Solar Powered Garden Light

This simple flyback/blocking oscillator LED power supply was adapted to and built into solar powered garden light by a talented experimenter in the United States known as "mrs biggs". The method of switching the power supply off during daylight hours, thus allowing the rechargeable cell to recharge, was taken from a circuit design by Nick Baroni, of Willetton, Washington, and published on the siliconchip.com.au website.  Here is the design from mr biggs' workbench.

Mr. Baroni's original circuit used a BC547, but mrpiggss found that a BC547C (the version he bought) would work as Q1 but not Q2 (see "A Note On Transistor Selection, below). In his version, mrpiggss used 2N4401 transistors for both Q1 and Q2, to keep the bill of material as simple as possible.  He also noted that if R1 was 15k, the sunlight gating function would be more sensitive, thus keeping the LED power supply off until it was darker than when R1 is 22k. The core for L1 came from ebay with no part number or supplier but it is the size of a penny and about 3mm thick. The wire came in a 3 pack of magnet wire from Radio Shack and and are pure copper.  Being green in color and 30 gauge, the wire is easy to handle and wind. The cores have 40 turns, 20+20 (wind 20 turns, pull out the center tap, then wind 20 turns more). and removing the coating is easier than with really thin wire. The insulation on this wire can just be burnt off with a lighter, and it comes off just like magic. No scraping or sanding, although he use a small emery board to give it some tooth for soldering.  Another very nice method of removing the insulation from magnet wire came from Christian Daniel of Germany.

     "Scratching away the insulation is difficult with thin wires, it's too easy to cut or weaken them. I prefer fine corund sanding paper. Or - for very thin wires - I use an Asperin(R) pill and press the wire with the tinned hot soldering tip on it : opacht ! Put your eyes and nose away ! The hot organic acid destroys the insulation and it can be tinne nicely."  The solar panel is a standard one-battery panel. it has no info on it but puts out 1.5 volts in full sun. No idea about the current rating.D1 can be nearly any silicon or germanium diode, provided it is rated to handle the solar panel's output current into a short circuit. For most of the panels used in garden lights, this means basically, any diode you can buy. A low power Schottky or germanium diode would have a lower forward voltage drop than a small signal silicon PN diode. The 1N4001 series, as used by mrpiggss is a good choice as well because its large junction area results in a relatively low forward voltage drop.

This is the "circuit side" of the printed circuit board. The components are mounted on the opposite side. It should be noted thatthe basing for the transistors corresponds to the 2N4401, and NOT the BC547.