One Stop Electrical Reference Centre

If you want to calculate Resistor, please visit Here!

Some good values to try:

As supply voltage:
For molex: 5, 7 and 12 volts
Batteries: 1.5 and 9 volts

As led forward voltages:
Red and green: 2 volts
Blue and white: 3.5 - 4 volts

Led current:
20mA will work for most regular leds.
Superbright leds can go from 30mA up to several amps.


Other CURRENT LIMITING RESISTOR CALCULATOR: FOR LIGHT EMITTING DIODES

The calculators on this page can be used to find current limiting resistors and currents for Light Emitting Diodes.

The first calculator determines the resistance for a desired LED current while the second calculates the current for a given resistance.

Voltages in Volts - Current in Milliamps - Resistance in Ohms

When selecting resistors - It is advisable to choose the next higher standard value.

A browser capable of running Javascript is required for this calculator.

If LED's are connected in series - ADD their voltage drops together to get the total voltage drop.

Entering a zero for the LED voltage drop will yield the resistance and wattage for a resistance only circuit.


Please Go here : Other CURRENT LIMITING RESISTOR CALCULATOR: FOR LIGHT EMITTING DIODES

Here's a simple problem: "How do you make an LED turn on when it gets dark?" You might call it the "nightlight problem," but the same sort of question comes up in a lot of familiar situations-- emergency lights, street lights, silly computer keyboard backlights, and the list goes on.

Solutions? Lots. The time-honored tradition is to use a circuit with a CdS photoresistor, sometimes called a photocell or LDR, for "light-dependent resistor." Photoresistors are reliable and cost about $1 each, but are going away because they contain cadmium, a toxic heavy metal whose use is increasingly regulated.

There are many other solutions as well. Look here for some op-amp based photodetector circuits with LED output, and check out some of the tricks used in well-designed solar garden lights, which include gems like using the solar cell itself as the sensor.

In this article we show how to build a very simple-- perhaps even the simplest-- darkness-activated LED circuit. To our LED and battery we add just three components, which cost less than thirty cents altogether (and much less if you buy in bulk). You can build it in less than five minutes or less (much less with practice)


What can you do with such an inexpensive light-controlled LED circuit? Almost anything really. But, one fun application is to make LED throwies that turn themselves off in the daytime to save power. Throwies normally can last up to two weeks. Adding a light-level switch like this can significantly extend their lifetime.

Here are our components: On top: a CR2032 lithium coin cell (3 V). On the bottom (L-R): the LED, an LTR-4206E phototransistor, a 2N3904 transistor, and a 1 k resistor. This LED is red, blindingly bright at 60 candela, in a 10 mm package. It casts a visible beam, visible for about twenty feet in a well-lit room. We got the LEDs and batteries on eBay, and the other parts are from Digi-Key, but Mouser has them as well. As we mentioned, the last three cost about $0.30 all together, and much less in bulk.

The LTR-4206E is a phototransistor in a 3mm black package. The black package blocks visible light, so it is only sensitive to infrared light-- it sees sunlight and incandescent lights, but not fluorescent or (most) discharge lamps-- it really will come on at night.

Our starting point is the simplest LED circuit: that of the LED throwie, which has an LED driven directly from a 3V lithium coin cell. From this, we add on the phototransistor, which senses the presence of light, and we use its output to control the transistor, which turns the LED on.

The circuit diagram looks like this; please ignore the messy handwriting. ;)

When light falls on the phototransistor, it begins to conduct up to about 1.5 mA, which pulls down the voltage at the lower side of the resistor by 1.5 V, turning off the transistor, which turns off the LED. When it's dark, the transistor is able to conduct about 15 mA through the LED. So, the circuit uses only about 1/10 as much current while the LED is off. One thing to note about this circuit: We're using a red LED. That's because the voltage drop across the transistor allows less than the full 3 V across the LED. The full three volts is really only marginal for driving blue LEDs anyway, so two-point-something really doesn't cut it. (Might be able to work around that with a cheap FET-- haven't tried yet.)

And now, let's build it. You can certainly put this together on a breadboard, but there's something more satisfying about the compact and deployable build that we walk through here.

First get the transistor and the resistor. The pins of the 2N3904 are called (left-to-right) Emitter, Base, Collector, when viewing it from the front such that you can read the writing. We're going to solder the resistor between the leads of the Base and Collector of the transistor. Unusual part: hold the resistor with its leads at 90 degrees to those of the transistor while you solder.




Read more


Stay safe when you do this.
After soldering, clip off the excess resistor lead that is attached to the transistor base (middle pin), as well as the excess length of the collector pin.

Next, we add the phototransistor. Note that it has a flatted side, much like an LED does. This pin on that side is the collector of the phototransistor. Solder the collector (flatted side) to the middle pin (the base) of the transistor, again at 90 degrees. The other pin of the phototransistor, the emitter, is left unconnected for the moment. (Here is an alternate view of what that should look like when you're done.)


Finally, we need to add the LED. To do so, we need to know which side is the "positive," or anode side of the device. Regrettably markings of LEDs are not consistent, so the best way to be sure is to test it with the lithium coin cell-- put the LED across the terminals of the cell and, when it lights up, note which side is touching the (+) terminal. (Usually, it's the one with the longer lead.) Solder the "positive" lead of the LED to the emitter pin of the transistor-- it's the one on the left, which doesn't have anything soldered to it. Trim away the excess lead of the LED that goes past the solder joint. Solder the other pin of the LED (the "negative" pin, or cathode) to the emitter of the phototransistor, the pin on the non-flatted side, which does not have anything connected to it yet.

By this point, there are only two pins sticking down below the components: One that goes to the resistor and collector (rightmost pin) of the transistor, and one that goes to the emitter of the phototransistor and to the cathode of the LED.


To test the circuit, squeeze the coin cell between these two terminals, positive side goes to the lead touching the resistor. You can't see the LED on here because these photos were taken with incandescent lighting-- it wouldn't turn on.

Bending the leads to contact the lithium cell a little more reliably, you can try it out a little more easily. In the photo on the right, I cupped my hand over the circuit-- so the LED turned on.

To make this into an actual "throwie," you still need to add some tape and a magnet, but that's quite easily done. This one makes a pretty good nightlight attached to the top of a doorframe-- when the room lights are off, it shines a bright, bright spot on the ceiling.

Where to go from here? While this little circuit can do something on its own, it would probably also be happy as part of a larger circuit. At a minimum, note that if you work with batteries that have lower internal resistance than the lithium coin cells, you should place an appropriate resistor in series with the battery before trying to operate this circuit-- or else you may put too much current through the LED. Certainly, this is one of the easiest and least expensive ways to control an LED with a photosensor.

Light Emitting Diodes (LEDs) have been around for years in red, yellow and green. New technological advances have given us incredibly bright blue and white versions--the white LEDs on our products page are state-of-the-art in brightness. The rated brightness varies by how wide the beam angle is. LEDs with a super-high brightness rating also have a very narrow beam angle.

Wider-angle LEDs have a lower brightness rating, but may put out just as much light. It's important to choose the beam angle to suit your needs.

• LEDs can last tens of thousands of hours if used at rated current
  
• No annoying flicker like from fluorescents
  
• LEDs are impervious to heat, cold, shock and vibration
  
• No breakable glass is used, and LED lights can be waterproofed for marine use

White LEDs are perfect for replacing small, inefficient incandescent bulbs in night lights, flashlights, path lights, task lights and exit signs. Try 6-9 white LEDs for reading and task lights, and 1-3 LEDs for flashlights and path lights.

Designing LED lighting

DISCLAIMER: None of us here are electronics experts. We've already corrected this page numerous times thanks to real electronics experts who have emailed us. What we'd really like is for a real electronics expert to completely re-write this page for us!

LED ratings are specified by current, not voltage. For longest life, we recommend you run them at 20-25 milliamps (ma). HOWEVER, in our LED flashlight conversions (and many commercial LED flashlights), the LEDs are run at 50-60ma, twice the rated current. One of our test LEDs ran at 98ma for over 200 hours without damage or appreciable light loss. So go ahead and experiment with running them at over rated current if you are willing to take the risk of a shorter life. In my opinion, a flashlight bulb that lasts 100 hours is a huge improvement and cost saver over the incandescent alternative which gives only 15-20 hours before it dies.


You must use some method of limiting current to your strings of LEDs. The easiest is simply using the right number of LEDs for your supply voltage. Each white LED gives a voltage drop of 3.6 volts. So, for a 115 volt DC light, you could use 32 white LEDs in series (115 / 3.6 = 32 +/-) with NO current limiting (they will limit themselves by their inherent voltage drop). In reality, though, there are many other circuit design issues you need to look at to build a reliable 115VAC home LED lighting fixture! We link to a few resources farther down on this page, and you can always Google up 'LED lighting circuits' for more information. Reverse polarity will not damage an LED unless the voltage is very high--it simply will not work, and will not pass current through.


However, be sure to check the manufacturer's rating for the specific LEDs you are using--there are some out there, particularly the latest models, that can be damaged by relatively low reverse voltages. The diagram below shows how the LED package is marked for polarity.


The next easiest is a simple resistor. The resistor does consume power, though, but is usually needed since an 'ideal' 3.6 volt source is rarely available. Use Ohms law

(Resistance(R)=Voltage(E)/Current(I)) to calculate the value and wattage needed: (R=E/I)

Each white LED gives a voltage drop of 3.6 volts. As an example, for a 12 volt light, you can run a maximum of 3 white LEDs in series at full power (3.6 x 3 = 10.8 volts drop). Subtract this from your supply voltage of 12 volts to get the additional voltage that must be dropped (in this case, 12 - 10.8 = 1.2 volts of additional drop needed). In this case, 1.2 volts of additional drop / .025 amps (25 ma) = 48 ohms.

Use the next highest value of resistor available, 50 ohms. You must also be sure the resistor can handle enough current. Volts x Amps = Watts; resistors are rated in watts. So in this case, 1.2 volts x .025 amps = 0.03 watts. A 1/4 watt resistor will work fine, but if you run a second string of 3 LEDs in parallel, each string would need its own 50 ohm resistor. It's important that each string has its own resistor....putting them in parallel with a single resistor is bad practice.

This method is cheap and works great, but there's one problem--voltages in a remote power system (or car, for that matter) tend to vary. In our home system, voltages range from about 12 volts when the batteries are low up to 14 volts when equalizing the battery bank. An LED lamp string designed to run at 25 milliamps at 12 volts would be pushing 64 ma at 14 volts, which would be very bright and PROBABLY last at least a few hundred hours...but then then when your batteries are low, the LEDs will pull only 10ma or so, making them very dim. If you are looking for maximum lifespan (which could be over 10 years of run time) and brightness that doesn't vary with your battery condition, try a voltage regulator circuit (below).

So, we highly recommend a simple voltage regulator chip for the safety of your LEDs. White LEDs are expensive, and it would be a shame to blow them out. Parts for a current-limiting circuit are very cheap--less than $2. Use the Ohm's law calculations above to select the resistor for the voltage you choose. Or, use the regulator in a current-limiting configuration to run the LEDs. You can also use an LM317 adjustable voltage regulator set to the exact current level needed by your strings of LEDs. See the circuit diagrams below.

We originally described using an LM7812 voltage regulator chip for this application, but it presents some problems--they generally won't start regulating until input voltage reaches 13.4v, and they have a 1.4 volt voltage drop, leaving you with under 12 volts at typical RE system voltages. Instead, the LM317 is a better choice, and you can adjust its output to fit your needs. Choose your current-limiting resistors as shown in the diagram below. This protects your LEDs from fluctuating system voltages.

You really need to use a multimeter for any LED circuit design and construction($10). If you have an RE system, you should already own a multimeter! and solderless breadboard ($5) for designing your home built LED fixtures. Both are available at Radio Shack. With the multimeter, you can check your polarity, voltages, resistors, and current draw before assembling the final version of your light by soldering. The breadboard allows you to make changes to the circuit without soldering, and makes it easy to transfer the working circuit to a soldered version--solder-in PC boards are available that exactly match the connections of your solderless breadboard.

- The LEDs Are ON When The Phototransistors Are Dark -(The Outputs Are LOW When The Inputs Are HIGH)

The circuit on this page is for a visible and infrared light detector circuitboard that has 8 detectors. LM339 voltage comparators are the active element. These detectors can be used as part of other light detector circuits shown on other pages at this site such as these Light Activated Detector Circuits at this site.

A) Basic Inverting Detector Circuit

The following diagram shows the basic circuit on the Inverting circuitboard.


Selecting A Value For The Input Resistor (R1)

The value of resistor R1 depends on the type of sensor and the desired sensitivity. See below for more details.

For phototransistors a value of 470K ohms will work for most room light situations. If the light is dim, selecting a higher value resistor such as 1 Megohm will give better sensitivity. This High Impedance Test Voltmeter circuit can also be used for testing phototransistors installations.

For CdS photocells it is usually best to install the cell and then measure its resistance under the normal lighting conditions. A resistor with a value that is 3 to 5 times the measured resistance of the cell is then selected for R1.
Selecting A Value For The Output Resistors (R4)


The value of resistor R4 is chosen to give a desired current flow though the LEDs See below for more details.

A 1K ohm resistor will allow about 10 milliamps to flow through a typical LED if the supply voltage is 12 volts. The value of the resistors at the outputs of the comparators can changed depending on the desired current through the LEDs.


B) 8 - Photo-Detector PCB Circuit

The following diagram shows the circuit that is on the printed circuit board. There are 8 independent photo-detectors with open collector outputs that can sink up to 15 milliamps each.



Circuit Notes

i) All of the comparators on the PCB are wired so that when the photosensors are dark, the output of the comparators will be LOW and the LEDs will be ON.

ii) The detection voltage level for the circuit as shown is set at 1/2 of the supply voltage. If a lower or higher detection level voltage is needed, the values of resistors R9 / R10 and R19 / R20 can be adjusted.

iii)This circuit does not need a regulated power supply and can operate on supply voltages of up to 32 volts.

iv)The 1K output resistors can be replaced by jumper wires if they are not needed such as for inputs to control or signals circuits that have their own current limiting resistors.

v) WARNING - If the polarity of the power supply for this circuit is reversed or the circuit is connected to an AC or DC source this circuit will be damaged. The maximum supply voltage for this circuit is 15 Volts.

Please go here to see the full model!

For this Section, Please read Infrared Light Photo-Detector Circuit First!

Basic Phototransistor Detector

In this circuit the light falling on the phototransistor will be from an Infrared Light Emitting Diode (IrLED) but otherwise it is the same as the phototransistor circuit shown above.

When the light falling on the phototransistor (Q1) is blocked, its conductance will decrease and the voltage across Q1 will rise. When the voltage rises above 1/2 of the supply voltage the output of the comparator will turn ON and the LED will be lit.

Basic Visible and Infrared Light Detectors

This page features basic, visible light photo-detector circuits that can be used to detect trains or other light blocking objects.

The sensors used for these circuits are silicon phototransistors or Cadmium Sulfide (CdS) photocells. Both of these sensors allow less current to flow when they are dark. (Phototransistors change their 'conductance' while photocells change their resistance depending on the intensity of the light falling on them.)

The phototransistor or photocell would normally be placed between the rails in the circuits on this page.

The Photo-detectors on this page use LM339 (Quad) or LM393 (Dual) voltage comparator, integrated circuits to detect the change in voltage across the sensor.

All of the circuits on this page are configured to have the LED's turn on when the sensor element is dark (covered by a train.) The LED's can also be made to turn off when a train is detected. This will be explained in the NOTES sections of this page.

The supply voltage for the circuits is specified as regulated 12 volts DC but this can be changed if needed. In some cases the values of some resistors may have to be adjusted to compensate.

Visible Light Photo-Detector Circuits

A) Basic Phototransistor Detector

In this circuit, when the light falling on the phototransistor (Q1) is blocked, its conductance will decrease and the voltage across Q1 will rise. When the voltage rises above 1/2 of the supply voltage the output of the comparator will turn ON and the LED will be lit.


The only critical part of this circuit is the value of resistor R1 which in most cases can be 470K ohms but may have to be increase if the room is dark or decreased if the room is well lit.



Increasing the value of R1 will cause the sensitivity of the sensor to decrease. This may be necessary when the light falling on the cell is not very strong or shadows can affect the phototransistor.

There are a number of phototransistors sizes and case styles. The smaller cases will be easier to hide but connecting wires may be more difficult.


B) Basic CdS Photocell Detector

In this circuit, when the light falling on the photocell (PC 1) is blocked, its resistance will increase and the voltage across PC 1 will rise. When the voltage rises above 1/2 of the supply voltage the output of the comparator will turn ON and the LED will be lit.

Due to wide variations in CdS photocells it is usually best to install the cell and then measure its resistance under normal lighting conditions. A resistor with a value that is approximately 3 to 5 times the measured resistance of the cell is then selected for R1. For example; If the cell resistance is measured at 400 ohms then a 1200 to 2200 ohms resistor would be used.

Increasing the value of R1 will cause the sensitivity of the sensor to decrease. This may be necessary when the light falling on the cell is not very strong or shadows can affect the photocell.

This circuit can be adapted for use in dark areas by placing a small light above the photocell.

Make your own single or multi phase bridge rectifier from diodes

The electricity generated by most wind turbine generators is alternating current (AC). To use this to charge batteries or power most lighting and devices directly, it must be rectified into direct current (DC).

The simplest bridge rectifier is made up of just four diodes (components which allow electricity to flow in just one direction).

Three Phase Bridge Rectifiers

Typically wind turbines do not generate single phase AC (displayed in the above example), but instead generate multi-phase AC - usually three-phase AC electricity.

Therefore, more diodes are required to rectify the three phases of electricity - in fact six are required and must be wired up as shown in the diagram below.


Why Make Your Own Bridge Rectifier?

It is possible to purchase complete bridge rectifiers cheaply with current ratings from below 1 Amp to as much as 35 or 50 Amps.

To make a three phase bridge rectifier is a simple case of wiring them together and then to each of the three phases of generated electricity.

Pictured above is an example of a 35A bridge rectifier. Bridge rectifiers rated above 35A or 50A suddenly become very expensive. Therefore if your wind turbine has a total maximum output current of more than 25-30 Amps, it is worthwhile making your own bridge rectifier with high power rated diodes wired as shown above but fitted to a suitably large heatsink.

Find out how best to connect batteries together into a battery bank

For any off grid renewable energy system the battery bank is probably the most important component. It doesn't matter how much power you generate - if it is not stored safely and efficiently then you will have no electricity when you need it. Batteries are also one of the most expensive parts of wind, solar, and hydro power generation systems so they need to be well cared for.

Unless you have a very small system you will need more than one battery - therefore you will need to connect the batteries to one another to form a battery bank. Left is an illustration from SmartGuage Electronics showing how this is often done:

The Problem
Because of the small amount of resistance in the cable used to interconnect the batteries, and from the connection between the cable and the battery posts, the battery closest to the installation is charged the most, discharged the most, and worked harder, whereas the battery furthest from the installation is charged the least, discharged the least, and worked the least.

The Reason
The power from the bottom battery has to pass through the main connection leads whereas the power from the top battery has to pass through the main connection leads and another four sets of interconnecting leads. Although the resistances are tiny - it is the fact that they are so small that makes them have such a big effect on the current flowing to each battery.

SmartGuage Electronics used a computer simulation in 1990 to calculate the following assuming a battery internal resistance of 0.02 Ohms, interconnecting lead resistance of 0.0015 Ohms per link, and a total load on the batteries of 100 amps:

• The bottom battery provides 35.9 amps.
• The next battery up provides 26.2 amps.
• The next battery up provides 20.4 amps.
• The top battery provides 17.8 amps.
  

...which means the battery closest to the installation is worked twice as hard as the battery at the top of the battery bank! These surprising findings have since been reproduced in real world situations.

Connecting Batteries in a Battery Bank
So it the example given above shows you how NOT to connect batteries to make a battery bank, how should you do it? It is actually very simple - instead of taking the negative AND positive feeds from the same battery (in the example above it was from the bottom battery) , you should take one feed from each end of the interconnected battery bank - e.g. +ve from the top battery and -ve from the bottom battery. See the image from SmartGuage Electronics..

With the same example load of 100 amps presented above the new loads on each battery are as follows:

• The bottom battery provides 26.7 amps.
• The next battery up provides 23.2 amps.
• The next battery up provides 23.2 amps.
• The top battery provides 26.7 amps.
  

To get the batteries perfectly balanced requires a different scheme involving a little more work and expense (more cables and connections required), but is only really necessary if you have very expensive batteries or a more than 6 or so batteries in your bank.

Warning: Be Safe When Handling Lead Acid Batteries
When handling lead acid batteries, great care must be taken. You should always wear gloves and safety goggles because if acid sprays or spills from a battery onto your skin or eyes you could sustain a serious and permanent injury. Invest in a bottle of surgical eye wash and leave it next to your battery bank at all times so you can flush your eyes immediately if you get acid in your eye.

Find out more about bridge rectifiers.

For most alternative energy applications, we require a direct current (DC) voltage to be generated - for example to charge a bank of batteries. However wind turbines and wave power generators create an alternating current (AC) voltage.

This is where the Bridge Rectifier comes in. The AC voltage generated is passed through a circuit of four diodes arranged as shown below and emerged converted into a more useful DC output.

Diodes allow electricity to flow in only one direction, but there is a small voltage lost across the a diode of 0.7V called the forward voltage drop.

If the diode is wired in the wrong direction then no current (actually a very tiny current) flows across the diode. However, if the voltage is too high and goes over the diode's maximum reverse voltage, the diode will breakdown and fail.

Every year electricity gets more and more expensive. Read on to find out how you can save electricity and money with this handy selection of energy saving tips.

Saving Electricity Tips
The biggest electricity users in the home tend to be heating and/or air conditioning, water heating, washing machines, tumble dryers, dishwashers, lighting, and the refrigerator. When looking to save electricity, spend the most time focussing on these big energy hogs to get the biggest effect for your efforts.

Here are a selection of tips to help you to reduce your electricity consumption:

1. Turn off your television, video, hifi, playstation, and other entertainment devices when they are not being used.

2. Do not leave your television etc in standby mode. Devices can use up to 90% as much power in standby mode as when they are on, so it is a serious waste of energy when a device is left constantly on standby. If you keep forgetting, consider purchasing a PowerSafer - a device which automatically cuts power to appliances when they go into standby mode.

3. Replace all of your inefficient incandescent light bulbs with energy efficient CFL bulbs. Replace halogen spotlights with much more efficient and longer lasting LED Spotlights.

4. Hang your clothes out to dry rather than using an electric tumble dryer. Ideally use a spin dryer before using the tumble dryer.

5. Cook many items at the same time when your electric oven is hot.

6. Use a microwave to reheat food or to cook small portions. Although a microwave uses a lot of power, it does so over a very short time and so saves energy overall.

7. Turn down your heating system thermostat. For every degree you lower your heat between 60° and 70° F you can reduce your heating bill by up to 5%. Wear an extra layer of clothing in the house so that you stay warm. Turn down individual radiators - for example, 16°-18° is warm enough for bedrooms whereas 20°-22°C is more comfortable in bathrooms. Rooms that are rarely used can have their heating turned all the way down or off.

8. Purchase energy efficient white goods (washing machines, tumble driers, fridges etc). Although they usually cost a little more initially, the cost savings in electricity will cover that many times over. As an added benefit, efficient items are usually better made and last longer than inefficient models.

9. Vacuum clean the condenser coils at the back or underneath your fridge freezer. Accumulated dust reduces their efficiency by up to 25% adding that cost to your electricity bill.

10. Keep your fridge full, but not so full that air cannot circulate properly.

11. Fold clothes straight out of the tumble drier while they are still warm to save on ironing.

12. Cool cooked food before you put it into the fridge.

13. Do not put uncovered liquids into the fridge. Their evaporation will make the fridge have to work harder.

14. Heat only as much water as you require for drinks and cooking. If you keep forgetting, purchase an energy efficient eco kettle.

15. Use a convection oven. A small fan inside circulates hot air throughout the oven cutting cooking times by up to 30%.

16. Don't preheat the oven for roasting.

17. Don't keep opening the oven door. Every time you do so, your oven loses 20°C of heat.

18. Put lamps in the corner of a room so that the light is reflected off two walls.

19. Turn down the temperature on your washing machine. Heating the water uses the majority of the electricity, so by doing a warm wash instead of a hot wash, big savings are possible. See Wash Most Clothes at 30 Degrees.

20. Defrost frozen food in the fridge since this helps to cool the fridge.

21. Running a full load in an efficient dishwasher will use less hot water than washing up by hand in the sink! Save money, save time, and save electricity.

22. Boil water in a kettle rather than on a hob to save 50-70% of the energy and to get your water boiled faster.

23. In the summer use ceiling fans on a fast setting instead of air conditioning to keep cool. In the winter, running the fans slowly will push warm air collected at ceiling height down to where you want it. (If the slowest setting on your fan is too strong, reverse the direction of the fan in the winter so that the accumulated warm air is blown up against the ceiling and bounces more gently down around the walls and into the living space.

Make a simple solar charger for 4 AA rechargeable batteries

In this article you will find out how to make a very simple solar battery charger for 4 AA rechargeable batteries using a small 6 Volt solar panel. A labeled photograph of the completed charger is provided at the end of the article.

Here is the parts list:

• 1 of 4 x 2700mah AA Rechargeable Batteries.
• 1 of 6V 250ma Solar Panel.
• 2 of 2 AA Battery Holder with Flying Leads.
• 1 of Blocking Diode.
  

The Solar Battery Charger Specifications
The long life rechargeable batteries used have a capacity of 2,700mah - therefore a charging current of 10% of this (i.e. 270ma) is safe. The nominal voltage of each battery is 1.2V, so four in series is 4.8V with a fully charged voltage of around 5.2V being normal. Therefore our 6 Volt 250ma solar panel is perfectly rated to be used as a charger for these batteries.

In order to prevent stored power in the batteries being released through the solar panel during the night, a blocking diode is used. Placed in the postive line from the solar panel this only allows electricity to flow from the solar panel to the batteries and not from the batteries to the solar panel.

Up to 0.7 Volts from the solar panel are lost as heat in the diode as electricity flows from the panel to the battery leaving us with a perfect charging voltage of around 5.30 Volts for the batteries.

Putting the Solar Battery Charger Together
To keep things simple no soldering is required to built this solar battery charger. Instead terminal strip is used to make the connections - only a small flat head screwdriver is required to secure each wire/component in place.

First of all the four batteries are put into the two battery holders and then the holders are wired together in series. To do this the positive lead from one battery holder is connected to the negative from the other. The remaining free wires - one positive and one negative - are our charging inputs.

The ringed end of the blocking diode (pictured above) is then connected to the positive battery input (red), and the other end to the positive output from the solar panel (red). The negative output from the solar panel (blue) is connected directly to the negative battery input (black) .

Pictured above is the completed Solar Battery Charger. The voltage measured across the points labeled B and C is the voltage coming in from the solar panel.

The voltage measured across the points labeled B and A is the voltage of the batteries. (Note that during charging the battery voltage measured will be higher than the true voltage of the batteries. Cover the solar panel to measure the true battery voltage.)

A suitable multimeter is required to make these measurements.

Monitor battery status with this easy electric circuit project - no skill required!

It is essential that the battery bank in a renewable energy system is well looked after. This means that the voltage of the battery bank must be known. It can be easily measured with a multimeter or a voltmeter, however a fun and inexpensive project is to make a very simple battery status monitor.

In a typical 12V system the voltage of the battery bank can fluctuate from 10.6 Volts (below this the battery is very dead) when heavily depleted and under load, to as much as 15 Volts when being heavily charged. A healthy full 12V battery bank would usually have a voltage of around 12.6 Volts when not under a load and between 13-14 Volts when being charged correctly - i.e. not too quickly. Therefore it would be interesting to have an indication of the status of the battery bank using an LED to show if it is being charged.

For this example we will use the arbitrary figure of 12.6 Volts to indicate battery bank under charge however this could value could be set lower or higher according to your own needs and system configuration.

The Battery Status Monitor Introduction
The status monitor uses a Zener Diode, a Light Emitting Diode (LED), and a Resistor - components which can be bought for pennies each.

Each Zener Diode has a specified Zener voltage. If the voltage in the circuit is greater than the Zener voltage, then the voltage drop (ie. the voltage reading across the diode) is equal to this Zener voltage. However, if the voltage in the circuit is less than the Zener voltage then no current flows. Therefore, if you put a Zener diode in series with an LED in a circuit, the LED will light if the circuit voltage is greater than the Zener voltage plus the voltage drop across the LED.

Making the Battery Status Monitor
We want our LED to light when the voltage of the 12V battery bank is 12.6 Volts or higher to indicate the batteries are being charged by our renewable energy set-up. Let's use the following components:

1 x 8.2 Volt Zener diode.
1 x standard green LED (Specfications: maximum current 30mA, voltage drop 2.5 V).

In order to make the LED last as long as possible we will not use the maximum current - instead we will aim for around 15mA.

The total voltage drop across the Zener diode and LED will be 8.2+2.5=10.7 Volts. A resistor is therefore required to prevent too much current getting to the LED and destroying it. The difference between the battery bank target voltage of 12.6 Volts, and the voltage dropped by the Zener and LED of 10.7 Volts is 1.9 Volts. These 1.9 Volts must be dropped across a suitable resisitor with a current of no more than 15mA - therefore using Ohm's Law we find that resistance = 1.9 Volts / 0.015 Amps = 127 Ohms coincidently the exact value of a manufactured resistor. Normally you would select the resistor with the nearest value above the resistance value calculated with Ohm's Law.

Therefore add to the parts list for this project:
1 x 127 Ohm Resisitor.

Choosing Correctly Power Rated Components
We now need to check the power dissipated in the Zener diode and resistor at different voltages so we can select suitably rated components, and check that the current flowing through the LED when the battery bank is at its maximum voltage is below the manufacturer recommended maximum.

If the battery bank were to hit 15.5 Volts the voltage drop across the Zener diode would remain fixed at 8.2 Volts and the drop across the LED at 2.5 Volts, therefore the voltage drop across the resistor would increase to 15.5-8.2-2.5=4.8 Volts. Again using Ohm's Law we find that the current through the resistor (and therefore through the LED) will rise to 4.8 Volts / 127 Ohms = 38mA. This is a little over the manufacturer recommended maximum current however, if your 12 Volt battery bank is regularly at 15.5 Volts the expense of replacing your little LED a couple of years early will pale into insignificance compared to the cost of the damage to your battery bank!

At a current of 38mA the power dissipated by the Zener diode will be 8.2 Volts * 0.038 = 0.31 Watts, and the power dissipated in the resistor will be 4.8 Volts * 0.038 = 0.18 Watts. Therefore a 500mW rated 8.2V Zener diode and a 1/4 Watt 127 Ohm resistor will be perfect.

Putting the Battery Status Monitor Together
There is no need to worry about soldering this together - the legs of the LED, resistor, and Zener diode can simply be twisted together.

The last item you will need is:
1 x Length of One Amp bell wire (split along its length into two pieces of wire).
..which is very cheap - or you can cannibalise any other single-core insulated wire you have lying around. With a maximum current of 40mA flowing through it the wire does not need to be thick, but it is best to always use insulated wire to prevent accidently short-circuiting the battery bank.

The short leg on the LED (pictured above) is the cathode and is connected with the wire to the negative terminal of the battery.

The cathode of the Zener diode (pictured above) is marked with a stripe (silver in this example), however Zener diodes are placed into circuits in reverse, so next connect the positive anode (long leg) of the LED to the positive (no stripe) end of the Zener diode.
Finally the cathode of the Zener diode is connected to one leg of the resistor, and the other leg of the resisitor connected to the positive terminal of the battery bank with wire.

The green LED of the completed simple battery bank status monitor will now remain lit as long as the battery bank voltage stays above 12.6 Volts.

Developing the Battery Status Monitor Further
This battery status monitor could not be more basic - it just indicates when the batteries have 12.6 Volts or more. However it would be very simple to extend the monitor to include an over-charge warning indicator when the voltage reaches say 14 Volts, and a battery healthy indicator which is lit as long as the voltage is over say 11.8 volts.

Simply recalculate the values of resistor and Zener diode required for each additional monitor and join each string of components in parallel to the battery to be monitored. The whole thing can be soldered together and built into a suitable box with the LEDs labelled so that everything looks tidy and you have a project to be proud of!