LDR Needed for Solar Powered Lighting

Have you tried going down to a local store and buying one of those self-contained solar garden light kits designed to give pathway lighting at night (the type with a single solar panel, battery and LED) and seeing how they do it? I believe the LDR is just a standard Cadmium Sulphide type controlling a comparitor, and a transistor acting as the switch. This should be available from local electronic stores (manufacturers include Philips, and likely many others).

I used one of these kits to do some development work. Couldn't believe how cheap they must be to make. I couldn't even source the battery for the same price as the individual solar light (which includes its own battery). I wonder how many millions of these there are being produced. Oh, the unit was not much help to me as the solar panel was garbage, but what do you expect for a few dollars (better than my "solar" powered calculator! Pulled it apart and found a battery, the solar panel was a strip of plastic that looked like a panel . Oh a whole $2 wasted, but don't worry, I didn't go hungry that night or anything). Oh well, I should probably stop before I start rambling .

To simplify your project, consider using the "solar panel" that you use to charge the battery as the on-off sensor. Apply the output of the Solar Cell (or array) to a comparator circuit with a capacitor on the input to inhibit response to "fast" changes in the solar cell output.

Using the solar cell will require the addition of a steering diode placed in series with the battery being charged, the sense point is the junction of the solar cell output and the steering diode. The steering diode should be of a low forward voltage type.

When using CdS cells (light dependant resistor of Cadmium Sulphide type), please consider that the CdS element will respond to temperature as well as most "radiant energy".

What the comercial folks do is quite simple, if you obtain an orp12 LDR and a transistor to take the required current a few milliamps, almost any npn type To5 type

then put a suitable resister in the base to +V rail the LDR in the base to ground rail

the load in the collecter to +V rail with serries (R) to limit current connect the emitter

to ground. Remember V/I=R, V/R=I, IxR=V OHM'S law. It is very simple realy.

Get the LED's the right way around. The emitter is shown by the tab on the transister

can, the base is the center connection the other is the collector. Most all sillicon transitors need .6V to turn on. A 100 - 220uf cap on the base to ground + side to base of Tr. Will slow down any noise on the base connection. .1uf ceramic will stop RF, noise. 1000uf low leakage type will if connected to the +V side and ground help

give a more even light. Adds some extra energy storage. Do not add more or the

photvoltaic cell will not like it. Try it out it won't break the bank.