Solar Charging and Switching Circuit for Outdoor Sculpture Installation

Over the winter, my friend Steve Atwood got a commission for a sculpture to be installed in the Wichita Falls, TX Kemp Center for the Arts “Art on the Green” sculpture garden from May 2011 – 2012.

Lure 22 V2.0 by Stephen Atwood at Kemp Center for the Arts, Wichita Falls, TX

He had in mind to continue a series of his sculptures based on the form of a fishing lure but wanted to enhance this sculpture with one or more LEDs, preferably that would come on only at night. We discussed a wide variety of options that we hope to develop for another installation in the future; but in the end, in the interest of time for this project, Steve found control modules that flash up to five LEDs at random and installed them behind a set of cones protruding from a recessed panel.

He asked how to make the LEDs turn on at night and also wondered whether he could power them for a year from a primary battery or whether he should use rechargeables.

Lure 22 V2.0 by Stephen Atwood at Kemp Center for the Arts, Wichita Falls, TX

About seven years ago, I had come into possession of some discarded solar yard lights, and out of curiosity had reverse-engineered their charging and control circuits. Since yard lights accomplish both functions — charging and switching — I figured the circuit would be perfect for the sculpture. I was able to find one and instruct Steve how to modify it for his needs.

Solar yard light schematic

The circuit is very simple and I find it rather elegant. During the day, the solar panel assembly (left — for want of a proper schematic symbol, I just drew another battery) charges two AA cells through a diode that prevents the battery from damaging the panel with reverse voltage at night. Additionally, through the R1 – R2 voltage divider, the solar panel pulls up the base of Q1, switching it off and allowing R3 to pull up the base of Q2, switching it off and switching off the load LED1.

At night, the panel’s output approaches 0V and R2 pulls down Q1‘s base, causing Q1 to conduct and pull down Q2‘s base (in a Darlington-like arrangement — I don’t know whether it’s still considered a proper Darlington with R3 pulling up the Q1 emitter – Q2 base connection), switching on Q2 and LED1. In fact, depending on the panel’s exact voltage, the load may switch on even before full darkness, and R1 – R2 can be tweaked to tune the turn-on point.

Control board from solar yard light, modified

Steve removed the LED from the control board and replaced it and the fly wires for the solar panel and battery with screw-terminal connectors for ease of installation inside the sculpture. He bought a new solar panel with a higher output voltage to charge the higher-voltage battery for the white LEDs he wanted to use (the yellow LEDs in my yard lights didn’t require as high a forward voltage) and milled a Lexan cover for it to protect the panel from hail, with an O-ring groove to protect it from rain as well.

With higher battery and solar panel voltages, Steve indicated the load was turning on before the ambient light got as dark as he wanted, so I told him how to locate R1 and replace it with fly wires to a 100K pot. After the swap, he said he was able to tune it perfectly and he was delighted.

Lure 22 V2.0 by Stephen Atwood at Kemp Center for the Arts, Wichita Falls, TX, night view

I’ve not had a chance to visit the sculpture garden and probably won’t while Lure 22 is installed. If anyone’s in the area, I’d love to hear from you how well it’s working and how well the electronics hold up over the course of a year outdoors.

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