A Solar Engine is a circuit that takes in and stores electrical energy from solar cells, and when a predetermined amount has accumulated, it switches on to drive a motor or other actuator.  A solar engine is not really an 'engine' in itself, but that is its name by established usage.  It does provide  motive  force, and does work in a repeating cycle, so the name is not a complete misnomer.  Its  virtue is that it provides usable mechanical energy when only meager or weak levels of sunlight, or artificial room light, are  present.  It harvests or gathers, as it were, bunches of low  grade energy until there is enough for an energy giving meal for a motor.  And when the motor has expended the serving of energy, the solar engine circuit goes back into its gathering mode.  It is an ideal way to intermittently power models, toys, or other small gadgets on very low light levels.

It is a great idea which was first thought up and reduced to practice by one Mark Tilden, a scientist at Los Alamos National Laboratory. He came up with an elegantly simple two-transistor solar engine circuit that made tiny solar powered robots possible.

Since then, a number of enthusiasts have thought up solar engine circuits with various features and improvements. The one described herein has proven itself to be very versatile and robust.  It is named after the day on which its circuit diagram was finalized and entered into the author's Workshop Notebook, Easter Sunday, 2001.  Over the years since, the author has made and tested several dozen in various applications and settings.   It works well in low light or high, with large storage capacitors or small.  And the circuit uses only common discrete electronic components: diodes, transistors, resistors and a capacitor. 

This Instructable describes the basic Easter Engine circuit, how it works, construction suggestions, and shows some applications.   A basic familiarity with electronics and soldering up circuits is assumed.  If you haven't done anything like this but are eager to have a go, it would be well to first tackle something simpler.   You might try the The FLED Solar Engine in Instructables or the "Solar Powered Symet" described in the book "Junkbots, Bugbots, & Bots on Wheels", which is an excellent introduction to making projects such as this one. 
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oh fun now to see if i can combine that circut with a crystal battery to make it build a larger cap for a flashlight.

hparikh13 months ago

what are the equipments to make this? can u give me the list?

tbudka6 months ago

My daughter and I made this circuit together on a breadboard. It worked well. We tried it with a vibrating motor from a cell-phone and found we needed to raise to turn-off voltage since the motor stopped spinning around 1.8V. You already had instructions for doing that on your nicely documented design. Thanks so much for this nice and well documented post.

TinkerJim (author)  tbudka5 months ago

It's so good to hear from a Father and daughter working together on a gadget like this ! Thank you for letting us know !!

baudeagle7 months ago

I was this article today on Reddit : I think that with your Easter engine design combined the information found here: this could make a nice home made cell phone charger. What do you think?

ZJ-Weaver1 year ago
Hi Tinker Jim. Awesome project. I'm working on a circuit to run nitinol SMA wires. A question: Does the power to the load flow from the solar panel or from the capacitor?
TinkerJim (author)  ZJ-Weaver1 year ago
The solar cell and the capacitor are connected in parallel, so when transistor QP turns on, both deliver power to the load. However, unless the solar cell is relatively large or in bright sunlight, most of the power driving the load will come from the capacitor.
Understood. Thank you for the reply!
Sassah1221 year ago
ynze1 year ago
Hi TinkerJim,

Thanks a lot for this I'ble! I spent the last days building solar engines, and yours gave me the final push to start it. I tried your circuit first of course. Later I built the "original" Sun Eater I (and it turned out it was made by a fellow countryman of mine :-)).

When comparing, I find the Sun Eater more efficient ("lively") than your circuit, but has more components as a trade-off. Is that your finding too?

Anyways, thanks a lot for your very well documented I'ble!

TinkerJim (author)  ynze1 year ago
Thank you for your comments on the Easter Solar Engine. I too made a SunEater and was very much pleased with it (in fact it was the inspiration for the Easter engine as mentioned in the Instructable) and it is still working daily on a windowsill!

As to your queries regarding "efficiency" and/or "liveliness", the two terms can take in quite a few different meanings. Efficiency would most precisely mean the ratio of energy delivered to the motor to the energy collected in the storage capacitor from the solar cell This is easy to quantify. But the word could also be used more loosely to refer to how short the operating cycle seems to be, that is, how frequently the device activates and goes through its on-off cycles. The word "lively" could also very well refer to this activation frequency. Or more simply,liveliness could mean the rapidity or strength of the way the motor snaps into action when it does turn on. These are quite different things, but we are apt to use the words "efficient" and "lively" for any or all of these characteristics in an interchangeable casual way.

The most important condition in attempting to make any sort of general comparative declaration, is that both circuits must be set up to have the same turn-on and turn-off voltages. Otherwise, the energy exchanges with the storage capacitor could be too different to draw any meaningful conclusions. This is most important because the energy stored in a capacitor is proportional to the square of the voltage across its terminals: Es = (1/2)• C • (V^2). Thus a small difference in voltage represents a much larger difference in energies.

Now if both solar engines are set up with the exact same turn-on and turn-off voltages, then they will be practically equally "lively". First, they will both collect solar energy for the same time before firing; this is because both circuits pass no current until the trigger strings conduct and turn on the first transistor. They will not run a load for exactly the same time, but if both have the same turn-off voltages, the difference will be small in typical applications. The difference arises precisely because the SunEater has a dual transistor output switch; these are set up as a complimentary pair which functions as a very high gain transistor. Hence, only a tiny current is needed to turn the pair on and they turn on hard (this could also be the "liveliness" you are impressed with). The single output transistor of the Easter Solar engine takes more current in the circuitry to turn a motor on (e.g. at 2.9V turn-on, the 3.3K resistor passes about 0.5mA into the base - note that this resistor can be increased to give a softer run to the motor, or decreased to give a more jolting or lively start).

Now, if the current draw of the output device for the two solar engines were the same and say constant, the SunEater would yield more on-time because less current is used in its circuitry to keep it on, making more available for the load to use up. But then on the other hand, the Easter Solar engine would go through its charge-run cycle more often than the SunEater!

Alas, the situation with a motor as the load is far more complicated! When a motor at rest is switched on from a voltage source, it takes a lot of instantaneous current, and then less and less as it gains speed. A capacitor is more than willing, eager in fact, to supply its energy at high current levels, so a lot of energy can be used up just in getting things moving. This would shorten the on-time.
Avasar100002 years ago
Can you provide some websites that stock the SIP's? Thank you!
TinkerJim (author)  Avasar100002 years ago
All the major electronics supply houses carry them, and I think many of the specialty and surplus electronics sellers do also.
BC-453 years ago
what kind of electronic can i find those kind of capacitors?
Most stuff have big enough caps to work in this.
Look for old VCRs, Tape players, ect.
The audio amps inside of these most of the time have big capacitors.
It looks like in the first picture he is using a super cap.
Just use any caps that say "1000uf" or bigger.
wildfire83 years ago
Where are some videos of these working?
TinkerJim (author)  wildfire83 years ago
I haven't made videos of these working.
Purple Guy3 years ago
As I am pretty much a beginner at electronics I was wondering:

Is their any way to make a more simple trigger which uses less components?
I want to be able to adapt it to suit my, simpler, needs and I don't really understand some of the circuit.

Thanks in advance to who-ever answers.
TinkerJim (author)  Purple Guy3 years ago
You can find a lot of straightforward information on various solar engine circuits at the following site:

TinkerJim (author)  Purple Guy3 years ago
Also, the book I mentioned in the Easter Engine Instructable,

"Junkbots, Bugbots, & Bots on Wheels" by Dave Hrynkiw & Mark W. Tilden

is a very good one for beginners in Beam Technology.

Another good introductory book in more general robot making is

"Robot Building for Beginners" by David Cook.

And for a hands-on introduction to making electronics gadgets of all kinds, you couldn't do better than

"Make: Electronics" by Charles Platt.
Mudbud3 years ago
Wow this is great! I can't wait to build one of my own, won't be for a while though cause my allowance is only 5 bucks a month :/

I'm making a new ible based off this!
cool. I might try that with an earth battery also.
TinkerJim (author)  Computothought3 years ago
Earth batteries should be a suitable source from which an Easter Solar Engine could collect usable energy. You'll need enough earth batteries hooked in series to offer a voltage slightly higher than the turn-on voltage of the Easter engine.
Did some experiments and was surprised at the amount of voltage generated. A whole backyard of cells might be very interesting. Have to go get some resistors tomorrow and build the engine.
GreenD4 years ago
haha look at that old school led in pic #4.

Ok so, just in general what type of diodes can you use for this?? How do you figure out the voltage required for diodes?? Sorry I'm noob!
TinkerJim (author)  GreenD4 years ago
Instead of being thrown out as being too dim and unwanted, the old LEDs are quite happy to be put to work in trigger strings!

The very common small signal diode 1N914 are the ones I use.  They work fine for this low voltage low current application.
Let me start by saying this is gadget with so many uses it's amazing. I'm also a noob so I have to ask. How can I tell the voltage of a random LED I find in old electronics? What can of test can I do to an LED?
TinkerJim (author)  zer0_da_hero3 years ago
To test LEDs for Easter Engine use, I just make up the whole circuit first on a solderless breadboard. With a Volt meter hooked up to the storage capacitor, I just note when the engine circuit cycles on and off. If it's not what is wanted, I just plug in a different LED or two.
jensenr303 years ago
subscribed!! 5 star! great project, man!
TinkerJim (author)  jensenr303 years ago
Thanks !
such a nice job friend
your brain is holymoly
it is best
caret4 years ago
Hi! what program did you use to draw the board above??? Sorry my bad English  :D

Thanks in advance!!!
TinkerJim (author)  caret4 years ago
It was done in VectorWorks.
Tigrezno4 years ago
Why are you using a "double latching" system? That part is not clear to me.

Is it possible to remove Q2?
TinkerJim (author)  Tigrezno4 years ago
It is not a double latch.  It is a single latch formed by two transistors.  Each transistor in the pair is set up to feed the base of the other. In that way, once the first transistor turns on, even a little, the second one gets turned on by it, and then it in turn turns on the first even more, and then the first turns on the second even more, and so on.Thus both transistors find themselves locked on.  This condition persists until the supply voltage drops below the combined diode drop inherent in the transistors.  Now read Step 1 again and follow along in the circuit diagram and it should be clear.
Robootzz4 years ago
 Hello! im trying to build this one myself. but i come across some problems, so i hope i can get some help. ive built it up like this, but my capacitor wont stop recharging. it just keeps going and going. i suppose it has to do with the diodestring. however i have the components that it says in the circuit. and is it true, that the more resistance i have in that string, the more the capacitor will charge?

TinkerJim (author)  Robootzz4 years ago
It might be that the solar cell isn't producing a voltage high enough to trigger the diode string.  Measure the voltage output of the cell you are using and make sure it is at least half a volt above the diode string trigger voltage as listed in the table in Step 3.  You can also try fewer or different diodes in the string.  It could be also that the circuit has triggered but the capacitor has a high internal resistance not allowing it to drive the motor but just slowly drain through it - this happens with capacitors that are intended for memory backup.  I often test solar engine circuits  by feeding from batteries through a resistor - about 1 or 2K.  Monitor the voltage at the capacitor to get a better idea of what is happening.  If you still have trouble, send a photo if you can or a sketch of your circuit setup with the values and types of components you are using. 
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