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06-July-2008
PC Controlled RGB LED
light
Using 1 Watt power LEDs,
the following example shows a PC controlled RGB light. It can
light up a room, strobe, dim, change colour, anything you can
think of programming it to do.
The 1 Watt LEDs are
expensive (only use quality), (around $8 dollars each), so a
full array of 14 LEDs isn't cheap, a smaller version
could be constructed using 6 leds. (2 red, 2 green, 2
blue).
About this
circuit:
PWM is used to control
the brightness of each LED pair, all 8 outputs of the Top16 module
are individually controllable.
Rather than use an inductor
and current control, resistors have been used to limit the
current, this is less efficient. however...efficiency has been
greatly improved by placing the LEDs in pairs (2 in
series).Because each LED drops around 3.4 volts across it
(varies with LED type), 2 in series will drop around 6.8
volts, this means the resistor has less voltage drop across
it, and wastes less of the power as heat. i.e, A 12 volt
power supply is used; 6.8 volts across the two LEDs and 5.2
volts across the resistor,..the less voltage across the
resistors, the less power (volts X current) they are
dissipating.
So why not use 3 or 4 LEDs in series ?,
Because the voltage drop across each LED is not exactly
known, it varies between LEDs and also during
operating life, more LEDs in series will give less control of
the voltage (across each LED and Resistor), giving less
control of the current, brightness and power dissipation.
2
LEDs maintains reasonable control of current, whilst doubling
the power efficiency of the circuit.
The resistors
should be 1 Watt each and chosen using the following
formula:
(Supply voltage) -
(voltage dropped across the LEDs) = (voltage across the
resistors)
(Total resistance
value to choose) = (voltage across the resistors) /
(max desired current through LEDs)
(Individual
resistor value in each pair) = (Total resistance value to
choose) X 2
* Larger 2 or 5 watt resistors
could be used to provide (Total resistance value to
choose), and use just one resistor instead
of two in parallel.
Expect the power
LEDs to have a voltage drop of 2.5 to 4.5 volts, with a
max current of around 300mA,
*Aim for a maximum
current of less than the maximum rated for the LEDs,
this way they are operating well within thief capabilities
and will reward you
with long trouble-free service.
Programming.
This is a matter of
controlling the PWM periods of the Top16 outputs, by using the
Top16 API (dlls available for linux or PC), and is up to
you.
PWM frequency = 7.8 kHz.
Resolution 0 to 255 (8
bit)
Enquiries ?, please contact: sales @
emx.net.au
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