A simple solar-powered electronic project.

I recently made a solar-powered night light, and I thought it would make a good electronics project for anyone who is starting to learn electronics.

I acknowledge and am grateful for the information provided by Evil Mad Scientist Laboratories here.

The circuit diagram is here. Considering it from left-to-right, a solar cell charges a 100F capacitor through the diode D1. The diode prevents the capacitor from discharging when it is dark.
The 4k7 resistor provides a sense voltage to the base of a PNP transistor T1. (Rule of thumb: PNP transistors conduct from emitter to collector when the base is pulled low)
In the dark, T1 allows current to flow (via a 100 ohm limiting resistor) to the Joule Thief.
The Joule Thief is a simple switching boost mode power supply, although its’ theory of operation is quite complex. For most purposes, I consider it as a “building block” or a single component. The double-wound coil, the 1K resistor, T2 and D2 (white LED) form the Joule thief. The LED is not actually on continuously, it flashes at some silly fast number of pulses per second. This frequency depends on the input voltage, the nature of the coil and the transistor T2.
I had made the Joule Thief separately, on a bit of strip-board. In these pictures here, here and here you can see it.
The control part of the circuit I made by connecting the components directly together, for simplicity’s sake. Using a bit of heat-shrink here and there to prevent short circuits.
The entire project is in a transparent plastic box, which may have been one of these. I used a few dabs of silicone sealant to secure some of components in place, a bit of sticky tape for the solar cell.
Finally, when soldering to the solar cell, use extra flux and heat the solder just enough – don’t hesitate to remove the soldering iron when the solder flows.

17/6/2010: A quick update.
If you don’t fancy winding your own coil, “acquire” a miniature isolation transformer from a dead PCB.
Something like this:
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2106368
Although they come in various different designs, I found one in a faulty ADSL line filter recently*.
Ideally it will have a 1:1 ratio of primary to secondary windings, but you may have to guess at the relative polarity of the coils, test it on breadboard first.

* The transformer I used came from a “Tehkal/Origo ASL 2019” (this page, about half-way down. It did explain why the connection would sometimes drop when the telephone rang.

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