■ FIGURE 4
work over the wide range that I
need. That is why the 555
oscillators were used. With 14 stages,
the 4,060 divides by 16,384 but I
am only using half the period. The
output is initially low and goes
high after 8,192 clock pulses. When
the output goes high, it feeds back
and resets itself to start another
time period. There is a power-on
reset through C2 which resets all
the flip-flops. The parts list is
Figure 3.
The variable time (R6) is less
than one second to almost 60
seconds. If the variable time
exceeds the fixed time, the pulse
width is the difference which will
be very small; you may want to
tweak R4 and R5 but don’t make
R5 less than 1K because when R6
is zero, the discharge transistor has
to pull current through R5. The
only reason for R2 and R4 is to make
the 555 output pulse wide enough
that you can see it on an
oscilloscope.
The components VR1, R1, D2,
and C4 are to protect the circuit from
the transients of the automotive
system which can reach 60 volts or
more.
MAILBAG
Dear Russell: Re: Inexpensive
Drafting Software, April ‘ 10, Page 26.
I have an old OrCad package
that works wonderfully for electronic
schematics, but for a general-purpose,
26
November 2010
SEVEN-SEGMENT
LED HELP
QI have a whole box of seven-segment LEDs of assorted sizes. Some are multiple character; some
have decimal points. They go from 14
pins up to many pins. Some are common
anode, some common cathode. I
haven’t been able to find the pinouts
anywhere on the Internet for LEDs in
general. I think it would be good if you
could tell us where to look to get that
info. Or better still, how to test to find
out where the segments are for each
character and whether it’s a CA or CC.
I tried this: Set bench power
supply to 1.5 VDC. Connect one
probe to any pin. Run the other
probe along the remaining pins until
something lights. If not, swap the
probes and do it again. One of the
two pins must be the common. Then,
determine which is which. Then,
identify the segments pin by pin. For
some reason, this doesn’t always
work. There must be a better way as
many of these are made in China or
some place like that and there is no
datasheet available or it’s in a foreign
all around drafting package that can
do mechanical or architectural
drawings, electrical schematics, and
flowcharts, I suggest TurboCad.
Version 16 is available from Internet
sites for under $80 plus shipping
and tax (if any). It has a huge library
of shapes for all purposes and it
language. Help bail out us “duffers.”
— C. P. Furney, Jr.
AA Mouser or Digi-Key catalog may be helpful. Find a similar device and ownload the datasheet,
then check if the pinout of your device
is the same. Otherwise, I don’t know
of any method better than what you
have done. However, I would use a
five-volt supply with 1K in series to
limit the current; 1.5 volts would not
light up some colored LEDs.
LED FLASHER AND
BEEPER
QI’d like to modify a 9 VDC LED flasher diagram by adding a mini-beeper to ‘beep’ in tandem with the
LED; also, I’d like to add a 5K (or
other value) variable resistor to select
the speed from ‘fast’ to ‘slow’ to
beep and flash in tandem.
Lastly, I would like a second
schematic that would do everything
just as above but be powered from a
3 VDC source. Thus, incorporate a
CMOS 555 timer instead! Can you
please help with these two schematics?
— Michael Williams
AOnly one schematic is needed because the LMC555 will operate from 3V to 12V. In Figure 4, R5
varies the speed from fast to slow and
R1 sets the maximum speed. Changing
speed also changes the duty cycle so
you don’t want to go too fast because
the beep and blink will get very short.
You could fix R3 at 390 ohms, and the
brightness of the LED will vary with
supply voltage. You could use the
SBZ-204 buzzer; I didn’t because I did
not know where you found it. NV
comes with a tutorial.
— Peter A. Goodwin
I have used AutoCAD LT with a
Via Schematic plug-in but it is cost-prohibitive — about $2,000 a few
years ago. However, I think I have an
excellent choice for the flow-charting
