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Flameless LED Candle Power Boost
By: Dave Johnson

 

I got an email from a Discover Circuits visitor a while back.  He loved the new battery powered flickering yellow LED candles but he wanted something which was much brighter and powered by longer lasting alkaline D cells. 

He wanted to make a light, shaped as a torch.  A plastic piece would be shaped to resemble the flame of a touch.  A flickering yellow LED light would give the illusion of a flame.

Most flameless candles are now quite simple circuits.  They consist of nothing more than a power switch, a battery and a special flickering LED. The special LED might work fine for a candle but it is not bright enough for the touch.  To boost the power of this flickering device, you can remove that LED from a flameless candle and insert into the circuit shown below.  This circuit extracts the flickering signal from the special LED and uses the signal to drive 5 super bright yellow LEDs at a peak current of 20ma each.

Flameless Flickering LED Candle Power Boost Circuit

Flameless Flickering LED Candle Power Boost Circuit

The flickering effect is produced by a pulse width modulation (PWM) circuit, internal to the special LED.  Each time that LED circuit turns on, current is drawn through the 560 ohm resistor R1.  The voltage drop across the resistor is enough to turn on the 2N3906 PNP transistor.  That transistor then produces an identical PMW signal at the collector, which swings over the full 3v peak to peak voltage.  The collector signal is then fed to the gate of a ZVN4306 n-channel MOSFET.  Acting as switch, the transistor routes current through 5 super bright yellow LEDs.  Each LED has a 47 ohm resistor to limit the peak current to about 20ma.  A fresh pair of 1.5v alkaline D cells should be able to power the touch light for about 8 hours.
 

Please send comments to me


November 2010     Issue 13

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