# Question about a solar cell circuit

Discussion in 'The Projects Forum' started by edcteam2, May 12, 2010.

1. ### edcteam2 Thread Starter New Member

May 12, 2010
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I am working with a group to design a bird that flaps its wings with the implementation of shape memory alloys, which must be powered by renewable energy (we chose solar energy).

Our main problem is that we need to find a way to release energy from the solar cell/capacitor combination to the SMA in a quick burst of current (~500 mA) to actuate the SMA, and then allow no current to flow through the SMA for half an hour as it cools. Then repeat.

What is the best way to design a circuit that would accomplish this?

Our current design was essentially a solar cell connected in series to a capacitor, which was in turn connected in series to the SMA (acts as a resistor in circuits), and then attached to a ground.

We have looked into an NE555 chip to be used as a timing device as well.

Thanks for any help!

Sep 7, 2009
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3. ### retched AAC Fanatic!

Dec 5, 2009
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All depending how accurate your timer needs to be will determine what timer you use.

The 555 gets a little fishy after 10 min or so. You may want to use a temperature sensor to determine when the SMA cools to an appropriate temperature before firing again.

4. ### Bernard AAC Fanatic!

Aug 7, 2008
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What is approx. operating voltage? What is resistance of the SMA? A 30 min 555 timer is doable, maybe triggering an SCR or another 555 & FET.

May 11, 2010
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I would do this with a low-power microcontroller. That would allow you to put other "smarts" into the behavior of the thing very easily... like doing the actuation when the power accumulated gets to a certain voltage... or responding to a button or a shadow, or whatever you can imagine.

A pin of the microcontroller can drive a logic-level gate of a power MOSFET to allow the current to flow from the capacitor through the SMA element.

What voltage are you storing the accumulated charge at? I'd guess you're using a supercapacitor. You can get a 100F capacitor for about \$20 at Digikey last I checked. This will allow you to store power up to 2.5V.

I'd use a TPS60211 charge pump to provide a 3.3V Vcc for the microcontroller. This has an ultra low quiescent current (2uA typical) and can run down to input voltages of about 1.6V... so if you have the uC only drain the supercap down to 2.0V, then there is plenty of reserve to keep the microcontroller powered up for a while, even with no new solar energy. The whole thing can run without a battery. If you want to have a coin cell to maintain the Vcc, it would make the system more robust. You can "or" a 3V Lithiun coin cell or two AA's into the 3.3V supply.

Simple circuit for an energy harvesting bird.

What is this project for? It sounds really cool.