How to... improve an LED tealight for brightness.

[Siliconous Skumins]

Nearly all the LED Tealights available cheaply, consist of nothing more than a single yellow LED, a small 3V coin cell battery (CR2032) and a plastic body roughly the same size as a real wax tealight. They are made using the *minimum* of parts, and are thus very cheap but not very "hackable".

The LED is a special type that has the circuitry built into the die of the LED (the bit that makes light), thus no external circuits are needed to make it flicker. Usually the circuit is actually the chip from a musical greetings card, but instead of producing sound, it is connected to the LED which varies in brightness in responce to the 'tune' (just as a speaker vibrates in responce to the same tune). To keep costs even further down, no current limiting resistor is used, instead the circuit relies on the internal resistance of the coin cell battery to limit the power to the LED. LEDS are current driven devices.

From wikipedia:

The current/voltage characteristic of an LED is similar to other diodes, in that the current is dependent exponentially on the voltage. This means that a small change in voltage can cause a large change in current. If the maximum voltage rating is exceeded by a small amount, the current rating may be exceeded by a large amount, potentially damaging or destroying the LED. The typical solution is to use constant-current power supplies, or driving the LED at a voltage much below the maximum rating. Since most common power sources (batteries, mains) are not constant-current sources, most LED fixtures must include a power converter. However, the I/V curve of nitride-based LEDs is quite steep above the knee and gives an If of a few milliamperes at a Vf of 3 V, making it possible to power a nitride-based LED from a 3 V battery such as a coin cell without the need for a current-limiting resistor.

These flickering LEDs are also available on their own, so if you need a few for a project, you can get them easily. Here is one link to a supplier :  

http://evilmadscience.com/productsmenu/partsmenu/189-candled

OK that's the explanation out of the way, onto the hacking.  


When you open a standard cheap LED tealight you will find an LED, two small wires, a switch and a coin cell battery. To make a brighter tealight you will need a better LED -brighter requires more power- , a bigger battery or "wall wart" mains adapter, a common low power transistor (eg 2n2222), and an appropriate rated resistor for the LED. The resistor value will depend on your supply voltage, the forwad voltage (Vf) of the LED (the voltage required to make it light up), and the current required for the LED to operate correctly.  
LED resistor calculator here:  

http://led.linear1.org/1led.wiz

For this example I will use a 1Watt Luxion Star LED, and a 5v 'wall wart' USB charge adapter - mainly because 5V is an easy voltage to work with, and USB charging adapters are easy and cheap to find. The resistor values are correct for the circuit with a 1 Watt *WHITE* LED running from a 5V supply. Different LED wattages and colours will require a different resistor, as will different power supply voltages. Refer to the specs for the LED (the voltage (Vf) and current (mA) values) and use the resistor calculator in the link above.

The circuit looks like this:




The flickering LED is connected to the +5V supply via a 56 Ohm resistor. This resistor limits the current to the flickering LED, and provides a voltage difference between 'ground' (negative side of the power supply) that can control the switching of the transistor (the "base" pin of the transistor - marked 'B' on the diagram). As the flickering LED changes brightness, the voltage it uses changes, and this is what controls the transistor to switch the power to the 1 Watt LED, which changes in brightness according to the flickering LED. The 1 Watt LED is connected to the +5V supply through the "emitter" ('E') pin of the transistor, and via a 4.7 Ohm resistor connected to the "collector" ('C') pin of the transistor, which limits the current the LED can pull.

The transistor is a very common low cost item that can be found at nearly every electronic parts suppliers. It is able to handle up to 1 Amp of current, so will be suitable for LEDs up to 3 Watts.

The NEGATIVE pin of the flickering LED can be identified by the flat spot on the base of the case - see diagram above.

If you can't find the exact resistors required, just use the nearest lower value resistor. If you can't source suitable resistors at all, then you can get a suitable value by using much high rated resistors in parallel to reduce the resistance to the required value.
Here is a calculator for parallel resistors:

http://www.sengpielaudio.com/calculator-paralresist.htm

And finally a resistor colour code chart to help with values:

http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/info/comp/passive/resistor/colourcode/colourcode.html



It's a very simple circuit, and there are many ways it can be adjusted and improved. Have fun.  

SS

[Aleister Crow]

Thanks. I've already got a couple of ideas. How well do you think this'll work on the 5V from my PC's power supply?

[Siliconous Skumins]

The 5V rail on a computer PSU is perfect, very well regulated with little drift in voltage. More than enough current to support a large number of high power LEDs.


If required, the same circuit with the resistor values shown can be used on higher voltages just by adding a simple voltage regulator with a 5V output, such as the LM7805. This part is good for 1A of current if used with a small heatsink and given plenty of airflow, and a voltage input between 7V (min) and 25V (max). Not very efficient for battery use though, but useable. The 7805 regulator is another common low cost part. More efficient (and easier / cheaper) to just change the resistors though.

I'll add a diagram with the 7805 in the circuit, if there is enough call for it. It is easy to figure out though (only three pins)...

SS

[Aleister Crow]

Perfect. I have a wooden PC case that looks like an old radio (an nMEDIAPC HTPC 8000 ATX, if interested) . I've been trying to figure out how to add a couple of flickering lights inside that'll be visible through the "speaker grills" on the front, and this looks like just the thing.

[Poppy Locks]

LED tealights are really quite fun things.  I've recently used a whole batch of them in my personal, portable furnace to a very good effect



And the whole thing in position....



Poppy

[Banfili]

This is very handy. Earlier in the year I acquired more LEDs than the average electronics tech could use in a lifetime. & I have a nice tea light lantern just waiting to be modded!

[MakerMike]

Very useful!  I've got an old gas lamp that I've modified with several flickering LEDs to look like a flickering gas flame, but it's barely bright enough to create ambiance.  Replacing them with a brighter (yet still flickering) LED module may be just the thing!

[Siliconous Skumins]

Very useful!  I've got an old gas lamp that I've modified with several flickering LEDs to look like a flickering gas flame, but it's barely bright enough to create ambiance.  Replacing them with a brighter (yet still flickering) LED module may be just the thing!

Indeed, this is what I intend to do with the several Victorian gaslights I've been collecting. It was the reason I came up with the circuit originally.    Clear silicone sealant is usually a bit opaque, and as such it works nicely to make a flame shaped diffuser if required. The conical shaped applicator tip on the tube can be used to make a mold that is roughly right shape, then when set, you can cut / add more silicone to get the exact effect you want.

SS