A Solar Engine is a circuit that takes in electrical energy from a solar cell, stores it in a capacitor, and after a sufficient amount has been saved up, switches the stored energy over to drive a motor.   When the running motor has used up a set portion of the capacitor's stored energy, the solar engine circuit switches the motor off and goes back to collecting and storing energy.  The  cardinal virtue of a solar engine is that it provides usable mechanical energy when there is not enough light - or not enough cell area - to run a motor directly or continuously from the solar cell.  It is an ideal way to intermittently power tiny robots, models, toys, or other small gadgets on indirect as well as direct sunlight or from artificial room light.

The Easter Solar Engine Instructable describes in some detail how to make a versatile and robust solar engine from common discrete electronic components: just transistors, resistors, diodes, and LEDs.  That Instructable also provides general background information on solar engines and tips on choosing the right motors, cells, and capacitors to use with solar engines, and so it can be a useful reference when making any type of solar engine.  

This Instructable describes an easy to make solar engine which has a special  IC as its heart, namely the CSS555.  This chip is a micropower version of the well known 555 timer IC.  It is pin for pin compatible with the basic 555, and has many quite remarkable additional features, but for a solar engine application, its incredibly low operating current of under 5 microamps is what makes it ideal.

The basic idea to use a 555 IC as the heart of a solar engine originated with Manfred Schaffran in 2003.  He used the  CMOS version of the 555, namely the 7555, which was the lowest power 555 then available.  Shortly after, Wilf Rigter came up with the idea of employing a photodiode as a voltage reference for triggering the 7555.  The circuit described here is basically the same but with the improved CSS555 IC which requires less than a tenth of the current of the 7555 and so gives very efficient operation and excellent low light operation.  This Instructable also includes a simple modification of the circuit which permits the motor to operate at a slightly higher voltage if desired.

Step 1: The Basic Circuit

The solar cell SC charges the storage capacitor CS and powers the CSS555 IC via pins 1 and 8 (the pinout diagram is included below).  Pin 4 is the reset which must be held high to enable the operation of the chip.  PD is a photodiode used as a constant voltage source with a fixed lighting level.

When the voltage in the capacitor CS rises to about 3 times that of the PD, Pin 2 is only 1/3 of the supply voltage to the chip.  In that case, the 555 makes the output Pin 3 of the chip go high.  This turns on the transistor QP and the motor starts.  RB is a 1K resistor just there to limit the current through the base of QP.  As the motor continues to run, the voltage of CS drops - unless the solar cell is large enough or the lighting strong enough to maintain the voltage while the motors runs.  The operation of the 555 is such that Pin 3 will now stay high as long as the voltage also fed to Pin 6 by the photodiode remains below 2/3 of the supply voltage to the chip.  Thus, if and when the voltage across Pins 1 and 8 falls to about 3/2 of the photodiode PD voltage, the chip turns Pin 3 off, the transistor cuts off, and the motor stops.  The capacitor CS then can charge up again and the cycle repeats.

Step 2: Stripboard Construction

The main components of the CSS555 Solar Engine are labeled in this photograph.  The CSS555 chip is available at Jameco.com.

The solar cell shown here is a thin film on glass type available from imagesco.com (#SC-02).  Although listed as an outdoor cell, it works very well on indirect and good room light.   It is rated at 5.2V OC and 21mA SC.  Cells rated as indoor type work well under fluorescent lighting, like calculator cells.  Whatever cells are used, they should be capable of putting out a volt or two above the turn-on voltage of the solar engine to ensure an adequate rate of charge for the capacitor.

The motor is a low current model suitable for solar operation.  The static resistance of motors that work well with solar engines is in the neighborhood of 10 Ohms.  A typical "toy" motor made to run on a battery or two is under 2 Ohms which is much too low for this kind of application; the capacitor will discharge through such a motor before it even begins to move.

The capacitor shown is 0.1 Farad rated at 5 Volts.  This solar engine has shown itself to work perfectly with capacitors ranging from 1000uF to 1.0F.  A requirement for any capacitor in any low power solar engine is that it have a low ESR - under 1 Ohm. 

Most ordinary LEDs actually function as photodiodes, that is, when light falls on an unpowered LED, it outputs a voltage and current.   A typical LED will usually put out around 1.2 Volts in bright light, but that varies among LEDs with some being a few tenths higher or lower.  The output current of an LED is extremely small, but enough for the high impedance of the Trigger and Threshold pins of the CSS555.

Now with an LED putting out 1.2V, the turn on voltage theoretically would be 3.6V and the turn off 1.8V.  With 1.4V from the LED, turn on would be at 4.2V and off at 2.1V.  This compares very well with measured voltages using a variety of LEDs: red tinted, clear red, green,yellow, small and large.

The output transistor in this circuit happens to be a ZVNL110A Mosfet, but an ordinary 2N3904 transistor works just fine.

This circuit is made on a small piece of Stripboard.  Some of the details of construction are visible in the accompanying photos.

Step 3: Experimenter Board Version

This photograph shows the circuit made on an "Experimenter's I.C. Protoboard" made by DATAK (# 12-607).  Details of  the  layout are shown in the lower photographs.

Step 4: Low Light Operation

This solar engine works reliably well across the entire range of lighting levels from outdoors to inside. In an experiment to check for dependable operation at the lower end of the lighting range, the engine was set up for a near extreme low-light small-cell test .  The solar cell was a little SC-2422 which gave just 36uA SC and 4.75V OC under the chosen light level.  The engine was first fitted with a 1000uF capacitor.  Near the 3.5V turn-on point, the solar cell was supplying just 15uA to the engine.  The engine cycled on and off perfectly.  Further, a 22000uF capacitor was plugged into the engine and the test repeated.  Charging this larger capacitor with the tiny cell was extremely slow (about 0.3mV/sec near the firing point), but the engine cleanly turned on and off when it was supposed to.  This test confirmed the reliability and efficiency of the CSS555 solar engine.  Hacking a meter into the circuit revealed that the chip itself was only taking 3.5uA during the entire charging process.

Step 5: Solar Whirligig

An amusing application of any  solar engine is to operate a "Solar Whirligig" as pictured here.  This video shows it in action.

Step 6: Walker the Robot

This is "Walker" -  a little robot that soaks up solar energy and every now and then takes off for a short but energetic stroll. His energy comes from a RU6730 polycrystalline cell atop  his head.  You can catch a glimpse of him in action on this short video.

Step 7: Modified Circuit

The maximum operating voltage of the CSS555 IC is 5.5V.  Therefore one cannot string two LEDs in series as input to pins 2 and 6 to run the motor at a higher voltage.  However, the following trick can be used instead, which is to supply power to the chip trough a diode or two or three. The circuit is the same as before, but two 1N914 diodes connect the chip to the supply line.  The drop across the diodes allows the voltage in CS to go higher than the PD would allow.  The Pin3 still clicks on and off at the voltage levels that the chip sees, just as it did before, but the voltage supplied to the motor from the capacitor is higher.

This circuit was tried with the following measured results (in bright light with a LED photodiode giving 1.42V):

# of diodes     Turn on voltage     Turn off voltage
        0                      4.3                        2.3
        1                      4.7                        2.7
        2                      5.0                        3.0
        3                      5.4                        3.3

Note that the voltages listed here are measured at the capacitor, CS.  Also note that the voltage drop across the diodes is less than the usual 0.6V or so because the current taken in by the CSS555 is so meager.
<p>please also load video... :)</p>
<p>plz also provide circuit diagram if u have ?</p>
<p>Look at Step 1 ...and Step 7 !</p>
<p>Link is in Step 5.</p>
<p>hey i need you to tell me please again what can i use to do the solar engine .... i mean the tools i need this for my school project and i think i cant understand such hard things , but i insist to do it so please write me the tools u've used ... if u respond to that i'll appreciate it :)</p>
<p>For someone starting from scratch, it is a good idea to invest in an introductory book on making electronics circuits and begin with a simple project or two. I made some recommendations along those lines in the last paragraph of the introductory step of the Instructable on the Easter Solar Engine. In addition to the book cited there, the following two are excellent for getting started in electronics: &ldquo;Robot Building for Beginners: by David Cook and &ldquo;Make: Electronics&rdquo; by Charles Pratt.</p>
<p>I used your circuit now the third time but now I do have a video: <iframe allowfullscreen="" frameborder="0" height="281" src="//www.youtube.com/embed/e9WRUs527yQ" width="500"></iframe></p><p>I did not have a CMOS 555 so I used a standard chip and a 2N2222 transistor which works as well.</p>
<p>can 2 ni-cd AA @ 1.2v 200mAh battery's be used instead of the capacitor be <br>used? I'm making a solar powered fan for a solar dehydrator. I plan on <br>putting a switch that lead to the motor for the ability to turn it on <br>and off, when not in use.</p>
<p>Solar engines are designed to work with capacitors. If you want to charge and use batteries with a solar panel, you'll want a &quot;solar charge controller&quot; circuit that is compatible with the charging requirements of the battery type. Alternately, you might consider connecting the fan directly to a suitable solar panel - when the sun shines, the fan runs.</p>
Can I use a NE555N Instead of the CS555 you are saying ?
<p>Here is what you need to consider: the bipolar NE555 requires a minimum of 4.5V and about 5mA supply current, whereas the CSS555 needs only 1.8V and under 5uA.</p>
Thanks for your fast reply ,<br>I've seen myself a few minutes ago that the circuit doesn't work and the led lighting , is that normal considering i used the wrong IC ?
<p>I know that if I plugged a bipolar 555 into my circuit, it wouldn't work. You might try a 7555 if you have one; it's been documented to work. But it won't be near as efficient as the CSS555.</p>
I looked everywhere for a CSS555 but <br>all i got was a TLC555CP ... Hmm i guess i'll see if this one works
<p>As stated in Step 2, the CSS555 is available from Jameco.com.</p><p>The TLC555 might work (2V, 170 uA min.), but if so would not be as efficient as say a ICL7555 (2V, 60uA min), and that would not be as efficient as the CSS555 (1.8V, 5uA).</p>
I Constructed it with the TLC555 and with a 2000&mu;F Capacitor bank and i have to say it gives short power bursts to a motor every 15 seconds !<br>Amazing instructable ! Thanks for your support ! :D
<p>Congratulations ! Glad you got one going !</p>
<p>I've used a NE555 and it works. OK, it's not very efficient but it works.</p>
<p>I've made it but don't have pictures yet :-) Very easy solar engine and the best is that it runs with my favorite chip, the 555. Well done!!!</p>
<p>Well here's a photo of mine , it doesnt look neat cause i made it on a breadboard but works fine</p>
<p>Could you go into more detail about motor selection? You say that you shouldn't pick a &quot;toy motor&quot; because the resistance is too low, but you also say that you should pick a &quot;low current&quot; motor. I generally have not seen too many motors that list their resistances in the datasheet. Is it just a simple V=IR calculation to determine the resistance? If so, which end of the voltage range should I use, for those motors that have a range of voltages listed.</p><p>Thank you for very much for your time and help, and thanks for writing this instructable! It's very helpful. I'm planning on using this circuit to spin a prism in a window. It's nice to have the rainbows spin when the light catches them. </p>
<p>Twirling a prism is an ideal application of this solar engine unit ! <br><br>The static resistance of motors is rarely quoted in mfgr's. or seller's specs. Measure it with a good VOM as a first step in looking for a suitable motor. Better is to test a motor for its suitability by charging a capacitor by connecting it to a battery or two (preferably through say a 5 or 10 Ohm resistor) and then touching the cap to the motor in question to see how it will respond. This will also check the adequacy of the capacitor you intend to use!<br><br>Step 4 of the Easter Solar Engine Instructable shows a selection of motors that work well with little solar engines. A Google search of &quot;solar motor&quot; will turn up some very likely candidates. </p>
Can I omit the engine and use this to charge a lawnmower battery?

About This Instructable




Bio: Emeritus Professor of Mathematics.
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