I have tested the QLF4 (PWM Quad LED Fader) circuit, and I can report that
it works very well.

Let me explain how the circuit works since the basic PWM principle can be
used in many other BEAM applications.

Attached is a section of the QLF4 circuit and the relevant waveforms to
match the description of operation. The frequency of the yellow triangular
waveform is much normally slower but is speeded up here to illustrate the
principle.

The left side of the circuit consists of 2 74HC14 oscillators one of which
operates at 2kHz and the other has a cycle time of 2 to 4 seconds. The
output pins of the inverters have squarewave outputs but the input pins have
a triangular waveform between 1V and 2V p/p which is the charge and
discharge curve of the capacitor voltage as it swings between the trigger
and reset thresholds of the Schmitt inverter. It is the BLUE 2KHz input
waveform and the YELLOW slowly changing reference voltage we will use to
generate the RED Pulse Width Modulation (PWM) waveforms that drive the LEDs.

In the middle of the circuit, one LM324 Quad opamp act as a comparator (a
LM339 can also be used with different pin out). The + inputs of all
comparators is connected to the blue 2kHz triangular waveform while the -
inputs are connected to the respective yellow ~0.3Hz triangular waveforms of
the slow oscillators. The slow yellow waveforms are used as ramping
reference levels to which the common blue 2kHz triangular waveforms are
compared.

If the rising blue 2kHz waveform is more positive than the yellow reference
level, the comparator output (red) goes high and supplies base current to
the output transistor. If the falling 2kHz waveform drops below the
reference level the comparator output turns off.

The yellow reference acts as a "slicing" level through the blue 2KHz
wavefrom and the resulting red rectangular output waveform of the comparator
will be positive for a longer time if the reference level is low and will be
off longer if the reference level is high.

The 2kHz rectangular waveform causes the average current through the 10
parallel LEDs to smoothly change between 0 ma and 100ma or from full off to
full on and back again.

Since the four slow (Pulsar) oscillators all have different cycle times, the
four strings of LEDs will independently vary in brightness at different
rates. Peak current can be as high as 400mA (all strings on) but the average
current should be about 200mA, so AAA or larger alkaline batteries are
required.

As for other applications, the LEDs can be can be replaced with motors to
provide PWM speed control.
The reference voltage level generated by the slow oscillators can also be a
potentiometer or a LDR/PD photo bridge. The speed of the motors will be
variable and proportional to the voltages at the reference level inputs.

wilf



----- Original Message -----
From: "Wilf Rigter" <wrigter@...>
To: <beam@yahoogroups.com>
Sent: Friday, May 23, 2003 11:50 AM
Subject: Re: [beam] Slow pulsar


> Attached is a hybrid PWM circuit, QFL4,
> with the lowest part count so far.

> wilf



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