I wanted to make a super high power solar light that puts out lots of light and turns off the LEDs automatically in the daylight. I wanted a very simple circuit, so this morning I put together this basic circuit that only uses three extra components for just that. See LED light sensor schematic below:
(to see a bigger version of any of these pictures click the image, then click the image on the following page, it is a weird wordpress issue)
When the sun is out the circuit only draws about 6uA from the battery. The 6uA draw is because the photo-transistor is conducting current from the 2.2M resistor to ground, this turns the MOSFET off since it does not have enough voltage on its gate. When the sun has gone down for the day the photo-transistor stops conducting current this turns the MOSFET on, drawing current through the LEDs to ground.
This light sensor is very sensitive to daylight, and turns on right around dusk. The sensitivity can be tweaked by changing the value of the 2.2M resistor, more resistive = more sensitive. If you increase the resistance too much the circuit will stop functioning because the leakage of the photo-transistor will pull the voltage at the gate down below the turn on threshold of the FET.
The LEDs are ~10,000mcd and I am running three of them in series for each string, my circuit is running two strings of LEDs. The 100ohm resistor limits the current to the LEDs to about 20mA, so with the two strings of LEDs this circuit draws about 40mA when the LEDs are on.
My circuit is powered from a 7A/Hr 12V SLA battery and is being recharged with a 1.5W solar panel. With the 40mA draw my LED solar light should run for more than 15 days without the solar charger, but with the solar charger this should run for years without any intervention.
I will be checking on the battery daily for couple of weeks and will try to determine how many more strings I should be able to attach. I also plan on adding a PWM circuit to the LEDs to pulse them with a higher current. With the PWM the circuit should draw less average current with the LEDs appearing brighter due to the persistence of vision from the human eye. More on this mod in the next two weeks.
Here is a picture of my front door with the two LED strings light pointed at both sides of it: (note the yellow looking LED in between the two strings is from another solar light, it is putting out 1/20th of the light)