using blue, red, and green LEDs
and triangle wave circuits, but they
are not as bright as I'd like.
Any suggestions out there?
#1 Go to www.BGMicro.com or
www.Goldmine-elec.com as both
offer an integrated three-color LED in
a T 1 3/4 package with an internal
controller chip that does exactly what
you want, for less than $3.00. They
are bright enough to light a tennis
ball-sized crystal in a dark room. Just
apply 4 VDC at 20 mA and you are all
done. Use several for a more
David D. Speck MD
NUTS & VOLTS
#2 I really like that idea — so I had
to build one! Instead of using a slow
triangle wave, I used a square wave
near, but not exactly at, the line
frequency to turn an AC lamp on and
off (see Figure 3). When the 120 Hz
square wave from the op-amp is high,
it turns on the high voltage power
MOSFET (just about any will work)
and the lamp sees the voltage from
the bridge. If the line voltage happens
to be high, the lamp is bright, but if it
is more near zero volts, the lamp is
dim. As the square wave drifts relative
to the line voltage, the brightness
goes up and down.
Originally, I used 60 Hz and a
half-wave rectifier, but the lamp was
off too much of the time. Moving to
120 Hz and a full-wave rectifier keeps
the lamp partially lit at all times.
Simply adjust the potentiometer until
the lamp is changing at the desired
rate. Use a non-polar 1 µF capacitor
op-amp and a good multi-turn pot to
set the frequency. The optoisolator is
not critical, as long as it can turn the
MOSFET completely on and off. The
full-wave bridge can be a molded type
or just four diodes rated for AC line
voltage. Make sure to use a fuse, too.
A quad op-amp could run four
lamps and the nine-volt battery could
be a molded power supply or the line
voltage can be dropped down with a
power resistor and zener, but
remember that the op-amp circuit
then becomes a shock hazard, too.
Circle #35 on the Reader Service Card.