How to start adding solar energy to your small electronics projects. Use the sun to power small solar night lights and batteries, garden lights and decorations for halloween.
Solar Circuits - 11
Solar Circuits - 12
The first part of a solar circuit is ... a device to collect sunlight. To keep things simple, we are using a single small well-made solar panel for all these circuits. The panel we are using for these circuits is this, BG Micro part number PWR1241, about $ 3 each. It is a monolithic solar panel of indium copper diselenide, apparently printed in a square of 60mm glass and coated with epoxy for hardness. At the back of the panel there are two solderable (thin) terminals, marked polarity. (Although you can weld directly to the terminals, be sure to relieve the stress of the connections, for example with a drop of epoxy on the wires.) In full sun the panel is specified to produce 4.5 V up to 90 mA, although 50 mA seems a more typical figure.
[Before going to our first examples, a word of caution: These are small simple circuits. In constructing these, we will deliberately overlook a number of small details and issues that are not important in these low powers, but could become critical if we were to expand.]
Direct management:
The most obvious way to use the power of a solar panel is to connect your load directly to the output cables of the solar panel.
Here are a couple of examples of this in practice
To the left, we have connected one of our small solar panels directly to a small engine taken from an old CD player. When you place it in the sunlight or near a lamp, the engine starts to rotate. To the right we have connected one of the panels to a high power blue LED. The reason we have used a high power LED here is that it can easily support 50-90 mA from the solar panel. A "regular" LED designed for 20 mA would be destroyed by that current. (The LED is the same type we used for our blinking high power LED circuit).
Interruption-resistant direct drive:
"Direct Drive" circuits work well for their design function, but they are fairly basic. They do not provide energy storage, and therefore are quite vulnerable to blink when a bird or a cloud passes over. For some applications, such as running a small fan or pump, that may be perfectly acceptable. In other cases, such as powering a microcontroller or other equipment, a brief interruption of the power supply can be detrimental. Our next circuit design adds a supercapacitor as a "flywheel" to provide continuous power for short interruptions.
Instead of adding a single supercapacitor, you might notice that we have added two. That's because the supercaps we had on hand are rated for 2.75 V - is not enough to handle the 4.5 V output of the panel when there is sunlight. To overcome this limitation, we use two of the series capsules, for which the voltage classifications are added, giving us a barely acceptable total score of 5.5 V. (Note: be careful to add capacitors of different values in series - the voltage ratings can scale in not obvious ways.) When first exposed to light, this circuit takes about 30 s to 1 minute to charge the capacitors rather than the LED can turn on. After it is fully charged, the circuit can be removed from sunlight and still drives the blue LED for about 30 s to 1 minute - a very effective flywheel for light applications.
Add a battery
While the break resistance is pleasant, a capacitor generally does not provide sufficient energy storage to power a solar circuit for long periods of time in the dark. A rechargeable battery can of course provide that function, and also provides a fairly consistent output voltage that a capacitor can not. In this next circuit, we use the solar panel to charge a rechargeable NiMH battery and also LED off the power, which will remain on when it darks out.
In this circuit the solar panel charges a 3-cell NiMH battery (3.6 V). Between the two is a "reverse lock" diode. This one-way valve allows the current to flow from the solar panel to the battery, but does not allow the current to flow back out of the battery through the solar panel. That's really a major concern because small solar panels like these can leak up to 50 mA in reverse direction in the dark. We use a 1N914 garden type diode for reverse locking, but there are also higher performance diodes available that have a "direct voltage" lower.
In this design we are continuously "slowly charging" the battery when there is sunlight. For NiMH batteries and sealed lead-acid batteries (the two types that are most suitable for this type of unmonitored circuit) it is usually safe to "charge" them by feeding them at a speed below something called "C / 10". For our cells of 1300 mAh, C / 10 is 130 mA, so we must keep our load below 130 mA; is not a problem since our solar panels only supply up to 90 mA.
The other thing to note about this circuit is that it is pretty damn inefficient. The LED is lit all the time, as long as the battery is at least slightly charged. This means that even when the circuit is under sunlight, it is wasting energy when turning on the LED: a considerable portion of the solar panel current is going to drive the LED, not to charge the battery.
Detecting the Darkness
We have recently written about how to make a dark LED detection driver circuit useful. That circuit used an infrared phototransistor. To add a capability of detecting darkness to our solar circuit is even easier, actually, because our solar panel can serve directly as a sensor to know when it is dark on the outside.
To perform the switching, a PNP transistor controlled by the voltage output of the solar panel is used. When it is sunny, the panel output is high, which turns off the transistor, but when it gets dark, the transistor allows the current flow to our yellow LED. This circuit works very well and is a joy to use, it would make a good upgrade to the dark pumpkin detection to make it go solar with this circuit.
A solar garden light circuit
While the last circuit works well to drive a yellow or red LED, it operates at 2.4 V (the output of the NiMH battery), it does not have enough voltage to drive a blue or white output LED. So we can add to that circuit the simple voltage boost Joule Thief to get a good design for a solar garden light: A solar-charged battery with a dark detector that leads to a Joule thief to run a white output LED .
Of course, you would like to give this a hard, weatherproof enclosure if it were to be run outside. (A mason jar comes to mind!) This circuit is actually very close to how many solar garden lights work, although there are many different circuits they use.
Adding a microcontroller
Our latest circuit examples extend previous designs by adding a small AVR microcontroller. We again use the output voltage of the solar panel to perform the detection of darkness, but instead take it to an analog input of the microcontroller. The microcontroller is potentially a very low current, efficient device that allows you to save energy by not running the LED all the time, but (for example) wait until one or two hours after dark and / or dim the LEDs on or off, or even blinking intermittently for very low average power consumption.
In this example, we have the PWM (pulse width modulation) output of the microcontroller that controls a Joule Thief style voltage pulse to run the white LED. (This is one of many, many different work designs for this type of impulse circuits.)
We also did a second version of this circuit, with two red LED outputs to make a creepy Jack-o'-lantern:
To finish it off, we carved a beautiful white pumpkin and added this circuit to make our programmable pump powered by microcontroller, dark detection and solar energy, that would fade the eyes and leave one by one. Note the long cables in the solar panel and the cables to the LEDs to reach.