Re: QCX: Display contrast changes when transmitting
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LCD Display flicker and contrast change is something that can be evaluated with
a simple test platform made up of an LCD and an Arduino.? Run the "Hello World"
program from Arduino IDE examples to get some text on the display.?
Experiments here seem to indicate that use of a potentiometer for setting display
contrast may not be necessary.? The more modern 1602 style displays seem to
work fine with just a 2.2K resistor from pin-3 (the contrast control pin) to ground.
If that is acceptable for your application then it will save the cost and board space
of the potentiometer.?
Drive to the LCD backlight can be made adjustable by using the Arduino PWM
output to drive the base of a emitter follower pass transistor between +5 volts and
the LCD backlight pin.? This then becomes a variable that can be set by software.
A more esoteric solution might involve a light sensor driving an ADC input to the
Arduino and software for automatically adjusting LCD backlight based on ambient
light intensity.? Put your finger over the sensor if you temporarily need higher
backlight output.?
Arv? K7HKL
On Sun, Oct 29, 2017 at 11:31 AM, Shirley Dulcey KE1L <mark@...> wrote: Flickering of the backlight brightness is caused by voltage drop in
the +12 power supply rail. In the QCX design the LCD backlight is
powered from the +12 rail through a resistor rather than from the +5
regulated supply; see the partial circuit diagram detail of the
microprocessor on page 115 and the explanation of why it is wired that
way on page 116. The input voltage will always drop on key down
because of the increased current demand; only a little if you're using
a bench power supply (because of resistive losses in the wires and
increased voltage drop in the reverse polarity blocking diode),
possibly a lot if you are powering the rig with batteries because the
output of the batteries will also sag. That effect is easy to see, and
it's not unique to the QCX; the backlight on the display of my K2 also
does it.
But that should not cause the contrast to change. The LCD panel itself
(other than the backlight) is powered by the +5 regulated supply, and
the display contrast setting is also derived from that supply through
a resistive divider (trimmer R47). If you are running the rig from a
low supply voltage it's possible that the 7805 regulator is falling
out of regulation, causing the +5 rail to vary excessively; normally
the regulator should keep it steady. You could also be experiencing RF
feedback that affects the operation of the regulator.
The first thing to try is to power the rig from a stiffer source:
either a good bench supply or a large battery (one with multiple amp
hours of capacity), with an input voltage to the rig of 10V or more.
If that doesn't clear up your display flicker you have a problem
somewhere; a significant change of contrast is not normal. If you have
an oscilloscope handy, probe the +5 rail and observe the voltage as
you key the transmitter; you should see only very small changes.
You're likely to observe a quick transient drop on key down and a
quick rise on key up if you're looking at times in the microseconds or
nanoseconds, and then the regulator quickly brings it back where it's
supposed to be. Those microsecond spikes are too short for you to see
the effect on the display.
On Sun, Oct 29, 2017 at 12:33 PM,? <peter@...> wrote:
> I noticed that the display contrast changes quite strongly (characters
> becoming dark) when transmitting, causing the display to flicker when
> sending CW. Is this a general problem, or is there something wrong with my
> unit?
>
> Otherwise vy happy with the little rig, amazing design!
>
> vy73, Peter HB9TVK
>
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