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Subject: Re: a cell charge design containing LED
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Date: 06/29/02 at 5:30 PM
Posted by: Will
In Reply to: a cell charge desing containing LED posted by Umut on 06/26/02 at 3:46 AM:
Without going into great detail (because I don't know enough to be able to tell you!), here's a basic outline of the system you need:The solar cell(s) [I think you would need two] charge the battery through a diode to prevent discharge during low-light conditions.The battery also runs a low-current op-amp that compares the voltage across the solar-cells (on the solar-cell-side of the diode) to some reference voltage, and when it drops below, use the output of the op-amp to switch on a transistor to enable the LED.The hardest bit will be getting the reference voltage to remain stable, since the whole system (solar cell *and* battery) are liable to produce varying voltages.The cheap 'n' cheerful answer would be to stick a simple potential divider across the battery, but the problem with that is that as the battery discharges the light-level required to switch on the LED also drops. That said, you could sight that as a "feature" - a battery with more charge can run the LED for longer, so we might as well switch it on before it gets so dark.If you wanted to fix the switch-on level precisely you'd have to use some form of voltage regulator, perhaps one of the low-current "voltage reference" ICs available quite cheaply. I think these are mainly designed to produce a 5V reference though (so you would be unable to drive them with 3V). There are also PSU ICs that have a variable output voltage, although you'd have to look into how much current these (and the associated external components) would draw - they might drain the battery quickly.If you get stuck, the device you have described is already available commercially, so you could get one of them and rip it apart to see how it works! (Actually, I have one in my garden so - since I'm unlikely to ever check this forum again as I only found it while searching for something else - if you'd like me to have a look inside it for you, drop me an e-mail. (I might not bother though, it all depends what sort of a mood I'm in. ;o) )Good luck!
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