Background : Red Grow Light
I have been experimenting with and using LED lighting of various types in greenhouses for many years now. I have had great success with LED lighting in propagators to shorten the germination time of light sensitive seeds such as begonias, impatiens and lobelia. About four years ago i decided to investigate using red and blue LED lighting to improve the development of small plants. Below is a record of the development of a simple prototype i have successfully used to improve the development of foliage on Geraniums, Begonias and Impatiens. Because of the very low heat output of the lamp it can be used very close up to the plant foliage without scorching or damaging it.
This prototype was developed with the intention of using it to experiment with the effect it could have on developing and established plants.
The picture above in the header of this page shows one of these lamps being used in the greenhouse to aid the development of Impatiens plants. One of the white LED lighting units that i also use can be seen on the right hand side. It was switched off to allow this picture to be taken. The red LED lamps proved to be very successful when using them in early Spring when the light levels were poor due to cloud. One of the downsides of trying to grow plants in Scotland!
This prototype was developed with the intention of using it to experiment with the effect it could have on developing and established plants.
The picture above in the header of this page shows one of these lamps being used in the greenhouse to aid the development of Impatiens plants. One of the white LED lighting units that i also use can be seen on the right hand side. It was switched off to allow this picture to be taken. The red LED lamps proved to be very successful when using them in early Spring when the light levels were poor due to cloud. One of the downsides of trying to grow plants in Scotland!
The circuit used to drive the LED's was simple and efficient.
LED's operate most efficiently and reliably when driven by a constant current.
A constant current circuit was implemented using a LM317 and one resistor. The current was set to 25mA, above the 20mA nominal value but below the 30mA maximum value.
It would not be advisable to run too close to the maximum of 30mA in the greenhouse due to the extreme range of temperatures that can be experienced during each day hence i chose 25mA as the operating current. Experimenting on a plug-in breadboard before constructing the final circuit showed that the LED's gave a good strong output.
Each LM317 drives a string of 6 red LED's. Each string consumes 25mA. The circuit board has 7 strings so the entire circuit board consumes about 175mA.
The circuit is driven from a surplus 17.5V laptop power supply. This gives a small overhead voltage above the minimum required to drive the circuit.
To make good efficient use of the available circuit board space the strings of LED's and the LM317 driving them were alternated.
This picture shows the circuit board in operation outside the case it is normally mounted in.
This picture shows the circuit board mounted in its clear plastic box.
To give maximum flexibility the case is small enough to allow it to be mounted horizontally or vertically either above or in among the foliage of a plant.
I had great success with these small prototype lamps using them with geraniums and in particular with tomato plants.
Buying the LED's in bulk kept the cost down and this one cost less than GBP5 including the case and circuit board.
As a result of the success i have had with these units i intend to build similar ones with more LED's in the same area to improve their light power output.
Update January 2017....
Over the past couple of years (2015/2016) i have had a great deal of success with these LED grow lights when used in close proximity to the plant foliage.
Although the light output is relatively small the lamps can be used in close proximity to the foliage due to the low heat output which does not scorch or damage it.
I have upgraded the casing of the lamps by lining them with reflective silver plastic film ensuring as much of the light as possible is directed at the foliage and does not escape through the back and sides.