How do the trees breathe?

By Alan Killan

How does a tiny computer control hundreds of lights to make a breathing tree?


by Alan Kilian, who designed and built the voltage controllers for “Winterlights: A Circle of Peace.” (This is written for non-engineers, the details are a little bit more complex than this explanation.) The way we vary the brightness of the trees is by sending them different voltages. If we send a lower voltage, the tree is dim. If we send a higher voltage, the tree is bright. Our tiny computer can create different voltages so it seems that we could connect the small computer to the tree and have the computer send different voltages which would make the tree breathe, but it's not that easy.


Our tiny computer can only produce a small amount of power and the trees take a medium amount of power to light up. Our computer could send voltage to 10 lights just fine, but the trees have hundreds of lights and that required more power than our small computer can produce. We need something in between the computer and the trees which can be told how much voltage to send and can send enough power to light up all the lights on a tree. We use a variable voltage power supply to send the voltage to the tree. It can send enough power to light up all the lights on a tree. But we need to make it listen to the computer to send different voltages as the tree breathes. Imagine our power supply is a person looking at a display that shows a voltage. We ask our person to turn a knob until the display say 12 Volts. If the display says 13 Volts, the person turns the knob down a little until the display says 12 Volts. If the display jumps down to 5 Volts, the person turns the knob up a lot until the display once again says 12 Volts. This is what our variable Voltage power supply is doing. We ask it to send 12 Volts out and it "turns a knob" inside itself to make the voltage match what we want it to be at 12 Volts.

How can we get the power supply to "listen" to our small computer's voltage suggestion and adjust itself to match our desired voltage? We added some electronic parts between the voltage being sent to the tree and the display. We can use these parts to fake the value on the display. Using this circuit, the computer can add in some voltage which will cause the display to read a higher amount of voltage than the tree is receiving. If the power supply is sending 12 Volts to the tree, and we have the computer send 1 extra volt which will get added to the display value, the display will now read 13 Volts even though the output is only 12 Volts. This will cause the power supply to turn the knob down a little until the display read 12 Volts. The result will only be 11 Volts sent to the tree. The display will read the 11 Volts being sent to the tree PLUS 1 volt send by the tiny computer to show 12 Volts. If we have the computer send an extra 6 Volts to the display, the power supply will turn its output down to 6 Volts sent to the tree plus the 6 Volts from the computer will add up to 12 Volts on the display which makes the power supply satisfied. The computer can send an up/down/up/down series of voltages to the display and the power supply will react by turn its knob up and down which will send a varying voltage (with a lot of power) to the tree lights making it breath. This is how we can use a tiny computer to control a bigger power supply and make the trees breathe.

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