Introduction: Kid's Green Tech - Solar Garden Lamp Kit

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The Kit:

This instructable is based on a kids science project that I am doing in a few days time with my daughter and a few of her friends.  It's simple and fairly quick to do so although it does involve some soldering I don't expect that we will have trouble doing this with our small group of carefully supervised 7-year-olds.

The idea for this project was inspired by this article over at Evil Mad Scientist although the circuit is modified to be a little more efficient.  Because it's made up from a simple set of parts it would make an ideal beginner's electronics kit.  I'm therefore putting this forward into the "kit" challenge as well as the education comp'.

This kit could be used for an introduction to science and electronics from ages of about 7 upwards.  It requires some simple through-hole soldering and employs batteries and a solar pannel, which might be at the level of a young junior school child.  However, it also uses a diode, which is a slightly more sophisticated concept, both NPN and PNP transistors and so could be the basis of an introduction to transistors and it uses a blocking oscillator in the "Joule Thief" part of the circuit which could be used to illustrate inductance at a higher level again.

There are many learning points at different levels from this kit:

At the lowest age group (say 6-9 years) I'm using this as an introduction to the concept of circuits, to soldering and to very basic components (see final step).  I'm not formally a teacher but I have been running young kids science projects for a few years now and they tend to be well received.

At a somewhat older age (say 10-14), this could be an introduction to discrete semiconductors - to diodes and transistors.  Towards the top of this group one might discuss NPN vs PNP transistors at least from a functional point of view.

At an advanced stage (sat 15+), this could be used to illustrate advanced electronic concepts such as inductance and the correlation between electric and magnetic fields.  A more theoretical consideration of semiconductor doping and the function of diodes, LEDs and NPN vs PNP transistors would also be suitable.  The LED and low Vf diode in fact allow discussion of band gaps and some quantum mechanics if appropriate.

Green Tech:

This is a nice, sustainable-energy kit, with all the power for the lamp being sourced from renewable solar energy.  It uses scavenged jam-jars as the enclosure but could also use some scavenged parts such as toroids from old CFLs


This is a small simple and low-priced kit with a wide range of learning points.  It's green, it looks cute and works at the end of it!

Edit March 2013:
I've been working with Joshua Zimmerman over at to make these little boards available through his site.  Leave a message if you're interested, or pop over there and see if he has them up yet.


Step 1: Kit and Parts

The parts that you will need are:


5V 70 mA Solar pannel (around 60x60mm)
Twin AAA-size battery holder
2 AAA-size NiMH rechargable batteries (around 1000 mAh works well)
Circuit board (see last step for Eagle files)
2N3906 general purpose PNP transistor (or equivalent)
2N3904 general purpose NPN transistor (or equivalent)
1N5817 low forward voltage schottky diode (general purpose - e.g. 1N914 - diode would probably work)
Slide switch
Ferrite bead/toroid (scavenge from an old compact fluorescent lamp if you only need a few)
LED (high brightness - diffused ideally but I only had water-clear so scratch it up with sandpaper)
22K resistor*
4K7 resistor*
1K resistor
1nf ceramic capacitor (some parts of this 'ible refer to a 2n2.  Either seems to work fine)
30 cm 22-guage solid copper wire (from an old ethernet or telephone cable works well)

Optional items::

Old (empty) jam or pickle jar to house your circuit (we will assume you are using this).
Sparkly things (e.g. acrylic jewels) for the bottom of the jar (makes it look pretty)
Glass paints (could be included in kit)
Small double-sided sticky pad (optional but useful)


Soldering iron & solder
Drill press or punch
Hot glue gun & glue (epoxy would be fine but slower).  I use low-temp hot glue with the kids.
Metal file
A little tape to hold things in place
medium grade sandpaper (tiny bit)
Helping-hands type tool also very useful

* These resistors may need adjusting depending upon the performance of your solar cell and LED.
The 4K7 and 22K make a voltage divider that controls the light level at which your LED comes on.  Increase or leave out the 22K for darkest switch-point.  Decrease the to switch on when it's lighter.  But be careful - depending on your solar cell you may need a pull-down to make the PNP switch on fully.  A 100K trim-pot would probably work well if you wanted to control this.

Step 2: The Circuit

As indicated, the circuit was inspired by this article at Evil Mad Scientist.  Thanks to Windell et al. for that.

The schematic is shown in the picture.

Essentially, the circuit can be divided into the charging part to the left, the light sensing part in the middle and the LED lighting part on the right.

During the day, the voltage across the solar cell is high and current flows through the diode to charge the NiMH battery.  Charging at up to C/10h amps (where C is the capacity of the battery in amp-hours) is supposedly safe for continuous trickle-charge.  So with 1000 mAh batteries we should be able to handle 100 mA.  Our 70 mA solar cell in practice generates 50-55 mA in UK direct summer sunlight so we are safe by a factor of 2 there - pretty much ideal for fairly quick charging but keeping the battery pack in good condition.

When it gets dark, the voltage across the panel drops.  This can consume significant current from the battery (so-called "dark current", which sounds like the evil side of the force to me).  Hence the diode.  I have used a low vF diode to reduce how much of or energy we burn getting past it.  We can tap into this voltage drop to turn on the light when it gets dark.  That's where the PNP transistor comes in. 

By making a voltage divider between the solar panel and ground and attaching this to the base of the PNP, we sink a very small emitter-base current when the solar panel stops pulling a voltage.  This allows a larger emitter-collector current to flow.  The voltage divider between the solar cell and ground can control the switch-point voltage and thus the light level at which our lamp comes on.

Once our PNP turns on, a current flows to the lamp circuit on the right of the diagram (and board).

From here we have a "joule thief" circuit for the LED light.  Explanation of this is rather beyond this summary but, once again, Evil Mad Scientist comes to our rescue: see here for a great Joule-thief article and here on Wikipedia for a more in-depth explanation.  The overall effect is that we light a 3V white LED from a 2.4 V rechargeable battery and can continue to use the battery as its voltage drops.  The capacitor is not an essential part of the circuit but it's great for efficiency.  Without it I was finding 100mA being drawn from the battery!  With a 1nf capacitor that drops to around 18mA but the LED is just as bright.

Finally, the switch isolates the joule-thief part so that we can continue to charge the battery but have the lamp turned off.  If you turn this off then the 5-10 mA that are generated in the shade might just allow you to charge the battery in the winter to give you light about one night a week!

Step 3: Add the Panel to the Jar Lid

As a first step, we need to attach the solar panel to the lid of the jar and pass the connecting leads through.  We want to do this in a way that will seal the hole so that we can leave our lamp outside without it filling with water or bugs!

We are starting by putting a hole in the jar lid.  To do this, I'm using a small drill-press but that's as much to get small girls comfortable with using power-tools as for any real need.  A punch into a block of wood would work well too, I'm sure.  However you do it, you'll want to clean up the hole with a file and thread the wires from the solar panel through.

Next, cover the solder points on the panel with hot-glue or epoxy and then glue the panel to the jar.  I'm using blue-sparkly glue so that you can see it but normal glue is fine!

Finally, make sure you fill the hole with glue to keep out those bugs!

Step 4: Lay Down Some Components.

Next, we want to start populating the board.*

Since my group will be taking turns at the solder station we'll do several components at a time.  On your own you might prefer to add them individually:

The resistors are easiest and can go in first - either way round.  Bend the legs out a little to hold them.  I am not using the 22K resistor to ground but if you include it then your light will come on at slightly higher light levels.

The diode is equally easy but needs to go with the stripe at the end shown on the board.

Then add the two transistors.  The PNP (marked 3906) goes to the top left and the NPN (marked 3904) goes to the bottom right.  Make sure the case goes the same way around as marked on the board (flat edge towards the bottom).

Finally for this step, add the LED.  You can leave as little or as much lead length as you wish but the longer lead (positive / anode side) goes nearest the right hand edge of the board.  I was expecting that to be marked on the boards but it didn't come out.  It's on the current version.

Now, for each component, carefully solder the leads to the bottom side of the board and clip them close with side-cutters.

*Throughout this 'ible, the pictures of the board are of my first "proof of concept" board which had a track missing (long story) and lacked the 1 nf capacitor.  The final board design is shown in a later step and is very similar but I haven't actually had them fab'ed yet.

Step 5: The Toroidal Transformer

The Joule Thief part of the circuit requires a small hand-wound toroidal transformer that we will make and add in this step.

I'm using ferrite beads around 9.8mm wide by 7.5mm deep with an 6.5mm diameter hole.  Whatever the size you use, you'll want enough wire for 6-8 turns.  For beads the size of mine, take about 20-30 cm of a pair of insulated 22-gauge solid copper wire (I use wire from an old 3-pair telephone cable).  Contrasting colours make life easier.  Push the wires through your torus leaving around an inch (2.5 cm) sticking out at one end.  Now loop the long ends round until you have made 6-8 loops spread evenly round your bead.  My beads are pretty much full after 8 turns of this wire.

I have made a few joule thieves and in my experience the ferrite bead is the most likely part to cause a problem.  Some types of beads work and some don't and I have not yet divised a way to tell before trying them.

Cut down the leads to an inch at most (say 2cm-ish) and strip the ends.  At this point it's handy to use a small sticky-pad to hold the torus in place.

Now take a wire of one colour from one end of the torus and the other colour from the other end and put them into holes 1 and 2.  The other ends go into holes 3 and 4 so that the hole in the torus now points across the board.  It should fall naturally so that the wires connect from holes 1 to 4 and 2 to 3, but check or it won't work!  Bend the wires out a little to hold them, turn the board over and solder it.

Step 6: Power Connections

All that is left to attach is the power switch, the battery and the solar cell.  These go in the marked spots towards the edges of the board.

Place the switch in its holes and hold in place with a little tape.  Turn over the board and solder.  The switch has much more thermal capacity than anything else we have soldered in this project so the solder tabs will take a moment to heat up - don't panic!

Same with the two power sources:  Red to the + terminal, black to the -, tape in place and solder.

You now have the complete circuit.  If you insert charged batteries and cover the solar cell you should see the LED light up.

Step 7: Final Assembly

Finally, hot-glue the board to the back of the battery holder with the LED pointing as you wish.  For a very wide-necked jar you could glue the battery holder flat to the underside of the lid and leave the LED sticking "up" (really down) from the board (not pictured). 

For most jars you will have to bend the LED past the end of the battery holder and glue the end of the holder to the lid of the jar (as shown).

If you used a water-clear LED you may wish to scratch it up with some medium grade sandpaper at this point to diffuse the light a little.

You can put some acrylic jems, pieces of metal, shiny plastic or glass (or indeed 10 carat flawless diamonds if you wish) into the bottom of the jar to scatter some of the light and give a pleasant effect.  Once they are inside, screw up the jar.

Finally, take some glass paints and paint a stained-glass effect onto the jar.  Or have your 6-year old do it.

A day of full UK sunshine should provide mor than enough charge for one night's light, and a full battery should hold enough charge for several nights, so in summer you might keep alight every night.  In winter that's not so likely, at least in the UK.  There is a surprising difference between the charge developed in shade (5-10 mA) and in full sun (50 mA+) so find a sunny spot if you can.

You now have a pretty, self-charging, LED garden light.

Step 8: Afterword

The Kit:

I am attaching to this step Eagle schematics and board files for the small PCB that underlies this project.  I have not had this final version fabricated yet but it is very similar to the prototype version (which works with the errors fixed) and the known errors with that are fixed in this version.  I am confident that this one works correctly.

The board is sized to be around the width of a AAA battery holder and has the components as widely spread as realistic for comfortable soldering.  At around 1" x 1.5", it's still a pretty compact little board and makes a compact kit, with the solar panel as the largest part.

I have not priced the components very carefully but as an approx guide:

£3.50 - 5V 70 mA Solar pannel
£0.50 - Twin AAA-size battery holder
£1.30 - 2 AAA-size NiMH rechargable batteries (1000 mAh unbranded)
£0.50 - Circuit board (based on getting 8 circuits on each of 10 (80 x 100mm outline) boards and snapping them apart)
£0.15 - 2N3906 general purpose PNP transistor
£0.15 - 2N3904 general purpose NPN transistor
£0.20 - 1N5817 low forward voltage schottky diode
£0.20 - Slide switch
£0. 50 - Ferrite bead/toroid
£0.10 - LED
£0.01 - 10K resistor*
£0.01 - 4K7 resistor*
£0.01 - 1K resistor
£0.03 - 1nf ceramic capacitor*
£0.01 - 30 cm 22-guage solid copper wire (from an old ethernet or telephone cable works well)

Total - £7.17 assuming jar & paints not included.

That's £7 to within the error of the calculation, with most parts purchased off e-bay in an amount to make 10 kits (but for 80 PCBs and resistors/caps from bitsbox).  Bulk cost would no-doubt be considerably less. 

Half of the cost is in the solar panel but you need a decent panel to make this work well.  Cheap commercial garden lamps have rubbish solar panels and are pretty useless.  Half of the remaining cost goes on the batteries.  I didn't buy the cheapest because I have been stung by cheap rechargeables before.  I bought unbranded NiMHs of high but not unbelievable capacity from an e-bay top rated seller.  They worked very well.  If you were making these kits in bulk then savings on the solar panel and batteries are clearly possible.


I have added a pdf of the current version of the worksheet that I am making to use with the kids in constructing our lamps.  It goes through the idea of a circuit and discusses batteries and solar power a little.  It does follow on from two previous workshops that I have done with them, making wind turbines and making bug-bots and so there are a few "do you remember" type points that you will not remember!

You will also note if you read the steps in the worksheet that it involves adding a couple of components in odd places.  This is because I had to order the boards based on only a proof-of-concept circuit in order to have them ready in time.  The instructions in this instructable are up-to-date and would apply to the final version of the boards shown in this step.

I hope you like the kit and that it's useful in bringing some practical science to more groups of kids.


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