Easy Electronics DIY Projects

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... and better-suited to each of us!

Monday, February 28, 2011

EASY MODS for the LED-strip Lamp

It is heartening to note that the LED strip lamp project has struck a positive chord with many hobbyists. Surely when compared with the few commercial products available at a steep price, the project is attractive both from the cost angle as well as from the ultimate saving one will make by way of reduced power bills.

Many had requested  the incorporation of a 'Hi-Lo' switching, so that the lamp could be kept on for dim 'mood' lighting and then switched to full brightness. I too thought this would be great, especially when  it is used as a 'wall-washer' or 'ceiling-washer' lamp above the TV.

Keeping to our motto of 'simple and easy', we will look at a solution. You might recall that 0.47 uF cap gives you about 47 mA current. Suppose you want to cut the current/brightness to half, go for a cap of 0.22 uF/400V, which gives you 22 mA. Please note that brightness and current do not have a one-to-one relation; the best way is to try various caps and then decide on "your brightness" and then wire that value in.

Incidentally, caps come in standard values of 0.47uF, 0.33uF, 0.22uF and 0.1uF.

Now for the Hi-Lo switching. Hunt around the electronics market and you are likely to get a mains-rated switch, either a rocker or more likely a slider, with a 'centre-off' position. If you do, consider yourself lucky and get that. Replace the original on/off switch with the new one. Look at the figure (A) and modify the wiring, and you are in business in no time!

Suppose you dont get a 'centre-off' switch? Go for a small rocker or toggle on/off switch, again rated for mains voltages. But this time you have to drill another hole or a slot near your on/off switch to mount this hi-lo switch. Wire it up looking at the diagram here (B) so that when the switch is closed, it adds a larger cap in parallel to the low-value one in circuit, which determines the low brightness. Please note that caps in parallel add up in value and so 0.1uF and 0.47uF will give you 0.57uF, pushing up the current to 57 mA.

The values I have suggested are typical ones. Surely they need some 'tweaking' and individual requirements will vary, depending on where you mount the lamp, the reflectivity of your walls/ceiling etc and your own 'level' of preferred low brightness. In the event that even with a 0.1 uF Low cap, you find that the lamp is bright, buy one more 0.1 uF of the same sort and put them in series. This will give you 0.05 uF and about 5 mA of current, but will push up the voltage rating of the 'total cap' to double that of the individual cap- a good ploy to prevent transient high voltages from 'eating' your cap.


I anticipate 'control freaks' going in for a multi-way switch and playing with a number of caps so that they  will have brightness levels that could be "dialled in" to suit any mood! Bravo to them, with the caveat that any switch you use should be capable of safely handling mains voltages.

Gentlemen, start switching...

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Sunday, February 27, 2011

EASY LED-STRIP LAMP


Soaring electricity charges and eco concerns have made many of us extra conscious of the need to use less energy. Of late we have been hearing a lot about the economy and long life of LED lighting, though they were often an expensive solution. The familiar Light Emitting Diodes are totally solid-state devices like the transistor which emit light when a low voltage is applied to them. They, unlike the incandescent bulbs, dont have any hot filament inside them to burn out or need high voltages like the CF Lamps, or their larger cousins, the common "tube lights".

Today a variety of LEDs in all colours, including blue, (which had evaded scientists' efforts till recently) have flooded the market. Popular are the white "hi-brite" LEDs which output usably bright light at a very low current of just a few tens of milliamperes. These days on the roads you get to see LEDs on a lot of cars and bikes as fancy decoration, and today you can buy/build LED replacements for brake lights, car interior lights etc, though high power LED headlights still are prohibitively costly.

Looking at what can be accomplished using LEDs available in the hobby market at average prices opens up various possibilities. In most homes our need is for general low-intensity lighting for most of the time. When we move to specific tasks, say like, cooking, eating, reading etc, we can switch to brighter "task lights". If you look at an LED you can see that they 'beam' their light in a narrow angle, and side scatter is very low. This property comes in very useful when you consider tasks such as viewing TV or working on the computer , where the light should not spill onto the screen, yet the keyboard should be clearly visible. The low power of the LED makes it easy to run it from the computer UPS so that you are not plunged into darkness the moment power fails. (I am not forgetting the extra bright CRT computer monitors that were very good "emergency lights", though they "ate up" the UPS battery fast.)  Today it is an easy matter to "cobble together" a quite good mains LED strip light at a reasonable cost and with a minimum of effort. We shall look at various "options" also to suit your individual requirements.

As I said earlier, LEDs work at very low voltages-- a couple of AA cells in series giving about 3 Volts or a Lithium "button cell"  will light up virtually any LED. A word of warning though--for 'testing' an LED it is okay to very briefly touch an LED's leads to the battery, but dont keep it lit without putting a small resistor in line with the LEDs, or the excess current will kill it in no time. This being a "hands-on" column, we will look at how to do things and look at the "theory" part later. I assume you have at least a small soldering iron (and good quality solder, and also that you are good at soldering well!) and other hand tools like scredrivers, pliers etc.

HANDS-ON

Now for the shopping list. LEDs first. Strips of pre-wired LEDs on a flexible printed circuit, complete with series resistors and an adhesive backing, are available now in most hobby electronics shops. This is the kind you see often on cars and bikes. Two varieties were available when I checked last--a plain strip and another with a water-proof gel coating. For ordinary use inside rooms, we need just the plain, cheaper variety. A 50 cm length would be ideal as it can be easily built to look like a 2-feet 'tube light'. Two lengths of 50 cm strips side by side will give you twice the brightness. So buy half a meter or one meter as your budget permits. Check to see that you are getting the bright, white or warm white (slightly yellowish, like Sodium vapour lamps) LED strips--at the shop they will test it for you after cutting a length. Buy four mains rectifier diodes of the 1N 4000 series, a couple of resistors of 470k or 560 k value and a good quality mini-rocker mains switch.

Now select a few good quality high voltage capacitors. Buy dependable brands and dont scrimp here --these are critical components that come under a lot of stress and we dont want them to break down easily and vapourize everything. Minimum voltage rating is 400 Volts, though it will be great if you could get 600 V. But generally they become larger the higher the voltage rating gets. Dont buy cheapos here. The values to go for are a couple of 0.47 uF and a 0.68 uF. While you are at the shop, get some lengths of plastic sleeving too for covering the "hot" (high voltage) leads for insulating them.


Now for a housing for your tube light. It may not be easy to get spare plastic housings made originally for fluoresent strip lights--if you can beg, borrow or steal (!), they are ideal. ( I was lucky to have an old friend, 'Wintek' Suresh, hobbyist and manufacturer of audio and other hobby kits, fluorescent lamps etc, who very kindly gave me a few housings for CFLs complete with diffusers--this contributed to the 'pro' finish of the project. Thank you, Suresh!)

 If not, walk into any electrical store and ask for a length of " cap and casing" wiring conduit, which is a click together, two-part plastic channel for household wiring. While there buy some good two-core wire and a two-pin plug for your lamp. Now you are all set to begin.
Examine the flexi strip of LEDs. The ones I looked at have three LEDs and a resistor wired together in series between the marked + and - prints. All the sets of three LEDs plus resistor are connected in parallel to the + and - tracks. Look at Fig 1 to get an idea. The black rectangular things are the resistors and they will have a value printed on them; 151 means 150 Ohms (that is 1, 5 and one zero), 221 is 220 Ohms and 331 is 330 Ohms. The lower values will give you higher currents and greater brightness. So check before buying if you have a choice. Otherwise dont worry too much. LEDs are polarized devices, though unlike a transistor, they will not be damaged if you reverse the connections, provided the voltage is low.

The circuit is extremely simple. But first, a serious word of WARNING. Whenever working with high mains voltages, give priority to safety, as otherwise it can be dangerous. Though the LEDs work at a low voltage, the circuit is "hot" and carries high voltages and are dangerous. Pay particular attention to insulating hot leads, covering everything in an insulating plastic enclosure etc.

{{{ EXTREME CAUTION IS ADVOCATED AND STRESSED!! }}}

If you want to light an LED you have to put a resistor in series to limit the current and bring it down to its working voltage and current. Suppose you are lighting an LED from a vehicle battery of 6V of 12V, you need only a small resistor in series. But when you are applying 230V AC (which will go nearly as high as 230 x 1.4 times at its peak) you will need to put a huge series resistor. But resistors dissipate energy as heat and this is wasted power. We are lucky we have AC mains; we can use the property of capacitors that offer a "resistance" to the flow of alternating current in order to do the work of a resistor without wasting energy as heat.

What you see in the circuit after the mains plug and the switch is the "dropper" capacitor, with a "bleeder" resistor in parallel, so that whatever 'charge' is held in the capacitor is bled away to protect the user if he touches the plug leads after it is pulled out. A higher value cap will give you higher current and higher brightness--but dont think of raising the value higher and higher to get fantastic brightness!  There are other factors involved here and so we have to work within limits.

Now wire up the four diodes into a bridge rectifier--careful about the diode polarities and be sure not to overheat them. Good soldering requires that you practice heating the components to be soldered and applying the solder, all in one brief instant. Do not attempt "painting" with hot solder as it will not give you reliable connections. Cleaning and tinning the individual leads earlier will make soldering easier.

The four-diode "bridge" rectifier converts the AC into DC; watch the polarity of the output, preferably connecting a red wire to the + and a black/blue wire to the -. Note that we are not using a filter capacitor here at the output, and so we get pulsing DC here, as we want to take advantage of the peak pulsing currents at the output to drive the LEDs.

Now make cutouts on the side of the plastic channel for the switch as also small holes for taking out the wires to the LED strips, taking out the mains wire to the plug etc. If the mains switch is a two-pole model, it will, when off, isolate both the live and neutral wires.  But dont worry if the switch is a single-pole unit, as we will insulate everything safely.We will also seal the ends of the channels with pieces of rubber--an eraser often will be ideal, or you can ask for an offcut from your local cobbler.

Now trim the +/- wires to the correct length and solder the ends to the flexi LED PCB, + to + and - to -, taking care to do the soldering in "less than a second". Carefully remove the backing and stick the LED strip in a straight line onto the channel top. If you are using two strips, be sure to have two extra +/- "pigtails" at the ends of the connection wires for the second strip.

Check all the connections and be careful about the high voltages and see that you have connected up the 0.47 uF cap in series temporarily. Now carefully switch on, and the LED strip should light up. If all is satisfactory, change the cap to 0.68 uF and see that you have more current and higher brightness. It is better to do these changes at night when you can judge the brightness easily. Use the strip with one value for some time and then change to the higher value and use it for some time and see. If you have a 150 Ohm resistor in the strip, go no higher than 0.68 uF. You can also try putting the second 0.47 uF cap in parallel with the first which will double the current. The 0.47 cap at 230 VAC will permit a peak current of 47 mA or an average of 33 mA, which is well within the rating of average LEDs. Try the various cap values with caution and see that you get a satisfactory brightness. Put a small piece of double-sided cello tape to hold the cap to the inside of the channel, tidy up the wires, close the channel halves and seal the ends of the channel.

You may use double-sided tape to hold the strip lamp say, to the bottom of your computer shelf above the monitor so that the light shines onto the keyboard correctly. The lamp can also be used for general lighting in a room, and with an old UPS, it will give hours of light during power outages. If turned towards the white ceiling, it will give a pleasant overall light, especially in a child's bedroom. It can be used as cupboard lights, lights for stairs/steps etc. Your imagination is the only limit, but the power saving will be considerable if you are able to replace a light that will be on for a long time with an LED strip.

MORE  HANDS-ON

Those adventurous souls who derive a lot of satisfaction from "home-brewing" as much as possible have other options. They could buy a few dozen 'hi-brite' white LEDs, drill a neat set of holes in one half of the plastic channel, wire up one set of three or four LEDs and a small 100 Ohm or 150 Ohm resistor in series, do the testing with different caps and resistor combinations for a best compromise, and then wire all the sets of LEDs and resistors(with the selected values). Then connect up all the sets together using red/black wires, and finally to the bridge rectifier output, as we did for the ready-made strip. Use Fig 1 as a guide here. For best peak brightness and efficiency, it is best to limit each series string to a maximum of four LEDs and one resistor. Be careful to watch the LED polarity and be neat and tidy in wiring and soldering. Your skills and care here will insure safety and long life for the lamp.

The handymen among us will be able to dream up custom housings and diffusers for the lamps, turning then into "designer lamps". Again, if  you can do good soldering, you can experiment with individual LEDs arrayed in different shapes so that they can fit into conventional light fittings. You may use a plastic /acrylic sheet or laminate and drill suitable holes to hold the LEDs. Neat and well-insulated wiring is crucial. Surely the completed project will give you, in addition to savings, a lot of satisfaction every time you switch it on.



Let there be light!
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