I recently took a break from BMOW’s multi-tasking support to work on something completely different. (Actually, I have a multi-tasking demo working in the simultator, but it doesn’t work on the real hardware yet.) The “something different” was a robo-flower: a pulsing light powered from solar energy that’s made to look like a flower from Junkyard Wars. The circuit was based on this Make Magazine article, but the construction design is my own. Here’s a photo of the result:

During the day, the solar cell charges the main storage capacitor. At night, the storage capacitor powers the circuit, making the LED in the flower’s center pulse organically every few seconds. I followed the circuit design from the Make article, with these exceptions:

• Replaced the 1000uF “pumm” capacitor (C3) with a 3300uF one. This was one of the suggested alterations.
• Replaced the rechargable battery with a 1F “super capacitor”. A 10F capacitor was a suggested alteration, but I couldn’t find one.
• Where their diagram shows resistor R3 connected to the positive power supply, I instead connected it through a diode to the negative end of the LED. This makes the LED turn off all the way at the end of each pumm, which I think looks better.

The robo-flower works, but I’m pretty disappointed with how it turned out. For one thing, the pulses of light are surprisingly dim, compared to what I’d expected. They reach a brightness I’d call “on”, but not bright. If you look at the circuit diagram, you’ll see that the pulses come from the 3300uF capacitor (charged to about 3v) discharging through an LED. With no current-limiting resistor, there should be a brief pulse of high current = high brightness, but that’s not what I’m seeing.

The bigger disappointment is that even after leaving the flower in the sun until the main 1F storage capacitor is fully charged (as measured with a voltmeter), the pulsing only lasts for about 15 minutes after the sun’s illumination is lost. That’s pretty lame. Other designs that use rechargable batteries pulse nearly all night, I think, so I was expecting much more. Yes, it’s a capacitor powering the circuit and not a battery, but 1F is a mighty-big capacitance.

If I get motivated enough, I may build another copy of the circuit on a protoboard, so I can experiment with it further, or measure the voltage at various points on the oscilloscope, to see if I can’t improve the behavior. Otherwise it’ll be one more piece of odd electronics junk littering my home. 🙂