This is a single-motor Sun-powered robot for those who do not have access to
solar panels and 1.5v pager motors. It uses readily-available components
and a brilliant piece of circuit-design to covert very a low voltage from a set
of solar cells into a voltage for a 3v to 12v motor.
The circuit for the Sun Roller is shown below. It consists of a Solar Engine Circuit and a voltage-multiplying circuit. The circuit does not need to have a flashing LED as the trigger, if you are using it in a well-lit area. The main purpose of the flashing LED is to ensure triggering every time the electrolytic reaches a suitable point for powering the motor. This can be between 8v and 20v for a 12v motor and the flashing LED can be substituted for an ordinary LED or by a single zener diode, as shown in the diagrams below.
We have chosen a high-power transistor for the motor driver as some of the low-voltage devices (such as BC338) were instantly destroyed as soon as the voltage rose to 30v.
The circuit is capable of delivering voltages in excess of 30v in full sunlight as the flashing LED fails to trigger when it receives a bright light and the voltage across the electrolytic keeps rising and rising.
The other reason for the high power transistor is the instantaneous current taken by the motor at high voltages. The stalled resistance of the winding of our motor was measured at 15 ohms and this represents over 1.3 amps when the main electro is charged to 20v.
In all, the high power transistor will prevent the circuit "blowing up." If you don't have a BD 679, any standard transistor can be fitted and the trigger voltage does not seem to alter.
Two things that need consideration are the number of cells you will be fitting and the voltage of the motor. This will determine the voltage of the trigger-zener and the value of the main electrolytic. The kit comes with 2200u however the circuit will accept higher values. 2200u provides a pulse of energy to drive the Robot about 1cm per burst.
A larger value will take longer to charge however the motor will produce a longer burst.
Smaller solar cells are available (rated at 100mA) and although they are cheaper and smaller in size, the charge-time will be longer. It's all a matter of adjusting for the best performance with the items you have on-hand and the amount of money you wish to allocate to the project.
HOW THE CIRCUIT WORKS
The circuit derives its power from a set of 4 solar cells and under good lighting conditions they produce approx 1.8v and 200mA. The cells charge a 100u electrolytic and this capacitor provides the high spikes of current needed by the voltage multiplying circuit.
Transistor Q3 is connected in a positive feedback arrangement that causes it to oscillate and produce a voltage across its main winding consisting of 100 turns.
This voltage is produced when the transistor turns off and the collapsing magnetic flux induces a voltage in the winding that is considerably higher than the applied voltage. The circuit oscillates at about 20kHz and the voltage spikes are passed through the high speed diode and charge a high-value electrolytic.
When the voltage across this electrolytic reaches a "trigger value", the Solar Engine Circuit turns on and delivers the energy from the electrolytic to the motor.
The "trigger voltage" is detected by the flashing LED and zener. These components exhibit an infinite resistance until a particular voltage called a "characteristic voltage" appears across them.
At this point current flows through them. The characteristic voltage for the zener diode is known (6v2) and the voltage for the flashing LED is about 2v. This gives a trigger voltage of about 8.2v
What actually happens is this: The voltage across the combination rises until 8.2v is reached. At this point the chip inside the flashing LED begins to operate and creates a delay-time of about 0.5seconds. The voltage across the main electrolytic keeps rising and and after 0.5s it will be about 9v.
At this point, transistors Q1 and Q2 are turned off and the only path for the very small current-flow is the motor, 33k, flashing LED and zener. Since the current-flow is very small, the voltage on both ends of the motor will be 9v and we can say this voltage will be on both ends of the 3k3, 1u and 33k.
The flashing LED "fires" and the voltage across the "trigger components" drops to 8v2. The 1u will be uncharged and since the negative end drops from 9v to 8v2, the positive end will fall by the same amount. This means the base of Q1 will fall and this effect will turn on the transistor. The base can only fall 0.6v as this is the maximum voltage drop that can occur between the base and emitter leads.
The other 0.2v will appear across the 1u.
The PNP transistor is now turned on and this causes the voltage on the collector lead to rise. This voltage is passed to Q2 and the motor is activated.
The motor now has a voltage across it and this voltage is passed to Q1 to keep it turned on. The two transistors are kept turned on while the energy from the main electrolytic is delivered to the motor. Eventually the voltage falls to a low value and Q1 cannot be kept on. The circuit turns off and the cycle repeats.
In place of the flashing LED you can use an ordinary LED and zener diode.
Refer to the circuit above to see how they are connected. The
100n and 33k resistor are removed.
This does not change the operation of the circuit in our situation however
it does limit the voltage across the main electrolytic to 15v and if the
flashing LED is receiving bright light, it will not trigger and the circuit
will "freeze." When the voltage across the electrolytic gets to 15v,
any higher voltage is leaked back into the Solar Charger circuit and gets
wasted across the primary winding of the transformer when the circuit turns
on. The flashing LED must be covered for this arrangement to work.
The author has made kits available for this project through Talking Electronics. A kit is the cheapest and best way to go. It provides all the components to get the project "up-and-running."
That's the advantage of buying a kit. All the hard work has been done for you and the cost is less than purchasing the components separately. There is not one electronics supplier that can provide all the components for this project and to buy them separately you would have to work with more than 20 different part-numbers. Then the problem arises if one component is not available. You have to start all over again! Just click the button below and the kit will be sent.
Some of the circuits in this series of Robots have been constructed without the use of a printed circuit board. Although this has some advantages in reducing the space taken up by the circuitry, it is not a fundamental of good electronic design. Being an electronics teacher I have to say the only appropriate way for me to present this project is on a printed circuit board.
Not only is it easier to construct on a printed circuit board but the chances of a mistake is considerably less. The board adds very little cost to the project but makes it much easier to assemble and fault-find, should a fault occur.
Lay out all of the components included in the kit. Make sure you can identify each item. Start with the printed circuit board and make sure you're familiar with the markings, which are straight forward.
The only item you have to "manufacture" is the transformer. It is wound the core of a 10mH choke. This provides an ideal bobbin. Take the 10mH choke and remove the blue plastic covering from the outside. Carefully snip the fine wire from the leads and unwind the coil as it will not be needed. Don't cut the leads off the ferrite as they are needed to mount the transformer to the PCB.
Unravel the 3 feet of 0.25 enamelled wire supplied in the kit without kinking it. This prevents the enamel from falling off the wire.
Scrape one end of the wire and solder it to one of the wires connected to the core. Make this wire with white-out as the start of the primary winding. Wind the 15 turns of the feedback winding onto the core and cut the wire. Scrape the end and solder it to the other wire on the core.
It doesn't matter if the windings are clockwise or anticlockwise, but make sure both windings are wound in the same direction.
Mark the start with white-out and leave at least a couple of centimetres of wire. The main winding consists of 100 turns. Leave a few centimetres of wire and twist the two wires together to keep the winding from unravelling.
Leave the completed transformer to the side for the moment and start building the circuit.
Start with the smallest or lowest parts on the PC board, which in this case are the resistors. There are 4 resistors to insert. Solder them and clip the leads as you go. This prevent the leads interfering with the other components as you carry out the construction.
The three transistors are next. Be careful with them as they are very heat-sensitive. Make sure the pin-out of the transistor you are fitting, matches the overlay on the board. The transistor-types we have chosen are commonly available and you should have no problem with obtaining a suitable type for each section.
There are two diodes on the board. A high speed diode (such as BY207), and a 6v2 zener diode. Both diodes have a line marked on their case near one end. This is the cathode. Follow the overlay on the PCB. An arrow intersects a line. The line corresponds to the cathode lead.
The four electrolytics, (in some cases there may be six electrolytics), are now to be soldered to the board. Be careful with soldering to prevent overheating.
The transformer can now be added to the board. Insert the leads connected to the core down the two holes so that the start winding goes down the hole market with a dot.
The wires of the main winding are fitted down two separate holes with the start of the winding going down the hole marked with a dot.
Insert the Flashing LED so that the shorter lead or the side of the package with a flat cut-out goes down the hole marked with the letter "k". Do not push the LED all the way down. It can be kept above the board to prevent it heating up during soldering.
The PC board is now complete. The external parts can now be connected. These include the solar cells and motor. There are four solar cells for this project and they have a rating of 0.45v each and an output of 200mA under direct sunlight. You can also use the smaller version, which is 100mA. When connected in series, the cells produce a maximum of 1.8 volts under direct sun light and less when the sky is overcast.
The 12v motor is now connected to the Sun Roller MkIII PC board. There is no switch for the circuit, as the energy is FREE!
The electronics do not do much until you build the body. This is a one motor circuit so there will be some designing involved for it to be made into a dragster-type vehicle. You will need to get a front wheel and axle and make sure the wheel spins freely. You will need to fit a small length of rubber tubing to the end of the motor to provide added friction and thus more traction for the Robot.
The body is made from a length of PC board. This material is easy to solder and create a chassis.
If a 4-cell solar panel is changed for 6 smaller cells, the size decreases and the performance also decreases. This is because the circuit needs the high current for its operation. The additional voltage provided by the extra 2 cells does not improve the performance.
The next project Robo Roller MkII represents a big advancement in