Photovoltaic Cells: Converting Photons to Electrons

Quiz Corner

Think you're an expert on solar cells? Test your knowledge with our Solar Cell Quiz.

The solar cells that you see on calculators and satellites are photovoltaic cells or modules (modules are simply a group of cells electrically connected and packaged in one frame). Photovoltaics, as the word implies (photo = light, voltaic = electricity), convert sunlight directly into electricity. Once used almost exclusively in space, photovoltaics are used more and more in less exotic ways. They could even power your house. How do these devices work?

Photovoltaic (PV) cells are made of special materials called semiconductors such as silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. PV cells also all have one or more electric fields that act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off to use externally. For example, the current can power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.

That's the basic process, but there's really much more to it. Let's take a deeper look into one example of a PV cell: the single-crystal silicon cell.

Photovoltaic Cells: Converting Photons to Electrons

Quiz Corner

Think you're an expert on solar cells? Test your knowledge with our Solar Cell Quiz.

The solar cells that you see on calculators and satellites are photovoltaic cells or modules (modules are simply a group of cells electrically connected and packaged in one frame). Photovoltaics, as the word implies (photo = light, voltaic = electricity), convert sunlight directly into electricity. Once used almost exclusively in space, photovoltaics are used more and more in less exotic ways. They could even power your house. How do these devices work?

Photovoltaic (PV) cells are made of special materials called semiconductors such as silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. PV cells also all have one or more electric fields that act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off to use externally. For example, the current can power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.

That's the basic process, but there's really much more to it. Let's take a deeper look into one example of a PV cell: the single-crystal silicon cell.

Anatomy of a Solar Cell

Before now, our silicon was all electrically neutral. Our extra electrons were balanced out by the extra protons in the phosphorous. Our missing electrons (holes) were balanced out by the missing protons in the boron. When the holes and electrons mix at the junction between N-type and P-type silicon, however, that neutrality is disrupted. Do all the free electrons fill all the free holes? No. If they did, then the whole arrangement wouldn't be very useful. Right at the junction, however, they do mix and form a barrier, making it harder and harder for electrons on the N side to cross to the P side. Eventually, equilibrium is reached, and we have an electric field separating the two sides.

The effect of the electric field in a PV cell

 

 

This electric field acts as a diode, allowing (and even pushing) electrons to flow from the P side to the N side, but not the other way around. It's like a hill -- electrons can easily go down the hill (to the N side), but can't climb it (to the P side).

So we've got an electric field acting as a diode in which electrons can only move in one direction.

When light, in the form of photons, hits our solar cell, its energy frees electron-hole pairs.

Each photon with enough energy will normally free exactly one electron, and result in a free hole as well. If this happens close enough to the electric field, or if free electron and free hole happen to wander into its range of influence, the field will send the electron to the N side and the hole to the P side. This causes further disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the path to their original side (the P side) to unite with holes that the electric field sent there, doing work for us along the way. The electron flow provides the current, and the cell's electric field causes a voltage. With both current and voltage, we have power, which is the product of the two.

Operation of a PV cell

 

 

There are a few more steps left before we can really use our cell. Silicon happens to be a very shiny material, which means that it is very reflective. Photons that are reflected can't be used by the cell. For that reason, an antireflective coating is applied to the top of the cell to reduce reflection losses to less than 5 percent.

The final step is the glass cover plate that protects the cell from the elements. PV modules are made by connecting several cells (usually 36) in series and parallel to achieve useful levels of voltage and current, and putting them in a sturdy frame complete with a glass cover and positive and negative terminals on the back.

Basic structure of a generic silicon PV cell

 

 

How much sunlight energy does our PV cell absorb? Unfortunately, the most that our simple cell could absorb is around 25 percent, and more likely is 15 percent or less. Why so little?

Besides Single-crystal Silicon...

Single-crystal silicon isn't the only material used in PV cells. Polycrystalline silicon is also used in an attempt to cut manufacturing costs, although resulting cells aren't as efficient as single crystal silicon. Amorphous silicon, which has no crystalline structure, is also used, again in an attempt to reduce production costs. Other materials used include gallium arsenide, copper indium diselenide and cadmium telluride. Since different materials have different band gaps, they seem to be "tuned" to different wavelengths, or photons of different energies. One way efficiency has been improved is to use two or more layers of different materials with different band gaps. The higher band gap material is on the surface, absorbing high-energy photons while allowing lower-energy photons to be absorbed by the lower band gap material beneath. This technique can result in much higher efficiencies. Such cells, called multi-junction cells, can have more than one electric field.

B E A M - R o b o t i c s - T e k

The Solar Engine

Introduction

A solar engine is a kind of control circuit that takes micropower and converts that micropower into more powerful pulses. These more powerful pulses are usually used to power motors, but can be used to power other motion-creating devices, such as, Nitinol wire. The first solar engine was built by Mark W. Tilden in late 1989[1]. Let's attempt to understand the need for such a circuit with an analogy that will illustrate the need and show the solution.

An Analogy

Let us suppose that we have a source of water and we want to use that water source to perform some work. For grins, let's say to grind coffee. Now our water source is not the most plentiful, but it does have a constant stream. So how can we devise a way to utilize this small, yet constant stream? The figure to the right depicts my solution, called a WaterEngine.

Now, let's go through the system briefly to describe the components and then we can look at the operation of the system as a whole. First we have (a) which is our source of energy, water droplets. Next, our collection device, (b) which funnels most of the water into the tank, (c) with a little spillage (loss of energy). Now once our tank gets full, we have a series of mechanical controls (d,e,f and g) that are activated when the tank is nearing the rim. This control mechanism controls the flow of water to the outlet, (h) and over the water wheel, (i), whereby the water's energy (potential) is converted into the mechanical energy required to spin a wheel via a belt.

So how does this control mechanism work? Well, when the tank's water level is nearing full, some water begins to leave the tank via the pipe, (d). The purpose of this "level detection" or "trigger" is to initiate the emptying of the tank, (c). So once the water reaches the triggering level, water flows through (d) and into the box (e) (with some minor spillage into (g). The device in (e) is essentially a spring valve that activates a larger valve in (f) to allow the water to flow from (c) through (f) and out (h). Now of course, once the water begins to flow, the tank's water level will lower and essentially cease the flow of water out (d), which triggers (e). However, our valve (e) will be activated by the flow of water coming out of (c) and through (f) up through (g). This mechanism will stay active until the flow of water stops activating (e) through (g).

The Circuit

Theory of Operation

Essentially the SE is a modified SCR (Silicon Controlled Rectifier) with supercritical feedback. I would like to point out that we are interested in the electron current, as opposed to conventional current (that is, the movement of positive charges, opposite to electron current), unless otherwise stated as so.

The following timeline are the events and circuit characteristics while charging and discharging:

• EVENT - Beginning of charging cycle
• Capacitor C1 begins to charge due to current from the solar cell and the voltage across C1 rises
• Q2e is positive, which is required for conduction. However, the base, Q2b is also positive - from the resistor through the motor. Thus, current cannot flow from Q2c to Q2e.
• In order for current to flow through Q1 (Q1e to Q1c), Q1b must be positive and Q1c must be negative. Otherwise, the collector-base [ N | P ] junction is basically a diode being reverse-biased. When there is no voltage connected to the base of an NPN then Q1 is equivalent to two diodes connected back-to-back, which do not conduct. (is this correct?)
• Once the voltage of C1 reaches the trigger voltage of D1, which is forward biased, current flows through D1. (why at that trigger voltage?)
• The current flowing through D1 establishes a source current from Q2b to Q2e. This in turn causes a greater current to flow from collector (Q2c) to the emitter (Q2e), thus allowing Q1b to see positive.
• The current flowing from the Q2c to Q2e, causes a similar flow of current in Q1 from Q1e to Q1b, since Q1b is now positive.

(Recall, that if a small voltage is applied to the base -- enough to remove the depletion layer in the emitter-base junction, current flows from emitter to base as in a diode)

• This small amount of current through Q1e to Q1b induces a larger flow of current from the emitter (Q1e) to the collector (Q1c).
• The current flowing from Q1e to Q1c goes throgh motor, M, causing it to go spin.
• EVENT - SE triggered, Beginning of discharging cycle
• EVENT - Motor turns on
• While the current is flowing through M, the voltage across C1 falls
• EVENT - voltage across C1 falls below trigger point of D1
• The base of Q2, Q2b is being feed current by way of D1 and also by way of R1. Thus, when the voltage falls below D1's trigger point, D1 stops conducting and current continues to flow into Q2b (and through to Q2e) by way of R1. Thus keeps Q2 on, which in turn keeps Q1 on, which allows the motor to run. And so on, until the motor resistance / load creates a high resistance whereby it essentially halts the cycle.
• Here you will recognize that this behavior is very much like that of a Silicon Controlled Rectifier (SCR), in that load is turn ON by one method and OFF by another. With an SE, it is initially turned ON by the trigger, D1; and turned OFF when the motor's inertia and resistance is too large.
• (note: Q1 is the main driving transistor switch that allows the current from C1 to flow through the motor M. Q1 can be doubled (see Darlington amplifier on p. 234 of gold electronics book) or replaced with higher current transistors - like the 2N2222.)

So how do these diodes and transistors work?

A diode is normal built by touching two different pieces of semiconductor together to form what is called a "p-n junction." Semiconductors are materials that are in between good conductors and good insulators. A pure semiconductor is a very poor conductor of electricity. With careful chemical processing, a semiconductor can be made into n-type semiconductor--a semiconductor that contains a small number of mobile electrons that permit it to carry electric current. With different processing, a semiconductor can also be made into p-type semiconductor--a semiconductor that contains a small number of mobile holes for electrons that permit it to carry electric current. It may seem strange that a hole for an electron can allow electricity to flow, but imagine a highway packed with cars (electrons) bumper to bumper. If there are a couple of empty places (holes) in the bumper to bumper traffic, then cars (electrons) can rearrange enough that the traffic can flow. Both mobile electrons and mobile holes allow these two chemically-treated semiconductors to carry current.

When an n-type semiconductor touches a p-type semiconductor, a diode is formed. The mobile electrons at the edge of the n-type semiconductor flow over the boundary (a p-n junction) and fill the mobile holes at the edge of the p-type semiconductor. This rearrangement creates a depletion region--a region near the p-n junction in which there are neither mobile electrons nor mobile holes. This depletion region normally won't carry electricity at all. But if you push electrons onto the n-type semiconductor, they will flow toward the p-n junction and replenish the missing mobile electrons. As these mobile electrons approach the p-n junction, they will repel the electrons that are filling the mobile holes on the p-type side of the junction and reopen the mobile holes. Electrons will begin to cross the p-n junction and current will flow through the diode. However, if you push electrons onto the p-type semiconductor, they will fill even more of the mobile holes there and the depletion region near the p-n junction will grow larger and more uncrossable. No current will flow through the diode. Thus a diode (a p-n junction) only carries current in one direction--electrons can only flow from the n-type semiconductor side to the p-type semiconductor side.

There are many types of transistors, so I will only describe an n-channel Metal-Oxide-Semiconductor Field Effect Transistor, or n-channel MOSFET. In this device, three layers of semiconductors are sandwiched together: an n-type piece (the source), a long, thin p-type piece (the channel), and another n-type piece (the drain). Two p-n junctions form between these three components and, since the junctions are arranged in opposite directions, they completely block current flow from the source through the channel to the drain. But a metal surface (the gate) that's separated from the channel by an extremely thin layer of oxide insulator can control the number of electrons on the channel material. If you put even a tiny bit of positive charge on the gate, it will attract electrons onto the channel and turn it from p-type semiconductor to n-type semiconductor. When that happens, both p-n junctions vanish and current can flow from the source to the drain. The MOSFET goes from being an insulating device when there is no charge on the gate to a conductor when there is charge on the gate! This property allows MOSFETs to amplify signals and control the movements of electric charge, which is why MOSFETs are so useful in electronic devices such as stereos, televisions, and computers.

-Source unknown.

The embryonic circuit of the BEAM robotics philosphy is an energy storage circuit known as a Solar Engine(tm) (abbreviated SE) - a simple relaxation oscillator. This particular design (called a Type-1 SE) stores energy from a small power source (e.g., a solar cell) in a large capacitor until a preset voltage threshold is reached, whereby the energy is released into a motor. This is the basic device, however many variations have stemmed from this design, incorporating various motor devices, triggering circuits and mechanical layout.

Following Tilden's philosophy, the circuits that I have constructed are made of recycled parts, calculator solar cells, transistors from radios (sometimes), motors from various sources (old volt-meters, pagers, toys, etc.) with efficiency, simplicity and mechanical beauty being my primary goals. It is like bringing life back into unwanted tech-junk.

A Note from the Author

There seems to be a lot of questions and problems in replicating the device on this page. Steven Bolt of the Netherlands has come up with a potential solution called SunEater. The best advice, from my experience, is to first create the circuit described following the author's specifications and design parameter. Experiment with the circuit once you have completed a functioning circuit. If you still have problems, E-mail the list or the author. A good course of study to those just beginning in BEAM robotics would be to start out with a solar engine based device, before moving on towards more complicated circuits, such as nervous networks, like the microcore.

Relevance to Nervous Networks

The Solarengine(tm) is just a transistor implementation of a Nv "Monocore" that can drive a motor directly 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 the motor 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 a quasi-chaotic phase/state-based oscillator threshold device that is self-attenuating with respect to environment in a dynamic fashion. See the FAQ for details on Nv Networks (NvNets).

Required Parts

Please when first building a SE, use the parts that I recommend. Experiment with part substitution once you have a working circuit.

M - a motor, I used anything I could find with an armature and a coil of wire. Motors with small power requirements usually work great (i.e., 3 volts at a low current rating). Coils (very fine wire with lots of turns), Nitinol wire (aka, "Muscle Wire") or diodes can also be used - however, a ten MegaOhm load is not a replacement for the motor. (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 over the storage cap remaining at less than 1.0V. Both SunEaters are immune to this problem, and will run such motors.)

C1 can be anywhere from 2,200?F to 47,000?F (or as high as 10 Farad). Depends on your application and timing requirements.

R1 can be anywhere from 1.8k Ohms to 15k Ohms. The smaller the resistance the better the starting power of M, but more energy is lost in the transistors.

Q1 - a NPN transistor, general-purpose like the 2N2222, 2N3904, etc. Cheap and easy to find.

Q2 - a PNP transistor, general-purpose like the 2N2907, 2N3906, etc. Also cheap and easy to find.

D1 - a theshold device, such as a small signal switching diode, e.g., 1N4728 to 1N4731. Since a single diode would trigger at [0.6 - 0.7] volts, we need to use multiple diodes in series. Arranging the diodes in series would give you an increased increased "trigger" voltage, thus for example, two diodes would trigger at [1.2 - 1.4] volts and three diodes in series would trigger at [1.8 - 2.1] volts. Variants:

• (1) can be substituted with a Zener diode-mc4742, here the Zener starts clamping at v+ 0.7 volts, eventually triggering the PNP transistor, then triggering the NPN transistor, and will lock so long as the motor is moving, OR,
• (2) a Flashing LED, which is a more efficient variant. See Zener diodes draw more and more power as they reach the "trigger" point. If the solar cell can't provide enough power to overcome this extra current draw, it'll stabilize at a point where the Zener "leaks" as much current as the cells provide, OR,
• (3) a voltage trigger, such as the 1381 (find them at Digikey, it is a 3 pin device, and monitors one voltage source).

Either variant can simply be replaced one-for-one.

Solar Cell - like a 24x33mm Sunceram Solar cell or a few solar cells swipped from calculators. I have also used several small photodiodes, which generate 0.4 volts each, in series they generate higher voltage (e.g., 2 in series generates ~0.8 volts, and 3 in series generates ~1.2 volts, etc.).

Construction

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

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

Debugging Information and Questions

Basic Debugging

Q: Is there a way to debug one of the 2-transistor/1 FLED Solar Engines?

A: (contributed by Richard Weait and Dave Hrynkiw)

• Check that your components are in the holes that they started in (especially if you share your lab with other lifeforms.)
• Check that they are in the holes that they are in. Breadboards can have reliability problems.
• Vcap <1.0 Vdc?
• Check V (solarcell-open circuit)
• Check NPN and motor.
• Vcap climbs to V solarcell but no trigger?
• Check connection to transistors
• Check LED and PNP.
• Vcap climbs and drops, motor doesn't move?
• Check motor shaft / efficiency.
• If you can see the FLED blinking, that means something's not right. Sounds like it may be in backwards. Cathode goes negative (the shorter leg, or the one closest to the chamfer on the side of the lip on the FLED). It could also be an indication of power-hungry motor - try an efficient motor, like a pager motor.

Just a few places to start.

Solar Engine Cycling

Q: My Solar engine takes forever to cycle (power up, discharge, power up,...)?

A: Sounds like a current-generating problem to me. Solarcells from calculators are notorious for good voltage but produce low-current. Try wiring up the cells so they generate about 4 volts, which should triple the current.

Large Capacitance, Low Current Generating Source

Q: I substituted a huge 2F capacitor for C and my solar engine doesn't work now?

A: You might have to wait days if you have a poor current (say, two 18 uA at 1.3V cells in series) generating solar cell (and supposing you have a blocking diode-to prevent the discharging of the capacitor through the solar cell). A solution might be to larger solar cells (say, 3.5V at 6mA) or gang them in solar cells in parallel for more current.

Testing Transistors

Q: How do I test Transistors?

A: Although transistors are very sturdy, they can make debugging a circuit quite painful if they are defective. For those of you with multimeters that don't have a "diode check" function, put your meter on its highest resistance scale. Testing an NPN, such as the 2N3904, with your positive probe on the base (middle lead), and the negative probe on the one of the remaining leads. If it shows infinite resistance, it is defective. Then switch the negative probe to other outside pin and check it too. If should show some resistance, about 28-34 ohms. In summary, an NPN transistor is checked with the leads arranged Negative, Positive, Negative. For a PNP transistor, do the same, except put the negative probe on the base (middle lead) and use the positive probe to check the outside leads. The same resistance values should show up.

In summary, a good resistor will have the following characteristics in terms of the resistances between the leads: emitter(e), base(b) and collector(c)

Note: Rbe means the base positive with respect to the emitter, Reb the emitter positive with respect to the base; and the other junctions have similar designations.

Why Not an SCR?

Q: Why do all the solar engine schematics I've seen use a PNP & NPN transistor to make a modified SCR? Why not just use a real SCR? Wouldn't it be a lot smaller?

A: An SCR is a single package designed to stay on when turned on. With the 2 transistor package, you can tune it for optimal performance (the bias resistor) depending on the application or motor. Also, you can substitute for larger power transistors (2N2222 NPN) and keep the smaller signal transistor (2N3906). And I'm not sure if there are any SCRs as easily available as the npn/pnp combo that run at such low power.

Solar Engine Not Firing

Q: (Using a battery) The circuit charges great to about 1.7 volts, then the FLED starts flashing. The circuit continues to charge to the max voltage, and the FLED is flashing happily, but the transistor never fires. I know the circuit works otherwise because when I short across the FLED the motor spins and everything starts over. I just can't get the FLED to fire it. I have tried replacing the 2.2k Ohm resistor with a 5.6k Ohm - same results, then with a 1.0k Ohm resistor - same results. I then replaced the 5.6k Ohm I was using to limit current from the batteries with a 1.0k Ohm- same results, the circuit just charges faster.

A: First, two comments:

• This is a transistor circuit which is a current amplifier.
• The FLED circuit requires about 0.75mA +/- 0.25mA to trigger the circuit.

Now, since the voltage accross the resistor connected to the battery is about 3.0 - 1.7 = 1.3 Vdc, the circuit is providing only 1.3/5.6K = 0.23mA, and the circuit cannot trigger. Reduce the 5.6K to 680 to 1000 Ohms and you should have no further problems.

Electro-Magnet Solar Engine

Q: Some questions on a Magbot-style solar engine, which generates a magnetic field with a large inductance coil every time the SE fires.

A:

Wire type for the coil?  
As fine a wire as you can get. Not heavy wire, probably too heavy and not enough inductance.

Core for coil?  
Air core, smallest hole possible.

Size of the coil?  
Largest diameter possible. Look for something about the size of nickel to a quarter.

Strength of magnet?  
Strong as possible. Try a rare-earth magnet.

Does the SE need a different resistor value, which also determines its efficiency and starting power?  
Depends on the inductance of the coil you use. Start low (i.e., 47 Ohms) and up the value until it starts working flakey. Then come down down about 25% in value.

Capacitor Internal Resistance

Q: Can anyone here tell me how to go about measuring the internal resistance of a capacitor?

A: There is a difference between self-discharge and internal resistance. The latter can be determined from the time it takes for the voltage the drop a certain part of the original value when the capacitor is shorted. (To find out the first, you also time the voltage drop, but you leave the terminals isolated.)

A couple of calculations that may be useful:

T = C * R

After time T, the capacitor is discharged to 0.37 * U (where U is the original value)

uC = U * e^(-t/T)

uC is the voltage after t seconds of discharge

It may be better to short the cap using an external resistor, and calculate what *extra* resistance the cap offers.

Debugging Tip

Double check the orientation of your FLED - cathode (side with chamfer) goes negative. Solarengines will work with the FLED in backwards too, but at reduced efficiency.

Ideas and Hints

·  A circuit that charges a large (many Farad) capacitor during the day and at night (at least for a few hours) flashes an LED. The trick is getting the circuit to activate at sunset.

·  A circuit that uses a coil of wire and a magnet to produce some sort of motion (use your imagination to produce something useful). Like Tilden's Magbot.

·  Aim for a more efficient motor and a low trigger voltage, this will keep you in the linear part of the charge curve.

·  Several medium to long bursts of energy are better than a whole lot of small ones.

·  In solar engines utilizing flashing LEDs (FLED), shielding the FLED from intense light makes it much more reliable. A light-emitting PN junction is also influenced by external light. This didn't used to be a problem, but lately there seems to be quite a few FLEDS that are sensitive to light, causing them to lock up. Try using some black electrician's tape or black heat-shrink to cover up the FLED.

Notes and references

[1] The first BEAM bot was "Solaroller 1.0". It was invented Nov 10, 1989 in Waterloo, Ontario at the University of Waterloo MFCF Hardware Lab (after hours) by Mark W. Tilden. It was made from two dead calculators, two dead Phillips cassette mechanisms, and several parts from Laser-printer cartridges. It has o-ring drive of the back motors, with a twist-castor as the front wheel. It would sit around for as long as 20 minutes before making a six-inch sprint, using a Happy-Birthday module as a push-pull trigger for the two 2N2222 style solarengines (very leaky, but all Tilden had at the time). Two weeks later, the first solaroller was born. Yes, backwards, but Tilden is not one for following instructions.

2003 BEAM SolarRoller Race Rules & RegulationsRules & Regulations (Same as 2002)  Last modified on February 4, 2002     Originally Created by Dr. Mark Tilden  Rules adapted for BEAM/WCRG Games by Dave Hrynkiw Official PDF Version of the 2002 Rules  1381 Solarengine Rosetta Stone  Flashing LED Solarengine Rosetta Stone  Note: The following is an extract of the official PDF version above. Competitors are strongly suggested to use the above PDF as your guidelines. 2002 Rule Change:  Masking off of solarcell surface area to meet the necessary size requirement is no longer allowed. There and Back Solaroller has been removed (too simple of a challenge!).   Object: Given a maximum single solar cell size of 806.5 square millimeters (1.25 square inches), make a self-starting 150mm (6") robot dragster to race one meter (3.3 feet) in full sunlight (or 1000 watts of Halogen lighting). Competitors will race each other down parallel 150mm (6") wide lanes. Fastest to finish, or furthest travelled in 3 minutes wins. Background: The Solaroller is a deceptively simple device which employs many of the features of small robot creature design. It is valuable in that it introduces the concept of the self-contained, one-motor-neuron Solarengine, the heart of many BEAM competitors. It is an excellent starter project for future roboticists and can be as minimal or complex as desired. The Solarollers only history is that it is a classic type "1" roving vehicle as described by Valentino Braitenburg in his book "Vehicles" (1984, MIT Press). That is, a vehicle that only moves forward, its speed proportional to the amount of light it receives on an optical sensor. Although simple conceptually, it can be quite difficult to implement because of the solarcell size restriction. 806.5 square millimeters (1.25 square inches) of even the most efficient solar cell is barely enough to turn most motors, let alone take the strain of actually moving the motor, solarcell, and wheels down a racing lane. This is where the designer learns the value of using what is commonly called an electronic "relaxation oscillator". The idea is to accumulate charge in an electronic storage capacitor while the device is at rest, and then to release it into the drive motor suddenly, causing the device to jump forward in steps. The stored energy spent, the capacitor returns to saving energy for the next time. This can be done using many methods from a simple two transistor circuit (detailed later) to a complex sequencer-FET arrangement. The problem is that most electronics require between 2 to 5 volts to operate, and most commercial solar cells only produce 0.5 volts. Amorphous solar cells are thereby recommended. These are found in many standard solar-cell calculators, and are characterized by a multi-cellular design; stacking 0.7v cells or stripes adjacent to each other to produce a low efficiency, low current 2 to 5 volts, which is exactly what is required.    The rest is innovation, calculation and solder-skills. The immediate advantage of building such devices is that they can be very small and thus very robust (see figure below). Anyone who has worked with space-quality solar cells knows that they are sharp, expensive, and as fragile as a potato chip. The amorphous cells are, although not indestructible, significantly tougher, cheaper, and easy to work with. Not surprisingly, it is also often cheaper and easier to buy and destroy a whole calculator for its solar cell than to buy the cell separately from a science shop.  Small means tough. No battery means that your Solaroller will only have to worry about mechanical wear and natural corrosion. Once a Solarengine is complete, it can be used for more than just racing. Devices from robot Venus-flytraps to self-turning solar Christmas tree ornaments have been successfully constructed, and competed in various competitions. The Solarengine is a counterintuitive learning machine. Most people have learned the simple idea of putting a battery to a motorized toy and then watch it whiz about. By contrast, the Solarengine is quiet, slow, and sedate. However, a robotic device using a battery must eventually have that battery replaced, much to the detriment of the environment (and the device if the battery leaks). On the other hand, a Solarengine device is slow but persistent, and will continue to work for many years regardless of human intervention. The lifetime is only limited by the years long decay-time of the cell, and the quality of adhesive holding the device together.    The Solaroller teaches designers to deal with micro amperes of power and efficient mechanical designs from the start. In battery powered toys, a shorted wire could lead to smoke, in a Solaroller, even a 100 kilo-ohm current load is a disastrous energy loss (in a battery powered toy, a high impact crash is inevitable. In a Solaroller, it can only happen if it manages to roll off the shelf. A small leash is recommended).    To help out first time Solar Engine builders, the following page is a high-detail "Rosetta Stone" for Solaroller construction. Such a device can be put together using the contents of the cheapest solar-cell calculator, a dead walkman, and possibly a radio. Chances are you (or a friend) have just these items in the back of a junk drawer, and there is much you can learn just by taking them apart. Solaroller Plan      The Racing Platform: The racing strip is composed of two side by side 1 meter lanes of clean, smooth, level, white melamine, glass mirror, or painted flat plywood, with 25mm (1") high x 12mm (1/2") wide melamine/arborite walls along both sides of each lane. The lanes will have exactly 150mm (6") between wall surfaces along the full travel length. Competitors start in a 150mm (6") square with their forward edge pushed up to the inside edge of a thin black or white starting line drawn on the walls and mirror surface. Exactly 1 meter away from the start line is the finish line. The finish square is also a 150mm (6") square and has no end wall. Due to heat distortion, the walls cannot be guaranteed perfectly straight. The light source for the table will be positioned so that no shadows fall from the sides of the vertical walls. Competitors will race into the light source and must be able to optimize a light source from perfectly vertical (90 degrees) to a late afternoon angle (30 degrees). Light may be limited by a single clear window, but otherwise should be unhampered. Due to the possibility of excessive heat buildup, it is advised competitors not use hot glue, soft, black plastics, or wax to hold their solarollers together. Melting has occurred on occasion, and hot glue has been known to hold a racer in place mid-way down the racetrack. Solaroller - The Race: The single-heat race begins when the judge says "go" and lets a charge build up in the two competitors. This is done by the judge lowering the conductive metal rail at the rear of the starting box that is shorting out the circuitry in the competing racers. Care shall be taken to insure that competitors are released fairly and with as little disturbance as possible. In designs where the aforementioned does not work, an alternate method of starting must be worked out with the judges. It is, however, the designers' responsibility to insure that a clean start can be managed, and this is best done by including a pair of shorting wires that fully extend to the rear of the 6" starting box. Once the contacts are open and the device   Designed by Free CSS Templates Make a Free Website with Yola.