12v auto on-off solar light

Discussion in 'The Projects Forum' started by fireofenergy, Apr 7, 2013.

  1. fireofenergy

    Thread Starter New Member

    Dec 17, 2011
    I want to make a solar light with four Cree XML high power leds (rated at 3A) in series that is turned on at night powered by a SLA battery. I searched about JFETS and MOSFETS and am confused about if the gate switches the circuit on or off (as I seen both on the internet). I know that the voltage drop is about .7v, therefore, should be able to power the four leds at about 2.9v each, assuming 12.4v from battery. This should provide about 700mA to each led as per Cree specs (and proper heatsinking) which is plenty to light up the yard and be like the most efficient light.
    I don't want to use a led driver although the light will be over twice as bright at first until battery voltage levels out because no driver can power four such leds in series from just a 12v source... (that I know of). I want brighter at first, anyways :D

    What I'm trying to figure out is: Can I use the solar panel voltage to turn off a MOSFET (or other suitable transistor type device that is rated at least 3A) via a small current to the gate? I don't know whether to search for N channel or P channel or which is the one where powering the gate turns off the circuit. I searched for JFET on Digikey and found that the highest rating was only 1 watt... :confused:
    I have made a smaller version using two transistors, one that bypasses the other when solar panel voltage is present via a 5,000 ohm resister, and one which allowed current to low power leds when the first transistor turned off at night (so I only know about circuits which turn on at the gate). I guess I just want to know which is the proper "high power transistor" and the simplest method of auto on-off for a much brighter led light. In this case, the four leds will provide about 1,000 lumens at "normal" battery voltage!

    Even though I will (slightly) resist the leds, I want a 3A rating which can handle high current should the battery still be charged up to 13v when the light is turned on...

  2. wayneh


    Sep 9, 2010
    So you've got an arrangement of PV panel, battery and LEDs that works fine, and now you want to turn off the LEDs when there's ambient light detected?

    I'd use a small, separate light sensor, not the PV panel itself. Panel voltage will vary with the load and this could complicate things. The sensor could be a small PV, but I usually see a CdS cell used for this. Cheaper I guess, but use what you have.

    I like comparators, so I'd set up a comparator to watch the sensor voltage versus some reference voltage, both powered by the battery but using very little power. You can adjust the reference voltage to get switching at just the right light level that you want.

    I'd use an N-channel MOSFET to switch the path to ground for the LEDs. LEDs negative lead attached to drain pin of MOSFET, ground applied to source pin. Use the comparator output to apply battery voltage to the gate pin to turn on the LEDs, ground to turn them off. (The MOSFET will have very little voltage drop across itself when turned on.) Almost no current is required to keep the MOSFET turned on, so most of the power loss of this strategy will be in the comparator, and this will be tiny compared to the scale of your system.

    Oh, be sure to choose a MOSFET rated to maybe 4X your expected current and put it on a heat sink. Just a good idea to add life without adding much cost.