Another Solar Charger/Light Project

Discussion in 'The Projects Forum' started by iONic, Aug 5, 2008.

  1. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
    The following circuit is a bastardized version of a SIMD1 / Solar Regulator, and it may be more true than not. That's why I am looking for opinions on its functionality as well as tweaking advice.


    Original Circuit Description

    SIMD1 / Solar Regulator
    The SIMD1 was enhanced in the Solar Regulator version which supplies a constant 2V after
    triggering and permits driving LEDs without using current limiting resistors with constant
    brightness. Note that you can use this Low Drop Out (LDO) linear regulator for various
    applications but like all linear regulators it is "lossy". This is not a problem for controlling LEDs
    since they would otherwise use "lossy" resistors but for motors it is a different story.
    Try this new SIMD1 / Solar Voltage Regulator for use with blinking LED circuits (pummers). It
    turns on when it gets dark, just like a D1 but the output voltage is regulated to about 2V
    (depending on the reference LED Vf).

    The SIMD1 / solar regulator circuit draws no current during charging and when turned on, it
    draws less than 100uA with a maximum 10ma output current. The regulator provides constant
    LED brightness during the discharge and turns off when 1F solar capacitor voltage drops below
    the LED turn on voltage. The LED used for voltage reference in the solar regulator feedback loop
    and the LEDs used for the flasher must be the same high efficiency type to match the forward
    voltage specs. This circuit is ideal for supplying voltage to a Bicore or 74HC14 LED flasher since
    it eliminates the LED current limiting resistors and greatly reduces current consumption of the
    HC flasher circuits.


    One interesting alternative would be to substitute a 5V NiCad (4 cell) battery for the 1F supercap
    which acts to increase the storage capacity many fold for use with flag waver motors,
    pendulums, etc. With higher load current, the 100K resistor may be replaced with 20K for up to
    50 ma output current. The quiescent current of the regulator remains very low and is
    proportional to the load current for high efficiency operation during discharge.

    The solar cell charges a 1F capacitor through a 1N34A Germanium diode to a maximum voltage
    of 5.5V. While the charging current flows through the diode the voltage at the cathode (stripe) is
    about 100 mV negative with respect to the 0 V line. This negative voltage is applied through a
    100K resistor to the base of a 2N3904 NPN transistor Q1 and holds that transistor off. This cuts
    off the base current for the 2N3906 transistor (PNP) Q2 and the output of the regulator will be
    zero volts.

    Rapid switching is very important in this type of circuit because a circuit that is half on draws
    power, draining the capacitor, but performs no useful work. On the SIMD1/ Solar Regulator, the
    output snaps on and off.

    At the end of the charging cycle, when the light on the solar cell decreases, the negative
    terminal of the solar cell starts to become more positive than the 0V line. The base of the NPN
    Q1 must be at about +500 mV (positive) with respect to the emitter which is connected to the 0
    V line, before it turns on and turns on the rest of the regulator. That usually happens in the
    evening but can be simulated by cupping your hand in front of the solar cell.

    When Q1 turns on, the PNP transistor 2N3906 - Q2 receives base current and it starts to turn on.
    The regulator output voltage at the collector of Q2 increases to about + 2V when the red LED
    starts to turn on and to supply current to the base of NPN transistor 2N3904 - Q3. When Q3
    turns on it "robs" base current from Q2. This in turn controls the base current for Q2 and the
    regulator output will stabilize at +2 V. The 10K resistor from the regulator output to the
    negative terminal of the solar cell provides positive feedback to the regulator turn on by loading
    the solar cell down so that it's output voltage drops even more and the regulator "snaps" on.
    Note that the red LED is used for reference voltage only and does not actually light up.

    The output voltage at the collector of Q2 remains at 2V while the voltage on the main capacitor
    can range from a full charge at 5.5V to 2.1V at the end of the discharge cycle. If no load is
    attached to the regulator the capacitor voltage will drop very slowly because of leakage and a
    small amount of current required for the active regulator (<50 uA). When a HC chip like a
    74HC240 or 74HC14 is powered from the 2V regulated output, the current for that chip is also
    very low.

    If the HC chip has a LED connected to the output which the same type of LED that is used for
    reference, then the current will be limited by a small voltage drop on the HC driver output. Since
    the regulated voltage is constant the brightness of the LED is also constant.
    When the voltage on the 1F cap drops below 2V, the regulator reference LED turns off and the
    base currents of Q1 and Q2 increase discharging the remaining charge on the cap and turning
    any attached circuit rapidly off. At some point the voltage of the solar cell even in dim light is
    higher than the remaining charge on the capacitor and if there is sufficient light (usually in the
    morning) the charging cycle repeats all over again. If the Sun is bright and the solar cell was
    shielded by your hand, then exposing the solar cell to the bright sunlight generates enough

    power to turn the regulator off and force the circuit back into the charging cycle.
    The PowerSaver Flasher uses capacitive output coupling to produce brighter shorter flashes and
    has a much lower average current drain than standard bicore or 74HC14 flashers. The PS Flasher
    with one LED circuit (2 LEDs) runs all night from a 1F cap charged to 5.5V. Up to 12 LEDs can be
    controlled with one 74HC14 flasher and probably would run for 2 hours from a full charge. Use a
    range of timing resistors between 1M and 4.7M for each oscillator to give a random light show
  2. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
    Hmmm, maybe I ought to delete this thread...
    Last edited: Aug 9, 2008
  3. HarveyH42

    Active Member

    Jul 22, 2007
    Why? I saved the schematic, but don't know when I'll get around to giving it a try. Most likely will modify it some for a really bright white LED. I've got a couple of Pummers going in my front window, looks like a camera flash.

    I'm busy finishing a couple of AVR projects, well more like tweaking, both are just starting points. One is a replacement for solar garden lights. Using an LED for the light sensor. Just enabled the Brown-out detector, in hopes the MCU won't freeze when the batteries are drained.

    The other project is an audio toy for my dog. It's been beeping periodically for over week on the same batteries, so that's good. The frequency range still needs some work. Dog doesn't seem to care or notice, but has be considerably less destructive while at work.

    Your circuit is a good addition, simple well written. You should leave it up, good school project for the kids to turn in. I have two super-capacitors, but no idea where I put them (some place where they wouldn't get lost, never works...). the 1N34a diode, don't think I've seen that before, but figure most anything will work.