TheSOLARENGINECIRCUIT

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Introduction A solar engine circuit is a control circuit that takes micropower (milliamps of current) andconverts that micropower into more powerful pulses. These more powerful pulses are usually used topower motors, but can be used to power other motion-creating devices, such as, Nitinol wire. (See thefull project on this amazing wire by clicking HERE). The first SOLAR ENGINE was built by Mark W. Tilden in late 1989. Let's describe the circuit.In BEAM folklore there are three types of SOLAR ENGINE: Type 1: Voltage based. This is your standard circuit for: Photopopper, FRED, Suneater etc. Type 2: Time based. This is trickier to do, since the changing voltage affects the time constants ofyour SE. The only type 2 SE that I'm aware of is the one inside a miniball - and it's not for the faint ofheart. Type 3: Current based. This SE turns on when the current going into the capacitor drops below acertain threshold. This SE has been a bit of a 'Holy Grail' for designers, because the SE will adapt to thespecific solar panel/cap/light level that you have available. In bright light (lots of current) the SE will turnon at a high voltage, and in low light it will turn on at a much reduced voltage, because there isn't asmuch current available.

THE SOLAR ENGINE CIRCUIT Type-1:

Theory of OperationThis circuit is the basis of BEAM (Biology, Electronics, Aesthetics, Mechanics) robotics philosophy. It isan energy storage circuit known as a Solar Engine(tm) (abbreviated SE). This design is called a Type-1SE. Low current from a solar cell is stored in a large capacitor and when a preset voltage-level isreached, the energy from the capacitor is released to a motor. The circuit starts by charging the electrolytic. The voltage across the electrolytic rises. The two transistors are turned off at the moment and they canbe considered to be absent from the circuit. The only three components "in circuit" are the diode, 1k8 and motor. Next, the voltage across the diode rises to 0.7v and stops at this level. This is the characteristic voltageacross a diode. It is called its forward voltage drop. (the resistance of the motor is very small and we can neglect it for the moment.) As the voltage across the electrolytic rises, the voltage across the 1k8 resistor starts to increase. This voltage also appears across the base-emitter junction of Q1 and when it reaches 0.65v, thetransistor begins to turn on.This creates a low resistance between the collector-emitter terminals of Q1 and allows current to flowinto the base of Q2.The voltage across the electrolytic rises slightly more and this causes Q1 to turns on harder. This effectively makes Q2 turn on a lot harder and now a very high current is able to flow through thecollector-emitter terminals of Q2. This current causes the motor to spin and the energy from theelectrolytic is released to the motor. Q1 was originally turned on via the diode but now the voltageacross the diode falls below 0.7v and Q1 is kept on by the voltage across the motor being passed to thetransistor via the 1k8 resistor. As the energy is released from the electrolytic, the supply voltage falls and when the voltage across themotor is less than 0.65v, Q1 begins to turn off. This action turns off Q2 and the motor stops.The cycle then repeats.The solar cell must produce a voltage greater than 0.7v + 0.7v = 1.4v so that the electrolytic will charge.If you add an extra diode in series with the 1N 4148, the solar cell (panel) will need to produce a highervoltage. A Note from the AuthorThere seems to be a lot of questions and problems in replicating the circuit above. Steven Bolt of theNetherlands has come up with a potential solution called SunEater. This is the next circuit in ourdiscussion. The best advice, from my experience, is to make the circuit above using the valuesgiven. Experiment with it. If you still have problems, a good course of study for those just beginning inBEAM robotics would be to start out with a solar engine based device, before moving on towards morecomplicated circuits, such as nervous networks, like the microcore.

Relevance to Nervous NetworksThe Solar Engine(tm) is just a transistor implementation of a Nv "Monocore" that can drive a motordirectly without the need for additional buffering (as in the more complicated Nv Networks).In fact a SE behaves very much like a nervous network. The SE has an upper threshold (trigger voltage)and a lower threshold (shut off), and the duration of the state change (pulse) is directly affected by themotor load. In addition the "off" state (really the time held below threshold) is affected by what's feeding

it (in this case the solar cell/trigger, in a nervous network - the previous Nv). So, in essence you have aquasi-chaotic phase/state-based oscillator threshold device that is self-attenuating with respect toenvironment in a dynamic fashion. See the FAQ for details on Nv Networks (NvNets).

Parts When first building a SE, use the parts recommended. Experiment with substitutions once you have aworking circuit.

working circuit.M - a motor. Motors with small power requirements usually work great (i.e., 3 volts at a low currentrating). (A word of warning: some small recorder motors have a built-in capacitor of about 20 nanofarad.The 2-transistor FLED SE won't work with such a motor - it will start to oscillate, sometimes audibly.Without a scope, the typical symptom is `hangup' in reset. The SE fires once, then stops, the voltage onthe storage cap remaining at less than 1.0v. Both SunEaters are immune to this problem, and will runsuch motors.)C1 can be anywhere from 2,200µF to 47,000µF (or as high as 10 Farad). Depends on your applicationand timing requirements.R1 can be anywhere from 1.8k Ohms to 15k Ohms. The smaller the resistance the better the startingpower of M, but more energy is lost in the transistors.Q1 - a PNP transistor, general-purpose like the 2N2907, 2N3906, etc. Also cheap and easy to find.Q2 - a NPN transistor, general-purpose like the 2N2222, 2N3904, etc. Cheap and easy to find.D1 - a threshold device, such as a small signal switching diode, e.g., 1N4728 to 1N4731. Since a singlediode triggers at [0.6 - 0.7] volts, you can use multiple diodes in series to obtain a higher voltage.Arranging the diodes in series will give an increased increased "trigger" voltage, thus two diodes willtrigger at [1.2 - 1.4] volts and three diodes in series will trigger at [1.8 - 2.1] volts. Solar Cell - like a 24x33mm Sunceram. Purchase solar cells with the highest current output for bestresults.

Construction For those that are good at soldering, the circuit is best put together in a free-form kind of structure, thatis, without any circuit board. See this alternative set of plans (called Solaroller) and picture below depictthe physical layout of the solar engine-the layout is right above the circuit diagram. I believe it is the bestlayout, and does not require a printed circuit board (PCB). As for the mechanical layout, the best adviceI can give is to start building, because your first will not be your best. When you have completed yourmechanism, don't rat-it for parts, move on to the next so that you can have a living history of yourprojects.

A solar engine using the free-formlayout without a PCB (without motor)

Making your own solarollerThe Solar Engine Circuit above is the heart of many solar-powered robots. It is called the BEAMSOLAR-ENGINE. The idea is that available solar cells do not supply enough power to overcome frictionin a typical robot design. So the power is stored until enough is available and then used to operate asmall motor. The result is a continuously operating (well, as long as there is light) drive system. Whencoupled with a clever design, it can produce robotic "plankton" which displays emergent behaviour. The original circuit was developed for the Robot Olympics and used in a Solaroller device, similar to theone in the pictures below. It covers one 1 metre in one minute, in very good sunlight using only a solarcell 2-1/2" by 1/2".

Click on the thumb nail of a Solar Engine circuit using a UJT and SCR to do

Click on the thumb nail of a Solar Engine circuit using a UJT and SCR to dothe same job as the Solar Engine described above. This circuit shows how youcan solve a simple task with a complex approach and using expensivecomponents!

SUN EATER I by Steven BoltAn improved design for a type-1 circuit is the Sun Eater. It has a very clever 2-transistor self-latchingarrangement that keeps the circuit ON until the voltage drops to 1.5v. The circuit turns on at 2.8v. Thisgives the motor more energy from the electrolytic at each "pulse" than the Solar Engine circuit above.

This circuit is also a voltage-controlled design. It turns on when the voltage across the electrolyticreaches a certain "threshold". But the advantage of this circuit is the "hysteresis" or wide differencebetween turn-on and turn-off. The circuit turns on at 2.8v and turns off at 1.5v. These levels areimportant as they control the amount of energy delivered to the motor during each "pulse."The Sun Eater robot constructed by the author with this circuit is shown below. It requires two of theSun Eater circuits.

The clever "latch" section of the circuit is made up of transistors Q1 and Q2. The secret to the operationof the circuit is the feedback line shown on the diagram. Everything is off at the beginning of the charge cycle and the only components to be considered are the100k, green LED and base-emitter junction of Q1. As the voltage on the electrolytic increases, the LED begins to pass current when the voltage across it is2.1v. The voltage on the base of Q1 rises and when it is 0.65v, the transistor begins to turn on. Thisbegins to turn on transistor Q2 via the 470k resistor and a voltage appears across the two 470k resistorsin series. The supply voltage at this time is approx 2.8vWhen the voltage at the join reaches 0.7v, transistor Q1 is turned on harder and now the voltage on theelectrolytic does not have to rise any further for the circuit to turn on more and more. Transistors Q1 and Q2 keep turning each other ON more and more by an action called positivefeedback and this causes transistors Q3 and Q4 to get turned on fully too. The whole circuit is fully turned on and the energy from the electrolytic is passed to the motor. As the voltage across the electrolytic drops, the circuit remains fully turned on until a point is reached

As the voltage across the electrolytic drops, the circuit remains fully turned on until a point is reachedwhen the voltage on the lower 470k resistor is less than 0.6v. This is when the circuit begins to turn off.The supply voltage at this time is 0.6v + 0.6v + 0.3v (the voltage collector-emitter of Q3) = 1.5v. Thisgives a hysteresis value of 2.8v - 1.5v = 1.3v With a single green LED to determine the switch-on level, the motor will start when the 47,000µ capreaches 2.8V, and stop when the voltage drops below 1.5v. This gives good economy for pager andsmall recorder motors. If your motor needs a higher voltage, you can put a diode in series with the greenLED. Each diode will increase the voltage by 0.7v.

Ken Huntington's 4-Transistor Solar Engine Circuit

This circuit can be called Sun Eater IA as it is an improvement on the Sun Eater I circuit shown above.It works exactly the same except the slight re-arrangement of the components allows an NPN powertransistor to be used. One less resistor is needed and one less capacitor but two extra diodes havebeen added to increase the upper turn-on voltage. The prototype layout on a PC board is shown below.

It is very compact and has connectors for the electrolytic, power and motor. The PC board provided inthe kit is pre-tinned and has an overlay on the top to identify the placement of each component. Clickbelow to order the kit. You will need a motor and solar cells to generate at least 3v. The kit comes with a1,000u electrolytic. THE SOLAR ENGINE CIRCUIT Type-3

Type-3 circuits are current controlled or current-triggered. This is another very clever way of detectingwhen the electrolytic has reached its maximum charge.At the beginning of the charge-cycle for an electrolytic, the charging current is a maximum. As theelectrolytic becomes charged, the current drops. In the type-3 circuit, the charging current passesthrough a 100R resistor and creates a voltage drop. This voltage is detected by a transistor (Q2) and thetransistor is turned ON. This action robs another transistor (Q1) from turn-on voltage and the rest of the circuit is not activated.As the charging current drops, Q2 is gradually turned off and Q1 becomes turned on via the 220kresistor on the base. This turns on Q3 and the motor is activated. The voltage across the storage electrolytic drops and thecurrent through the 100R rises and turns the circuit off. The electrolytic begins to charge again and thecycle repeats.

Click HERE for descriptions and photos of a number of solar robots from BEAM ONLINE. These photosshow the variety of designs that can be created with SOLAR ENGINE circuits.

For an excellent set of photos on BEAM Solar Robots, click HERE - Scott's Robotics Page.

Page 3 covers the semiconductors used in Robotics. Page 4 covers more Solar Engine circuits and the use of photo-detectors to detect light source.

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