the better, last thing I want is complaints every year because of dead batteries, at least if longer than a year I can give some sort of guarantee.
 

MrChips

Joined Oct 2, 2009
19,923
A quick calculation says we cannot do it with a super cap. We will have to go with a rechargeable battery. I am looking at a 3.7V 400mAh Li-ion battery.

The solar panel I have is 2¾" x2¾" and outputs 3.5V in full sunlight.
 

Thread Starter

Fishy_safc

Joined Aug 12, 2019
12
That's fantastic, the memorial I'm using is 3" thick so the panels will fit snug on top. it's also 21" wide so if it's beneficial to have 2/3 panels that could be done with creating a problem.
 

Reloadron

Joined Jan 15, 2015
4,902
When I have seen the starry night effect done in theater applications it was accomplished using white lights fading in and out behind gel filters at various speeds. If I were to look for a starry night effect I would likely start with a color picker like this one to get a few color themes for the stars. I would use RGB LEDs making it easy to generate the wanted color(s).
I'll drill straight through the granite to place the LED's in, and the PCB and wires I plan to run in to a watertight junction box out of sight at the back of the memorial.
After drilling and seating the LEDs in a pattern possibly a liquid acrylic could be left to harden. I am guessing when a LED fails you will want to replace it. The fade in and fade out can render a nice twinkle look when things are set right through a little experimentation. Anyway, something to think about.

Ron
 

Dodgydave

Joined Jun 22, 2012
8,679
I would go with garden ornament lights, ready made white leds with a solar panel and nicad built in, or you're looking at bigger solar panel and a 3.6V Lithium-ion battery.
 
A "3.7V" Li-Ion cell is fully charged at 4.2V and is almost discharged at only 3.5V so the 3.5V solar panel will not work.
There are Li-Fe-PO4 cells available that are fully charged at 3.65V.
 
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