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.
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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Signing UpStep 1: 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.
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.
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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
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.
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.
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.
http://library.solarbotics.net/circuits/se_t1.html
"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.
I'm making a new ible based off this!
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!
The very common small signal diode 1N914 are the ones I use. They work fine for this low voltage low current application.
Thanks in advance!!!
Is it possible to remove Q2?
thanks
I too had been working on a SolarEngine n was confused regarding many points. I need someone to prove my points regarding motor selection and turn on voltage. And you did it so very neatly!
Keep up the good work!