Introduction: Simple Solar Circuits

Each spring I gather solar lights my neighbors tossed in the garbage after the lights have stopped working. The ones that only need minor repairs, I repair, and the ones that need major work I strip for parts and reverse engineer the circuit boards. Most of the circuit designs used in automated decretive garden lights are simple and easy to reverse engineer.

Although it can be more work, I repair the solar cells.

Garden lights incorporate three basic circuits, the charging circuit, the dark detecting circuit that turns the LED driver on and off, and the LED driver. Some LED drivers incorporate a voltage multiplier or voltage booster in the LED driver circuit since 1.2 volts is insufficient to power the ultra-bright LEDs.

Now to get started adding solar power to your small electronics projects and use the sun to power your battery powered night lights, garden lights, and other automated decorations or projects. The circuits are easy to build and to get working. They are fun to build and to teach your kids, how to work with light.

In the last step I control a 5 volt motor with a 1.2 volt battery and the solar light IC.

Step 1: Parts & Tools

Most of the circuits in this Instructable work as long as you are in the ball park so it is easy to substitute parts and get the circuits to work.

Transistors; just about any general purpose low power transistor, can be used for these circuits.

2N2222, 2N3904, 2N4401, S9013, S8050, BC546, BC547, or similar NPN transistor

2N2907, 2N3906, 2N4403, S9012, S8550, BC556, BC557, or similar PNP transistor

Diodes; just about any general purpose, switching or other low power diodes, can be used for these circuits, however Schottky diodes have lower voltage drops and work very well.

1N4001 to 1N4007 series, 1N914 to 1N4448 series, and 1N5817 to 1N5819 series.

Resistors; you will need an assortment of resistors for these circuits most of them only need to be ¼ watt, once in a while depending on the circuit you build a ½ watt resistor for circuits over 3 volts. The resistors do not need to be exact so if the schematic calls for a 50Ω resistor, a 47Ω or a 51Ω resistor will work. There is a lot of room to play in these circuits.

50Ω, 100Ω, 150Ω, current limiting resistors for the LEDs.

1kΩ, 2kΩ, 5kΩ, 6.8kΩ, 10kΩ, 15kΩ, 22kΩ, 47kΩ, 100kΩ, 1MΩ, most of these resistors you will only need 1 resistor of each for a circuit but it is always nice to have extras.

Photo Resistor, if you salvage garden lights like I do you should have plenty.

1 ultra-bright LED more if you are doing more than one project, colored LEDs if you like, just for fun and children like pretty colors.

1 switch

Assorted batteries and holders

Assorted Solar Cells

1 bread board for testing.

1 multi meter

Capacitors; a must for the voltage multipliers.

1.2nF, 100pF, one of each.


Two 0.47mH

One 22mH

If you make the circuits in the garden light IC datasheet you will need the parts listed in the datasheets.

Step 2: Testing the Solar Cells

The first part of a solar circuit is the solar cell or other device for collecting light and making use of it; I have quite a collection of solar cells and solar panels, most of them salvaged from solar garden lights rescued from the garbage. Many of them were repaired by me and they range from 1.5 volt solar cells to 6 volt solar cells and 20 mA to over 100 mA.

Now that you have solar cells it is time to find out what you can do with them. You do this by checking the voltage and the amperage produced by the solar cell. On a good sunny the best as you can get, adjust the cell as close to a 90⁰ angle to the sun. Just a small cloud across the sun, or the cell not facing the sun at a 90⁰ angle can affect the cells output.

Never check the voltage or the current of the solar cell unloaded, that means do not just attach meter leads to the solar cells leads. Unloaded the meter misinterprets the current going through it as voltage and gives you a much higher voltage than the solar cell is producing.

Start by connecting the solar cell to a resistor, the resistor can be any size. I chose a 51Ω resistor because I wanted to use the same resistor for checking the current. Then measure the voltage across the resistor, now you get a much more accurate output voltage between 1.5 to 3 volts.

Next test the current; it is always good practice to never test a power source’s current without a load, dead shorts tend to be detrimental to electronics. With the 51Ω resistor attached to my circuit I got a fairly accurate current of 25 to 65 mA.

Solar cells are less affected by dead shorts; most solar cells convert less than 8% of the suns energy to electricity. If you dead short a battery the current will climb until something blows, if you dead short a power supply the current will climb until something blows. With a solar cell if you connect the amp meter to the cell without a load, the current will climb like a battery or a power supply but the current will stop climbing once it reaches 8% of the energy of the sun. That doesn’t mean this is safe to do in all cases, just some solar cells will not be damaged by it.

Since the solar cells were salvaged from solar garden lights most fell into two groups; 1.5 volt and 3 volt cells, however in the two groups the currents varied, 25 mA, 35 mA, and 65 mA.

Step 3: Battery Charging Circuit

Now you have the basic specks of the solar cells it is time to look at the batteries that are charged by these solar cells. The batteries come in 1.2 volt NiCads with a capacity of, 200 mAh, 300 mAh, 600 mAh and 1000 mAh.

When you match the battery to the solar cell all you need for a charging circuit is a diode. To charge the high capacity of a NiCad battery or battery pack it is recommended to charge the battery at the rate listed on the battery label. But when you don’t have these instructions follow the C/10 charging rate.

To achieve a complete charge of a NiCad battery it must be charged at a rate equal to or greater than C/10. Where C = cell capacity in mAh. For example: A 1000 mAh cell requires 1000/10 or a minim 100 mA charge rate or greater. Charging at a lower rate than C/10 will not result in a completely charged battery.

Although a current-limiting resistor between a solar panel and a battery is technically needed, it is not necessary if the battery will not be overcharged. In our case, the solar cells will not overcharge the battery. These solar cells should be able to charge one 1.2 volt, battery, or two 1.2 volt batteries in series at a rate of 20 mA for 200 mAh battery, 30 mA for a 300 mAh battery, or 60 mA for a 600 mAh battery.

The charging circuit for these batteries is simple, a solar cell connected to a diode then connected to a NiCad battery. The diode isolates the batteries from the solar cell so that when the sun is not out the solar cell will not drain the batteries.

You can use almost any switching diode for this circuit, and you can use a much more efficient circuit with a lower voltage drop than a diode, but you would be hard pressed to do it with the same ease or price as a 1N5817 schottky barrier diode.

Step 4: Dark Detecting With a PNP Transistor

Dark detecting LED driver circuit, to add darkness detecting capability to a solar circuit is easy, because the solar panel can directly serve as a sensor to tell when it’s dark outside. To perform the switching you need a diode between the transistors base and its emitter, (PNP Transistor) or the collector, (NPN Transistor). The diode isolates the base of the transistor from the batteries so only the solar cell powers the transistors base.

In this circuit I use a PNP transistor as Q1 that is controlled by the voltage output from the solar panel. When it’s sunny, the output of the solar cell is high at the transistors base, which opens the transistor and switches off the LED.

When it gets dark; the solar cells voltage drops to zero, the current flows out the transistors base and through the solar cell to ground, this closes the transistor letting the current flow through the LED switching it on.

This circuit works very well for low power applications when there in not enough current coming out of the base to damage the solar cell. However with circuits that produce higher currents coming out of the PNP transistors base, you can burn out the solar cell.

Step 5: Dark Detecting With NPN Transistors

With higher currents you do not want the current passing from the base of Q1 through the solar cell or you risk burning out the solar cell. When you use a NPN Transistor the current travels from the solar cell to the base of Q1.

This circuit uses the solar cell for dark detection, this charges the batteries and turns the LED on when the solar cell is in the sun, or turns off the LED when the solar cell is in the dark not charging the batteries. When the solar cell is producing power, the power is applied to the base and the collector of Q1, the transistor switches to closed, and lights up the LED. When the solar cell is in the dark and not producing power, no power reaches Q1s base and the transistor is open turning off the LED. This is a good charge indicating circuit however it doesn’t make a good nightlight since the sun must be out to light the LED.

Step 6: The LED Driver

This circuit is the LED driver using a NPN transistor, when the switch is closed power goes to the base and the collector of Q2 lighting up the ultra-bright LED. When the switch is open no power gets to the circuit and the ultra-bright LED is off.

When you combine the LED driver circuit without the charge indicating LED and the dark detecting circuit; the ultra-bright LED will come on when the solar cell is not charging the circuit. Now when light is on the solar cell it powers the base of Q1 closing Q1 and reducing the voltage to the base of Q2 to near zero volts opening Q2 and turning the ultra-bright LED off. When the solar cell is in the dark there is no power to the base of Q1 opening Q1 and increasing the voltage to the base of Q2 closing Q2 and turning the ultra-bright LED on. Now you have an automatic on and off light.

This circuit has one disadvantage, if you miss calibrate R1 and R2 the ultra-bright LED can come on with a very low drop in sunlight, or only come on in total darkness. To calibrate the light level the ultra-bright LED turns on and off, adjust the value of R1 up or down until the ultra-bright LED changes state at the desired light level.

Step 7: Photo Resistor Circuit

This circuit is a little different than the circuits that use the solar cell for a dark detection; this circuit uses a photo resistor for the dark sensor in place of the solar cell. Now the diode is placed right after the solar cell so Q1 and Q2 are powered by the battery. The advantage of this circuit is the dark sensing LED driver can be one location and the charging circuit with the solar cell can be in another location.

The value of R1 changes with the light, its value goes down as the amount of light goes up and its value goes up as the amount of light goes down. This action of R1 varies the power applied to the base of Q1 and allows Q1 to control the ultra-bright LEDs on and off cycle.

Since the value of R1 changes with light and R2 is fixed, to calibrate the dark sensing circuit you adjust the value of R2 up or down to adjust the light level that turns the ultra-bright LED on or off.

To switch Q1 from a NPN transistor to a PNP transistor you need to swap R1 for R2 and R2 for R1 for the circuit to function as an automatic light.

Step 8: 1.2 Volt LED Driver

No matter what circuit you use 1.2 volts is just not enough to power the ultra-bright LEDs, you need a Joule Thief or Voltage Booster built into the LED driver.

This circuit increases the voltage so the 1.2 volt batteries will power the ultra-bright LEDs. The circuit doesn't deliver a DC voltage to the LED but a high-frequency pulse. This creates the same brightness from the LED as a constant DC voltage while needing less than 50% of the energy enabling a single 1.2 volt cell to be used. Since this circuit does not have a resistor for the LED it further increases the circuit’s efficiency.

Step 9: 1.2 Volt Solar Light

Now that you have a 1.2 volt LED driver it is a simple matter of attaching the dark detecting circuit to the LED driver. Ether of the dark detecting circuits will work, when the solar cell or the photo resistor is in the light Q1 is closed reducing the base of Q2 to near 0 volts opening the transistor and shutting down the LED driver.

Step 10: Solar Light ICs

Solar light ICs are very handy, they have the dark detection circuit and the voltage multiplying LED driver built into one small four pin component. Using the solar light IC all you need is the solar IC, an inductor, and the ultra-bright LED to make the circuit. Add the battery and the solar cell and you have a solar light.

I haven’t had much luck finding the datasheet for the solar light ICs and the three I found are not in English. That aside the inductor controls the power to the LED.

Step 11: Controlling a 5 Volt Motor

When I first started experimenting with the IC I used a PC817 optocoupler to connect the solar IC circuit to a more powerful LED. The solar IC would continually trigger and turn on the LED until I added a 1N4148 switching diode to the optocoupler input. Now the 1.2 volt solar IC turns the more powerful LED on and off cleanly.

Turning an LED on with a solar LED was not very impressive so I went to a 5 volt motor by removing the ultra-bright LED and its resistor and connecting the motor in its place.

If you watch this video you can see the circuits in action.

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