Growlight experiments using LEDs

We are going to do some experiments trying to lengthen the day light-wise on salad leaves, getting them jump started to harvest earlier. Britain is a funny place to grow things, the maritime microclimate means it is relatively warm for the latitude. The growing season can be limited by temperature and/or light. Because we are warmer than typical for the latitude there might be some mileage in tackling light. Britain is basically warm and dark from a plant POV. You tackle temperature using a greenhouse, or in our case a polytunnel.

the mounts let us set the lights higher as the seedlings grow
the mounts let us set the lights higher as the seedlings grow
plant-level view
plant-level view

Trouble is we have no power on site, so this is an LEDs and leisure batteries job. It’s easy enough to turn off the lights once real light is bright enough[ref]I went off this after reading the Philips material and some other web research in favour of a straight time switch[/ref], but a leisure battery gets trashed if I just let it run down too  much, ideally I want to pull the load once it’s down to 11.5 V or so. This gets complicated when you have something charging the battery too, but the whole point I am trying this is because we are around the Winter Solstice so there’s not much light about. So the only way that battery is going to get charged is for it to be recovered, changed out, charged and reinstalled. So once the battery gets lower than 11.5V it’s time to pull the load and keep it pulled until the battery power is reset. Even if the battery level creeps up above 11.5V off load that load should still stay off.

the batetery, timer and low-voltage cutoff
the battery, timer and low-voltage cutoff

What LED colours?

LEDs give the opportunity to favour certain wavelengths – leaves are green because plants don’t use much green, which gets reflected back for us to see.

Red and a little blue seem to be the principal colours used in this Yorkshire facility
Red and a little blue seem to be the principal colours used in this industrial Yorkshire LED4CROPS facility

Red and blue seem to be the light colours wanted, with more blue in the initial stages, red for the flowering stages. This seems to be starting to be big business, though I do somewhat wonder at the approach taken. Many of the woes of industrial agriculture stem from its arrogance in separating variables and hitting one particular aspect for all its worth – chemical farming addressing nutrients but destroying soil micro-organisms that have cycled nutrients for millennia resulting in veg that increasing lacks trace elements (the McChance and Widdowson The Composition of Foods longitudinal research)  and tastes of bugger all being one example. There’s a difference between trying to push things a bit but still working with natural light and growing stuff in windowless warehouses and hubristic statements like

We are beginning to understand that growing crops in this way can improve their quality in many different ways, from their shape and colour to their flavour and nutritional value. We could, for example, increase plants’ vitamin C content.

Hmm. Maybe if we could answer why the mineral content of industrially grown foods has been falling and often tastes bland compared to 30 years ago or more I’d have more confidence in that statement. Can’t argue that yield has gone up due to industrial farming, but quality?

Back to the LEDs – we will always be short of power, though at least we are growing plants that can grow okay in the UK – salad leaves, just trying to advance them. Many people who use grow lights are trying to grow five-pointed leaf plants that aren’t typical UK horticulture. We should be having an easier job 😉 We are dealing with seedlings, which also makes life easier – we can get the LEDs much closer to the plants, a few centimetres. The RHS publication Science and the Garden: The Scientific Basis of Horticultural Practice seems to support this on page 211 which is just as well given our power limitations

1512_rhsOther sources on the web

LEDgrowlights seems to have to good stuff on what to look for in a grow light. Philips have got into this field, their 18W LED grow-lamps seem to be targeted at the flowering stage and it’s clear that red, deep red and blue seem to be where the action is, and NASA also seems to favour red and blue, indeed they seem to have had grief with just red in their early test.

Constructing a battery monitor/power manager

This is easy enough because I am looking for the battery voltage to drop below 11.5V after which I will shut off the lights. By putting this on our RF network I get the status reported back, and by using the Ciseco RFu I get an almost-free arduino chip so I can throw in a temperature monitor, as well as manage down the shocking 7mA quiescent current of the Arduino by sleeping it most of the time[ref]the ATMega 328 does have a watchdog timer but it’s a slight git to use with the Arduino[/ref].

Measuring the battery voltage depends on the reference voltage which is supplied by a KY5033 linear regulator (Texas LP2950 fixed 3.3V) There are actually pretty good, within 2% across -40 to 100C, which is better than the 5% tolerance of my resistor divider, which I want to arrange so the output is 3.3V/2 when I apply 10.24V, which can be done with a 115kΩ top resistor into a 22kΩ lower resistor. The total string draws 12/137 mA which is about 100uA, and the source resistance is about 18k, above the 10k recommended in the datasheet but I am only looking for about 8 bits of resolution. I could put a capacitor across the ADC input to improve that, but I can live with the error.

Jeelabs shows I could take this a lot further, but it’s good enough. I will be powering a string of LEDs drawing about half an amp for half a day or more, so as long as someone gets to the battery in about a week after it shuts down I will only have drawn another 0.02Ah from this cause, plus a bit more from the temperature and RF reporting for a few seconds every 15 minutes.

system overview
system overview

Results – failed

because we couldn’t keep the power up long enough, it just needed too many changes of batteries. We were also fighting the fact that the lighting blocks some of the daylight, so it was probably overall a reduction of light.

I have had success using three 11W CFL lamps about 60cm above some seedlings at home where power is available, so the idea of manipulating light is sound. But it’s not low-power, unfortunately, even with LEDs.

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