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Picture of The Easter Solar Engine
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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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Step 1: Easter Engine Circuit

Picture of Easter Engine Circuit
This is the schematic diagram for the Easter engine together with a list of the electronic components that make it up.  The design of the circuit was inspired by the "Micropower Solar Engine" by Ken Huntington and the "Suneater I" by Stephen Bolt. In common with them, the Easter engine has a two-transistor trigger-and-latch section, but with a slightly different resistor network interconnecting them.  This section consumes very little power in itself when activated, but allows enough current to be taken out to drive a single transistor that switches on a typical motor load.

Here is how the Easter engine works.  Solar cell SC slowly charges up the storage capacitor C1.  Transistors  Q1 and  Q2 form a latching trigger.   Q1 is triggered on when the voltage of C1  reaches the level of conductance through the diode string D1-D3.  With two diodes and one LED as shown in the diagram, the trigger voltage is about 2.3V, but more diodes can be inserted to raise this level if desired. 

When Q1 turns on, the base of Q2 is pulled up through R4 to turn it on also.  Once it is on, it maintains base current via R1 through Q1 to keep it on.  The two transistors are thus latched on until the supply voltage from C1 falls to around 1.3 or 1.4V.

When both Q1 and Q2 are latched on, the base of the "power" transistor QP is pulled down through R3, turning it on to drive the motor M, or other load device.  Resistor R3 also limits the base current though QP, but the value shown is adequate to turn the load on hard enough for most purposes. If a current of more than say 200mA  to the load is desired, R3 can be reduced and a heavier duty transistor can be used for QP, such as a 2N2907. The values of the other resistors in the circuit were chosen (and tested) to  limit the current used by the latch to a low level.
 

Step 2: Stripboard Layout

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A very compact embodiment of the Easter engine can be constructed on ordinary stripboard as shown in this illustration.  This is a view from the component side with the copper strip tracks below shown in gray.  The board is only 0.8" by 1.0", and only four of the tracks must be cut as shown by the white circles in the tracks. 

The circuit depicted here has one green LED D1 and two diodes D2 and D3 in the trigger string for a turn-on voltage of about 2.5V. The diodes are positioned upright with the cathode end upward, that is, oriented toward the negative bus strip on the right hand edge of the board.  An additional diode can be easily installed in place of the jumper shown from D1 to D2 to bump up the turn-on point. 

The turn-off voltage can also be raised as described in the next step.

Of course, other board formats can be used.   The fourth photo below shows an Easter engine built on a small general purpose prototyping board.  It is not as compact and orderly as the stripboard layout, but on the other hand it leaves lots of room for working, and space for adding diodes or multiple storage capacitors.  One could also use just plain perforated phenolic board with the necessary connections wired and soldered below.   

Step 3: Trigger Voltages

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This table shows the approximate turn-on voltages for various combinations of diodes and LEDs that have been tried in the the trigger string of various Easter engines.  All of these trigger combinations can be fit onto the stripboard layout of the previous step, but the 4-diode and 1 LED combination would have to have a diode-to-diode joint soldered above the board.

The LEDs used in making the table measurements were older low intensity reds.  Most other newer red LEDs that have been tried work about the same, with maybe a variation of only about plus or minus 0.1V in their trigger level.  Color has an influence: a green LED gave a trigger level of about 0.2V higher than a comparable red.  A white LED with no diodes in series gave a turn-on point of 2.8V.   Flashing LEDs are not appropriate for this engine circuit.

A useful feature of the Easter engine is that the turning-off voltage can be raised without affecting the turning-on level by inserting one or more diodes in series with the base of Q2.  With a single 1N914 diode connected from the junction of R4 and R5 to the base of Q2, the circuit turns off when the voltage drops to around 1.9 or 2.0V.  With two diodes, the turn-off voltage measured approximately 2.5V; with three diodes, it turned off at about 3.1V.  On the stripboard layout, the diode or diode string can be located in place of the jumper shown above the resistor R5; the second illustration below shows one diode D0 thus installed. Note that the cathode end must go to the base of Q2.

Thus it is possible to effectively use the Easter engine with motors that do not run well near the basic turn-off of about 1.3 or 1.4V.  The solar engine in the toy SUV in the photos was made to turn on at 3.2V and turn off at 2.0V because in that voltage range the motor has good power.

Step 4: Capacitors, Motors, and Solar Cells

Picture of Capacitors, Motors, and Solar Cells
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The capacitor used in the toy SUV is like the one shown on the left in the illustration below.  It is a full 1 Farad rated for use at up to 5V.   For lighter duty applications or shorter motor runs, smaller capacitors give shorter cycle times and, of course, shorter runs.  The voltage listed on a capacitor is the maximum voltage to which it should be charged; exceeding that rating shortens the life of the capacitor.  Many of  the super capacitors intended specifically for memory backup have a higher internal resistance and so do not release their energy rapidly enough to drive a motor.

A solar engine such as the Easter engine is fine for driving motors that have an internal static resistance of about 10 Ohms or more.  The most common variety of toy motors have much lower internal resistance (2 Ohms is typical) and so will drain all the energy from the storage capacitor before the motor can really get going.  The motors shown in the second photo below all work fine.  They can often be found as surplus or new from electronic suppliers.  Suitable motors can also be found in junked tape recorders or VCRs.  They can usually be singled out as having a diameter larger than its length.

Choose a solar cell or cells that will provide a voltage somewhat higher than the turn-on point of your engine under the light levels that your application will see. The real beauty of the solar engine is that it can collect low grade apparently useless energy and then release it in useful doses.  They are most impressive when, from just sitting on a desk or coffee table or even on the floor, they suddenly pop to life.  If you want your engine to work indoors, or on cloudy days, or in the shade as well as in the open, use cells designed for indoor use. These cells are usually of the amorphous thin film on glass variety.  They give a healthy voltage under low light, and the current corresponds to the illumination level and their size.  Solar calculators use this kind of cell, and you can take them from old (or new!) calculators, but they are quite small these days and so their current output is low.  The voltage of calculator cells ranges from 1.5 up to 2.5 volts in low light, and about a  half a volt more in the sun.   You'll want a number of them connected in series-parallel.  Wire Glue is excellent for attaching fine wire leads to these glass cells.  Some solar rechargeable keychain flashlights have a large cell that works well indoors with solar engines.  At the present time,  Images SI Inc.  carries new indoor cells of a size suitable for directly driving a solar engine from a single cell.  Their "outdoor" solar cell of the same type works quite well indoors as well.

More commonly available from many sources is the crystalline or polycrystalline type of solar cell.  These types put out a lot of current in sunshine, but are specifically intended for life in the sun.  Some  do modestly well in lower light, but most are pretty dismal in a room lit by flourescents. 

Step 5: External Connections

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To make the connections from the circuit board to the solar cell and motor, pin tail sockets taken from inline strips are very convenient.  The pin sockets can be easily emancipated from the plastic setting in which they come by careful use of nippers.  The tails can be snipped off after the pins are soldered in the board.

Solid 24 gage wire plugs into the sockets nice and secure, but usually externals are connected via flexible stranded hookup wire.  The same sockets can be soldered to the ends of these wires to serve as little "plugs" that fit into the sockets on board beautifully.

Board sockets can also be provided into which the storage capacitor can be plugged.  It can mount directly into the sockets, or be remotely located and connected via wire leads plugged to the board.  This makes it possible to easily change and try different capacitors until the best one is found for the application and its average lighting conditions.  After the best value of C1 is found, it still can be permanently soldered in place, but rarely has this been found necessary if good quality sockets are used.

Step 6: Applications

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Perhaps our favorite application of an Easter engine is in the toy Jeepster SUV illustrated in Step 3.  A thin plywood  bottom was cut to fit the body, and large foam wheels were made to give it a "Monster Wheel" look, but in operation it is quite docile.  The underside is shown in the photo below.  The axles are set to make the car run in a tight circle  (because we have a small living room) and the front wheel drive setup greatly helps it stick to the intended circular path. The gear train was taken from a commercial hobby motor unit shown in the next photo, but it was fitted out with a 13 Ohm motor.
 
A 1 Farad super capacitor gives the car about 10 seconds of run time each cycle, which takes it almost completely around a 3 foot diameter circle.  It takes a while to charge up on cloudy days or when the car happens to stop in a dark spot. Anywhere from 5 to 15 minutes is usual during the day in our living room.  If it finds direct sunlight coming in a window, it recharges in about two minutes.  It travels around in a corner of the room and has logged many revolutions since being built in 2004.

Another amusing application of the Easter engine is "Walker", a robot-like creature that waddles along by means of two arms, or rather, legs.  He uses the same motor and gear train setup as the Jeepster with the same 76:1 ratio.  One of his legs is purposely shorter than the other so that he walks in a circle.  Walker also carries a blinking LED so we know where he is on the floor after dark.

An simple use for a solar engine is as a flag waver or spinner.  The one shown in the 5th photo below can sit on a desk or shelf and every now and then it will suddenly, and rather wildly, spin a little ball around on a string thereby attracting attention to itself.  Some embodiments of these simple spinners had a jingle bell on the string.  Others had a stationary bell mounted nearby so that it would get smacked by the flailing ball - but that tends to become annoying after a few sunny days!



Step 7: NPN Easter Engine

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The Easter engine can also be made in the complementary or 'dual' version, with two NPN transistors and one PNP.  The complete schematic is shown in the first illustration here.  The stripboard layout can have the same component locations and the same track cuts as the first or 'PNP' version, the essential changes being switched transistor types and reversed polarity of the solar cell, storage capacitor, diodes and LEDs.  The NPN stripboard layout is shown in the second illustration and incorporates an extra diode D4 for a higher turn-on voltage, and a diode D0 from the base of transistor Q2 to the junction of resistors R4 and R5 for a higher turn-off voltage as well.
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kasperfish2 months ago

Great instructable! Thank you!

lfc4442 months ago
Hi, how can i connect two of these together? Is it possible to use one capacitor for power while the other charges and then switch back again? Thanks for the great info too!
bneo99 made it!3 months ago

Made one with 1N4001 diodes, and used different resistors in series to get the correct values, works nicely!

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TinkerJim (author)  bneo993 months ago

Nice Job !!! I like the way you used those tiny resistors on the backside. Thanks for sending along the photos.

MichaW8 months ago

I am not sure which way to put in the diode an led. Maybe you could clearify? thanks

TinkerJim (author)  MichaW8 months ago

The schematic in Step 1 is clear on these points. You just have to distinguish the anode from the cathode ends on the diodes and leds you want to use. The cathode end of a diode is usually marked with a band. The cathode side of an led is usually indicated by a flat portion on the lens.
The references cited in the Intro step will be very helpful for questions such as these. In any event, put the circuit together on a solderless breadboard to make sure everything is working correctly before you warm up the soldering iron.

atilladolphun10 months ago

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.

hparikh11 year ago

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

tbudka1 year 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)  tbudka1 year 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 !!

baudeagle1 year ago

I was this article today on Reddit : http://www.ohgizmo.com/2010/01/09/ces2010-rca-airn... I think that with your Easter engine design combined the information found here: http://hight3ch.com/free-electricity-from-thin-air... this could make a nice home made cell phone charger. What do you think?

ZJ-Weaver2 years 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-Weaver2 years 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!
Sassah1222 years ago
hi
ynze2 years 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!

Ynze
SunEater_I.gif
TinkerJim (author)  ynze2 years 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.
Avasar100003 years ago
Can you provide some websites that stock the SIP's? Thank you!
TinkerJim (author)  Avasar100003 years ago
All the major electronics supply houses carry them, and I think many of the specialty and surplus electronics sellers do also.
BC-454 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.
wildfire84 years ago
Where are some videos of these working?
TinkerJim (author)  wildfire84 years ago
I haven't made videos of these working.
Purple Guy4 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 Guy4 years ago
You can find a lot of straightforward information on various solar engine circuits at the following site:

http://library.solarbotics.net/circuits/se_t1.html

Thanks!
TinkerJim (author)  Purple Guy4 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.
Mudbud4 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)  Computothought4 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.
GreenD5 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)  GreenD5 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_hero4 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.
jensenr304 years ago
subscribed!! 5 star! great project, man!
TinkerJim (author)  jensenr304 years ago
Thanks !
such a nice job friend
your brain is holymoly
it is best
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