In my lastest project i want to build a little solar lamp. So i went to my local electronics distributor and want to buy i little solar cell. The solar cell has an output of 2V and the battery an maximum output of 1.2V, which is quite to low to drive any standard white LED.
Sure there is an easy way to solve this problem: Get 3 AA-batteries, a resistor, a diode, a 4V solar cell, perhaps a switch and you are done! Stop! Why not use a circuit to drive the LED and you the components we still have?
All we need is a step-up converter. I show you the basics of a step-up converter and how you can make i quite nice step-up converter on your own with standard components. It is quite easy.
Step 1: Basic Circuit Knowledge and Simulating
The basic circuit can be easily found. It is based on an inductor, a switch, a diode and a capacitor. The switch shouldn't be a real switch. You need a rectangular signal at this point. Working with this low voltage no timer like a NE 555 will work and you have to find another way to create a rectangular signal. I use instead a classic multivibrator. It works with low voltages.
The components for the step-up converter with a multivibrator are
1x 1.5mH inductor
2x BC337 (or equal) NPN transistors
some resistors (10k and 27k)
1x LED (white or something you like)
At first I do some testing stuff and simulate the circuit in Spice. While we want to drive a LED, we can remove the diode and capacitor. The LED rectifies the signal on its own and you will not recognize a ON/OFF-switching of the LED. Now we can add the multivibrator. First I try to use a full astable circuit – this circuit works not well - and search for best frequency. I found alternatives in the internet which might be more compact and more beautiful. I found my favorite solution on a website (German: http://www.b-kainka.de/bastel36.htm ).
The basic concept is still in this circuit. The astable circuit create a “rectangular” signal. When the right transistor conducts, the inductor generates a magnetic field. The left transistor starts to conduct and the base emitter current of the right transistor equals zero. Now the magnetic field of the coil inducts a voltage which is add to the voltage of the battery cell (Lenz's law). The coil is an energy storage. You can use any LED – the circuits manages the voltage and current.
Caution: Step-up converters can rise the voltage very high. If there is no load at the output, the components will be damaged. Remember to discharge capacitors and remove the input voltage before you touch something!
Step 2: Build the Step-Up Converter (1)
In the next step we build the step-up converter. I use a soldering iron and build a permanent solution for my solar lamp.
To build this circuit you need a soldering iron, a side cutter, a breadboard (with stripes) and the components from step 1.
Solder the first transistor on the board like on the assembling diagram (or feel free to modify). Be careful with the orientation! The arrows at the diagram show the copper stripes.
Caution: A soldering iron has a temperature of 662°F (350°C). Don't touch the iron parts! Some solder material may contain toxic elements.
Step 3: Build the Step-up Converter (2)
Add the missing inductor, capacitor and resistors. I use a axial inductor. There are many footprints for inductors – make sure they fit your board. At least you can add the LED. The forward voltage and current of the LED is automatically managed by the converter. Be careful with the direction off the LED or it might be destroyed! Connect the board to a AA-Battery at the + and – in the diagram and you are done.
Step 4: Testing and Performance
I connect my board to my adjustable power supply to check the performance: The LED starts to glow at 0.7V. At 1.2V – the Voltage of a NiMh-battery – it lights up bright. I use a 20.000 mcd white LED. The illumination is not as good as the directly driven way – but it shines very long. Using a 1100mAh battery at full charge the LED shines in my tests over 4 days and more! Even old batteries can be used – before they move to trash.
Step 5: Usage in a Solar Lamp
You use your new board directly in a solar lamp. You need a 2V solar cell, two standard diodes (1N4148) and a battery clip. One diode connects solar cell and the battery. The other diode is a used add your board a enable port – yeah thats right! - By connecting the solar cell and base of the left transistor the LED starts to light up, when there is no light. The left transistor conducts and a little leakage current flow trough the 10k resistor and the LED. It is about 0.2mA – nothing to worry about. The circuit generates in this mode no rectangular signal and the LED gets not enough power to light up. You can use shottky diode to set beginning of lightning to a lower illumination.
Step 6: Showcase
I use this combination for a little table top solar lamp. If you are interested on a instructable of the mechanical part, leave a comment.
Hint: There are ready to use converters at the market – like the PR4402 or QX5252 – for this applications, which can give you perhaps a higher illumination. My board is still quite cheap (less than 1€) and is build quickly with standard components.
Thank you for reading my instructable! If you have any questions, leave a comment.
Hopefully you get the content – my english skills are terrific :)