simple solar circuits

7/29/2019 Simple Solar Circuits

1/11

Simple Solar Circuits

Simple Solar Circuits:How to get started adding solar power to your small electronics projects. Use the sun topower small solar and battery powered night lights, garden lights, and decorations forhalloween.

The first part of a solar circuit is... a device for collecting sunlight. To keep thingssimple, we're using a single nicely made small solar panel for all of these circuits. Thepanel that we're using for these circuits isthis one, part number PWR1241 from BGMicro, about $3 each. This is a monolithic copper indium diselenide solar panel,apparentlyprintedon a 60mm square of glass and epoxy coated for toughness. On the
http://www.evilmadscientist.com/article.php/solarhttp://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=12269http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=12269http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=12269http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=12269http://www.flickr.com/photos/oskay/2963105579/http://www.flickr.com/photos/oskay/2963105787/http://www.flickr.com/photos/oskay/2963105123/http://www.evilmadscientist.com/article.php/solar


2/11

back of the panel are two (thin) solderable terminals, with marked polarity. (While youcan solder directly to the terminals, be sure to stress-relieve the connections, e.g., with ablob of epoxy over your wires.) In full sunlight the panel is specified to produce 4.5 V atup to 90 mA, although 50 mA seems like a more typical figure.

[Before we move onto our first examples, a word of caution: These are small simple

circuits. In building these, we will quite intentionally gloss over a number of minordetails and issues that are unimportant at these low powers, but could become critical ifyou were to try to scale up.]

Direct Drive:The most obvious way to use power from a solar panel is to connect your load directly tothe output leads of the solar panel.

Here are a couple of examples of this in practice:

On the left, we've hooked up one of our little solar panels directly to a small motor takenfrom an old CD player. When you set it out in the sunlight or bring it close to a lamp, themotor starts to spin. On the right we've hooked one of the panels right up to a high-powerblue LED. The reason that we've used a high-power LED here is that it can easilywithstand 50-90 mA from the solar panel-- a "regular" LED designed for 20 mA would

be destroyed by that current. (The LED is the same type that we used for ourhigh-powerLED blinking circuit.)

Interruption-resistant direct drive:The "direct drive" circuits work well for their design function, but are rather basic. Theyprovide no energy storage, and so are quite vulnerable to blinking out when a bird orcloud passes overhead. For some applications, like running a small fan or pump, thatmay be perfectly acceptable. For other cases, like powering a microcontroller or other
http://www.evilmadscientist.com/article.php/ledblinkerhttp://www.evilmadscientist.com/article.php/ledblinkerhttp://www.evilmadscientist.com/article.php/ledblinkerhttp://www.evilmadscientist.com/article.php/ledblinkerhttp://www.evilmadscientist.com/article.php/ledblinkerhttp://www.evilmadscientist.com/article.php/ledblinkerhttp://www.flickr.com/photos/oskay/2963103643/http://www.flickr.com/photos/oskay/2963103767/http://www.flickr.com/photos/oskay/2964148148/


3/11

computer, a brief power interruption can be disruptive. Our next circuit design adds asupercapacitor as a "flywheel" to provide continued power during brief interruptions.

Instead of adding a single supercapacitor, you might notice that we've actually addedtwo. That's because the supercaps that we had on hand are rated for 2.75 V-- not enoughto handle the 4.5 V output of the panel when sunlight is present. To get around thislimitation, we used two of the caps in series, for which the voltage ratings add, giving usa barely-okay total rating of 5.5 V. (Note: be careful adding capacitors ofdifferentvalues in series-- the voltage ratings may scale in non-obvious ways.) When firstexposed to the light, this circuit takes about 30 s to 1 minute to charge the capacitorsenough that the LED can turn on. After it's fully charged, the circuit can be removed

from the sunlight and still drive the blue LED for about 30 s to 1 minute-- a veryeffective flywheel for light duty applications.

Adding a batteryWhile interruption resistance is nice, a capacitor generally does not provide sufficientenergy storage to power a solar circuit for extended periods of time in the dark. Arechargeable battery can of course provide that function, and also provides a fairly
http://www.flickr.com/photos/oskay/2963946250/http://www.flickr.com/photos/oskay/2964148164/


4/11

consistent output voltage that a capacitor cannot. In this next circuit, we usethe solar panel to charge up a NiMH rechargeable battery and also LED off of the power,which will stay on when it gets dark out.

In this circuit the solar panel charges up a 3-cell NiMH battery (3.6 V). Between the twois a "reverse blocking" diode. This one-way valve allows current to flow from
http://www.flickr.com/photos/oskay/2963103995/http://www.flickr.com/photos/oskay/2963306721/


5/11

thesolar panel to the battery, but does not allow current to flow backwards out of thebattery through the solar panel. That's actually an important concern becausesmallsolar panels like these can leak up to 50 mA in the reverse direction in the dark.We're using a garden-variety 1N914 diode for reverse blocking, but there are also higher-performance diodes available that have a lower "forward voltage."

In this design we are continuously "trickle charging" up the battery when sunlight ispresent. For NiMH batteries and sealed lead-acid batteries (the two types that are mostsuitable for this sort of un-monitored circuit) it is generally safe to "trickle" charge themby feeding them current at a rate below something called "C/10". For our 1300 mAhbattery cells, C/10 is 130 mA, so we should keep our charging below 130 mA; not aproblem since our solar panels only source up to 90 mA.

The other thing to notice about this circuit is that it's pretty darned inefficient. The LEDis on all the time, whenever the battery is at least slightly charged up. That means thateven while the circuit is in bright sunlight it is wasting energy by running the LED: asizable portion of the solar panel current goes to driving the LED, not to charging thebattery.

Detecting DarknessWe havewritten recentlyabout how to make a useful dark-detecting LED driver circuit.That circuit used an infrared phototransistor. To add a darkness detecting capability toour solar circuit is even easier, actually, because our solar panel can directly serve as asensor to tell when it's dark outside.
http://www.evilmadscientist.com/article.php/darkpumpkinhttp://www.evilmadscientist.com/article.php/darkpumpkinhttp://www.evilmadscientist.com/article.php/darkpumpkinhttp://www.evilmadscientist.com/article.php/darkpumpkin


6/11
http://www.flickr.com/photos/oskay/2963104397/http://www.flickr.com/photos/oskay/2963104397/http://www.flickr.com/photos/oskay/2963306745/


7/11

To perform the switching, we use a PNP transistor that is controlled by the voltageoutput from the solar panel. When it's sunny, the output of the panel is high, which turnsoff the transistor, but when it gets dark, the transistor lets current flow to our yellowLED. This circuit works very well and is a joy to use-- it would make a good upgrade tothe dark detecting pumpkin to make it go solar with this circuit.

A solar garden light circuitWhile the last circuit works well for driving a yellow or red LED, it runs at 2.4 V (theoutput of the NiMH battery), it does not have sufficient voltage to drive a blue or whiteoutput LED. So, we can add to that circuit the simpleJoule Thiefvoltage booster to get agood design for a solar garden light: A solar-charged battery with a dark detector thatdrives a Joule Thief to run a white output LED.

Naturally, you'd want to give this a tough, weatherproof enclosure if it were going to berun outside. (A mason jar comes to mind!) This circuit is actually very close to howmany solar garden lights work, although there are many different circuits that they use.

Adding a microcontrollerOur last circuit examples extend the previous designs by adding a small AVRmicrocontroller. We use the voltage output from the solar panel again to performdarkness detection, but instead take it to an analog input of the microcontroller. Themicrocontroller is potentially a very low current, efficient device that lets you savepower by not running the LED all the time, but (for example) waiting until an hour or

two after darkness and/or fading the LEDs on or off, or even intermittently blinking forvery low average power consumption.
http://www.evilmadscientist.com/article.php/joulethiefhttp://www.evilmadscientist.com/article.php/joulethiefhttp://www.evilmadscientist.com/article.php/joulethiefhttp://www.flickr.com/photos/oskay/2963306769/http://www.evilmadscientist.com/article.php/joulethief


8/11

In this example we have the PWM (pulse-width modulation) output of themicrocontroller driving a Joule Thief style voltage booster to run the white LED. (This isone of many, many different working designs for this sort of boosting circuits.)

We also made a second version of this circuit, with two red LED outputs to make aspooky Jack-o'-lantern:


9/11


10/11

To finish it up, we carved a beautiful white pumpkin and added this circuit to make ourmicrocontroller-driven, dark-detecting, solar-powered programmable pumpkin, whichfaded its eyes in and out one at a time. Note the long leads on the solar panel and wires tothe LEDs to reach.

We hope that you might find this introduction to simple solar circuits helpful; let's seethose solar jack-o-lanterns!
http://flickr.com/photos/lenore-m/2962900613/http://flickr.com/photos/lenore-m/2962900613/


11/11

simple solar circuits

Documents