Saturday, 13 September 2014

Making a LED Growlight Panel

This is another build for the greenhouse electrics.  Growlights.

I've been reading up a bit about the light requirements of plants.  There's a lot of very technical stuff out there, but basically it seems that blue-ish light is best for seedlings & green vegetative growth and red-ish light for germination, flowering and fruiting.  

One of the many images from a Google search on the subject of plant light
requirements.  Unfortunately, this was one of several in a wide ranging
cut-and-paste job, and so I don't have the link for an acknowledgement....

Since the plan is to supplement the available sunlight in the winter when we start off the leeks and onions etc, then mostly blue seems the order of the day with just a splash of red.

It gets even more technical when they start talking about specific individual wavelengths within the blue and red regions for optimum growth, e.g. 439, 469, 642 & 677 nanometres, but blue is blue and red is red as far as I'm concerned - there must be at least some of the light emitted that touches on these optimum wavelengths.

This linked datasheet shows the actual wavelength curves for the red and blue '3528' LEDs, so it seems we're at least in the right ballpark for growing conditions.

And for good measure, we'll also include some white LED strips which cover the whole visible spectrum.

Therefore, I bought nine LED lighting strips, each of 1 metre length, rated at 12 V and said to draw around 0.25 A (i.e. ~3 W per strip or 27 W in total).   There's four blue, three white and two red.  

These particular strips comprise the smaller '3528' size LEDs, with 60 LEDs per metre.  There are larger '5050' LED strips available which are brighter, but they consume more energy and I don't think we'll have sufficient battery re-charging capacity from the solar array in the depths of winter.

At around 5 lumens per individual LED, our panel should emit around 2,700 lumens in total.  The lighted area is around 1,000 mm x 150 mm, or 0.15 square metres, enough to illuminate two-and-a-half of our rectangular plant cell trays.  

Therefore, we should have ~18,000 Lux (1 Lux = 1 lumen per square metre).  This compares to around 10,000 to 20,000 Lux from the sun on a brightish white-cloud day, although in direct sunlight at noon there can be up to 110,000 Lux.  But again, it seems we're not far off what's required for growth.

Moving on to the construction; an odd piece of white contiboard sawn to length, a wipe over the surface with meths, and then the strips were simply stuck down on the board using the self-adhesive backing.

I wired the strips up to terminal blocks at one end ...

... and gave the panel an initial test on the kitchen worktop using a CCTV power supply.   This supply could only provide 1.2 A and not the 2.5 A they should pull directly off the battery array.   Even at reduced brightness, it was difficult to photograph very well, but you get the general idea....  LED strip colours are laid W-B-R-B-W-B-R-B-W.

three of the red LEDs not working.....
Three red LEDs are off on one strip - bugger !   Apparently, these things are internally wired and controlled locally in groups of three - there are cut lines marked at 3 pitch intervals on the strips, so I suppose this means that the duff section could be cut out and replaced, but I've no spares and so for now I'll just live with these three on the blink.

Although it said '3M' on the self-adhesive backing paper, these are Chinese-built strips, and I'm a bit concerned about the effectiveness of the adhesive in cold and damp winter greenhouse conditions, given that the panel is suspended upside down over the area to be lit.  So I've drilled the board between each strip at four locations along the length and added individual tie-wraps over each strip, for good measure.

tie-wraps across each strip, at four positions along the panel....

The LED strips were a few quid each and came ready-wired for immediate connection to the power supply.   You can also buy these things in 5 m and 10 m lengths, which works out much cheaper on a per-metre basis, but then you'd need to cut them yourself and fit a special terminal connector to make the wiring - these connectors are also available to buy.  

This panel's now basically finished and ready to install, but there's still a bit more work required on the greenhouse wiring beforehand.

Update 22-Sep-14

We've now temporarily installed the LED panel in the greenhouse, on a timer switch.  

the timer switch.....

light panel rigged up above the upper greenhouse shelf...

We've installed it now so we can test the solar panels and batteries through regular charge and discharge cycles, on a daily basis.  The initial timing regime will be to switch on the panel between 06:00 to 12:00 and then again from 15:00 to 19:00, the idea being to try and provide around 13 hours of daylight equivalent in the 7 to 8 hour long winter day.

The panel is currently suspended simply by tied-off wires but we'll need to rig up a pulley system so we can lower the panel to ensure it's kept just above the top of the planters, and then gradually raise the lights to keep them as close as possible to the top of the plant shoots as they grow.

I bought the 12 V timer switch about three years ago, it was around £15 from memory.  On eBay today, they're available for just six quid !  

And interestingly, all the red LEDs are now working.   Must be a dodgy chip or contact somewhere, or maybe it's just that taking the power straight from the batteries allows the light strips to pull whatever current they need.

Update 25-Sep-14

I thought we'd take a more technical look at the winter daylight hours at our location.  There are many sunrise-sunset calculators available for free on the internet, and some even allow you to download the data in a file format which can be manipulated in a spreadsheet.  So here's the graphed dataset for the upcoming winter for where we are... (click for a larger image)

In the third week of December, we're down to just 7 hours 19 minutes of daylight at the lowest point (this graph ignores the dawn and dusk twilight hours).   

Throughout the period, the latest sunrise time will be 08:26 and the earliest sunset at 15:42, although these extremes actually occur on days over a week apart.

Anyway, I've now reset the timer switch to operate the lights from 06:00 to 09:30 in the morning, and from 14:45 to 19:00 in the afternoon.  This always gives us an hour's overlap between the time the sun is up and the growlights are in operation.

So we'll continue testing on this revised basis and hope we can get enough sunlight on the panels to keep the batteries working and the growlights on....


  1. Disco lights for the plants - sweet! :-)

  2. Did you ever find out whether this brings things on earlier? We use bottom heat and top light (tastelessly provided by three CFLs run through a timer and a photosensitive switch in series) to bring on some seedlings which are already out in our polytunnels.

    I'm considering your method, but putting the strips on perspex with big gaps between the strips so I can suspend the sheet of lights low over the seeds to benefit from the 1/rsquared gain, and natural light can still penetrate between the strips of LEDs

    There's no point in doing that for a mains powered rig because the cost of power is low, but out polytunnels are at a site with no power so the 12V version would score, maybe with a microcontroller just to extend the day a couple of hours.

    But it'd be nice to see a with/without control to see if it's worth the faff ;)

    1. At the minute, no I can't tell if there's been any benefit from these lights, although I built another one which has been indoors over the garage windowsill all winter, above our palm tree shoots, and they seem to be doing OK.

      The greenhouse version still switches itself on every evening but I've nothing underneath it at the present time, it's still too cold here - this should change in the next few weeks, when I put the onions / leeks out in there that are currently sprouting in the warmth of the house. They can tolerate the cold better when they're a little larger, and besides we'll need the space on the kitchen windowsill to start off the tomato and cucumber seeds in early March...

    2. Love that 12V timer, I got one of those too. Did see a site on how to hack a mains timer and use the timer bit, much cheaper since mains timers are made by the squillions. Oh well, must try that added light sometime. I think we already have seeds in the polytunnels so I could try and illuminate one of those patches. Probably getting a bit late for it to have much effect.

      Early March eh, I guess it's cold up North - we have had ours on the go for a couple of weeks now. Our telemetry system tells me the soil temperature in the polytunnels is 7.5 now and hasn't dropped below 0 for the last week. I'm sure I read somewhere that Britain is light limited not temperature limited for the growing season length, presumably because of the buffering of the maritime climate and the Gulf stream making us warmer than the latitude would typically be.

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