QUESTIONS & ANSWERS
LED10 LED9 LED8 LED7 LED6 LED5 LED4 LED3 LED2 LED1
Q5 C CP P0 1 Q Q3 4
MR Q Q1 2
■ FIGURE 4
middle of the negative-resistance
slope, at point B. An example of a
practical Lambda circuit is the sine-wave oscillator shown in Figure 3. All
it takes is a Lambda diode and a tank
circuit; this circuit should oscillate at 1
kHz with the values shown, but it can
be easily extended to 100 MHz with
the right tank values. The 1K potentiometer is used to set the bias of the
diode because not all JFETs will land in
the sweet spot at the same voltage.
have all the LEDs lighted with only one
going off in sequence, use the bottom
LED circuit instead. For a stunning visual effect, use both the top and bottom
LED connections and interlace the LEDs.
Being an artist, I’m sure you can come
up with some interesting combinations.
QI would like to attach 8 to 10
LEDs to a board and have them
flash on and off in sequence. I
have often seen such a display
in art galleries. Can you suggest a way of
doing this — perhaps using a 555?
— John Joyce
North Vancouver, BC Canada
QI have a power transformer
that can deliver 125 volts at
500 mA, and I want to use it
to construct a variable 0-150
volt regulated power supply for some
tube applications. Do you have a good
circuit for this?
ATrying to get 150 volts DC
from a 125 VAC transformer
is not easy because you don’t
have a lot of headroom. For
good regulation, you need some kind
of voltage difference between the input and output voltages. Even without
a load, the best you can hope for
from this transformer is 175 volts — and
that decreases as the load increases.
Fortunately, you can get more current
as the voltage is lowered; you should
get maximum current somewhere
around 120 volts and under.
With that said, here’s the best
design I could come up with (Figure 5).
The LR8 voltage regulator is a three-terminal device — like a 78L05 — that
has an adjustable output voltage range
of 1.2 to 450 volts. Unfortunately, the
AThis is an easy design — if you
use the 4017 decade counter as shown in Figure 4. This
chip forces each output high
in sequence each time the clock is
pulsed by the 555 astable oscillator, and
creates the “walking LED” effect you’re
looking for. The Rate control determines
the rate of the walk. If you would rather
■ FIGURE 5
0 - 150V
150-Volt Variable Power Supply
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