Solar Freaks

Hi everyone, I am in the process of making a solar lantern which uses LEDs. I have a 12V battery and want to use an LDR (light dependent resistor) to measure the ambient light around the lantern so that i can make it auto-dimmable. the LEDs come in a set of 3 surface mount LEDs in series with a current limiting resistor, and i want to hook up about 17 of these modules in parallel...
So my question is, does anybody know of a good LED driver to use, or could somebody point me in the direction of a relatively simple circuit that i could build? the LED modules are rated at 12V, 20mA so i would need about 350mA at 12V...

Any suggestions would be hugely appreciated!!

Thanks a lot!

Mertmate.

Hi Mertmate,

Pulse the LEDs at a rate faster than humans can see. LEDs can turn on and off very rapidly, but it is hard to get them to dim "just right."

The pulse modulation will create the dimming effect. The effect is not linear - they only really put out less light as they are more off than on (less than 50% PWM duty cycle). PWM = pulse width modulation.

I think what you want is something like this: http://www.electro-tech-online.com/electronic-projects-design-ideas-reviews/22394-light-sensor-leds.html Scroll down to see his simpler second schematic. The light is sensed during the time that the LEDs are off. It doesn't take very long to take a reading.

That way you can save money (no light sensor needed).

Thanks very much DCH!
I have a a whole bunch of LDRs availible to me so i think i will just use them, Not 100% about the LED as a sensor idea and how it all works so dont wanna run into any troubleshooting due to lack of understanding...I guess its a bit old school, but, "stick to what you know". I tested a circuit today, using PWM to dim the lights and you were correct, they didn't dim all the way, but i just thought that was the PWM at 5%? Im happy with whats going on though...

Now i need to convert my voltage drop across the LDR into an ADC reading... I'm getting between 5 and 12V at the reference, but my micro can only accept max 3V, 20mA... any suggestions as to how i could acheive a 0-3V reference from 5-12V...i would just use a bigger R1 but the bandwidth's are not equall (3V Vs 7V)???
12V|--------R1-------Vref------LDR-----|0V
and i would like to fed theVref into the micro's ADC which will then cause interrupts in the Timer mod = Change PWM = Change light level....

Thanks very much and have a great day!!!:)