|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.
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
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.