Simple Solar Circuits

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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.

https://www.instructables.com/id/Reverse-Engineering-1/

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

https://www.instructables.com/id/Repairing-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.

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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 multi meter

Capacitors; a must for the voltage multipliers.

1.2nF, 100pF, one of each.

Inductors

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.

http://www.powerstream.com/NiCd.htm

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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155 Discussions

I have a Walmart solar light that is not detecting daylight. I suspect the chip, but its not one that can be found D1803. Its close in design I think to the ANA608. I have the data sheet. I'm going to try and find that one.There's only the diode, capacitor and resistor on the board. Of course the chip and LED too... Matches one 608 schematic. The board is labeled 3416b-K10AX v1.0... That also could not be sourced. I found one hobby supplier (for the 608) in England (Flytron.com), and also (aliexpress.com). Trying to stay away from Chinese sources if I can... Even ebay is claiming possible tarrifs (:-(...Might you have any others? Seems the chip is also used in drones for LED night lighting!

You are missing a prefix that is JD1803 the ANA608 is different.

Thanks..There was something in front of the D but I couldn't make it out... Too close to the edge.

Love this instructable and right on time. I have an outdoor motion sensor that uses 9v batteries and transmits via 900mhz to turn on flood lights. Problem is the batteries last about a month and I'd like to improve that (as it requires ladder to change out battery). Built a solar panel array putting out 12-14v and about 34mA to charge a 9v 300mah battery. The arrary works fine. When I put it in series with the motion sensor and using a 1N914 diode, it works for a week and then the battery is drained. I have a problem with the load vs battery, and I think your example of using a PNP Transistor is the ticket, but struggling to determine sizing of some of the components. Any help in the attached diagram would be appreciated to keep my family more safe. Thanks.

What are the specs of the PIR?
You may not need the 100 ohm resistor if the PIR runs on 9 volts and the 5.6 k can be replaced with a 1 K.
How many hours of full sun daylight do you get?
Right now I get about 55 hours of full sun day light a month.
To run that you need about 220 hours of full sun daylight or more solar panels.

Wow! thanks for the quick response. The PIR is a SkyLink ID-318 https://www.skylinkhome.com/docs/manuals/lc/mID318v2.0.pdf
The PIR does run on 9v.
I don't have a lot of experience with circuit design, so sizing the resistors is a bit of a learning curve, so recommending to go to the 1K is helpful.

As for sun, I average about 5-7 hrs a day and hope that will provide adequate voltage. I've got a 20kW (92 Kyocera panels) solar array on my house, and the image shows solar performance for 12/12/19 as an example, and this is the time of year it's the darkest. The key in my mind is the load consumption, which I'm not really sure how to accurately measure. I've put a fully charged 9.02v battery in and tested a few days later to see the voltage around 8.48v. My hope is that with the solar panels, it will keep the battery topped off. Don't know if I should be concerned about overcharging?

If you think the 2N3906 is the right PNP Transistor to use, I'll order them tonight. I appreciate the help and will report back my progress to other readers. The ID-318 works great to place a remote motion sensor far enough away to provide advanced notice of traffic to light up the area before you drive up to it. My wife & daughter really like it, and make me aware when it's not working, so you'll make two girls happy. Thanks

You need C/10 for 10 hours to fully charge a battery.
300/10 = 30 mA for 10 hours.
You are supplying 34 ma for 5 hours that is 170 mAh.
To fully charge the battery in 5 hours you need to supply 60 + mA

You’re absolutely correct and I did not properly explain. The location will get 10+ hours of sun at ~12v and 34mA, but only 5-6 hrs of “full sun” giving ~14v+. I tried increasing the number of panels and mA output, but had problems with batteries overheating.

I’m looking forward to wiring this up when the parts come in. Wish me luck!

Well, so much for good luck. I wired it up as the diagram I sent you indicated with a 1K ohm resistor in place of the 5.6K, and removed the 100 ohm as you recommended, but I get 1.54v at the battery terminal with 14v from the sun.
The PIR is 9vdc @ 10ma, but the PNP 2n3906 doesn’t seem to “switch”.
I just don’t get it, but welcome any suggestions.

14 at the solar cell and 1.5 at the battery.
Ether the power from the solar cell isn't getting to the battery. (Bad diode or joint)
Or the power is being shunted away.
Check the diode and joints. if that doesn't work move the PIR.

Was out of town for the holidays, but back and thanks for the reply. I know you've got other things to do, but wanted to say thank you for your help. So, I moved the connections on the PCB board as your drawing indicated, and getting now getting 16.68v at the battery terminals, and saw the battery voltage go from 8.70v to 8.79v within a few minutes. Once the sun went down, the PIR is unfortunately not functioning. I decided to leave it up today, as Sunday here has been a beautiful sunny day, and check it again once the sun goes down. If the PIR is not working, I will check voltages across the leads and report back.

Yea; that is why I always build on a bread board first, easy to modify for circuit quirks.
Many of the circuits I build do not work in circuit simulators like "Electronics Work Bench" because the programmer only uses positive logic when they program the program or they miss things like DC in and pulsing out of a component.
PIRs can be picky.

There are no visible ICs, non-LED diodes, or transistors on the PCB I have for a garden PV light, but I can not tell if they are lurking under the round black mound visible on the B side photo. I have no training in circuits, please excuse my diagrams. 2 questions: A: is there a component hiding under the black mound? B: where do the conductor tracings go under the black mound? Thanks, F

The IC is under the black dot. It is custom so the exact wiring is a guess, but it should be something like this with the diode making sure the current only goes one way from the battery and not through the solar cell.

Thank you. You are doing a good thing here.

Hi, I've tried making a few of these circuits with an NPN transistor but am struggling with the dark detection sensitivity; it comes on early when it's still light outside. Any ideas on how to adjust the light operating sensitivity? Thanks. - Grant

How light is it outside when the lights turn on?
In steps 4 and 5; a lot of when the light turns on or off depends on the solar cells sensitivity to light, or rather its lack of sensitivity to light. The point the solar cell goes from producing a current and turns into a resistor.

You may need to tinker to match the circuit to the solar cell.

Cure 1.
Some transistors are more sensitive than others so changing the transistor may work.
NPN; 2N2222, 2N4401, 2N3904, BC547, S8050, or a S9013. might help with the circuits in step 6.
PNP; 2N2907, 2N4403, 2N3906, BC557, S8550, or a S9012. might help with the circuits in step 4.

Cure 2.
If changing the transistor in the circuits in step 4 or 5 do not stop the lights from lighting up to early you may need to go to the circuit in step 7 Photo Resistor Circuit. In those circuits you can control the on and off times by changing the value of the 100 k ohm resistor.

That's brilliant - thanks for the advice! So just so I'm clear; altering the 100k resistor is the way to alter the LDR sensitivity, but there's no way to do this with the Solar Panel as the dark sensor, because we're governed by the actual sensitivity of the panel?

Yes you have it.
I have hundreds of transistors and solar ICs so I can play with components but not every one can do that.
Think of the LDR circuit like a voltage comparator circuit and a schmitt trigger all in one. When the voltage at the base of the first transistor reaches one value the transistor is closed and the LED is off. At a different value at the transistors base, the transistor is open and the LED is on. You can also use a NPN or a PNP transistor as the first transistor.

Ah, that's making more sense. My solar panels is a 6v 330 mA from eba y and not that sensitive with the simple circuits. I'll have a go with the ldr designs and see how i get on, as my simple circuit pnp gets a bit warm. Which ldr and circuit would you recommend for the least voltage drop, and also so that the leds only comes on when it's really dark?