Dear Russell: Re: Convert Logic Circuit to
Microcontroller, November ‘ 11, page 23. In your
column, you indicated that “a microcontroller
can only do one thing at a time,” but the
Parallax PX832A Propeller with eight parallel
processors and 16 counters/frequency
generators such as is used in the column by
Jon Williams could easily handle the type of
work Mr. Sellen was looking to do with
Response: Thanks for writing, Gregg. I had
no idea this was possible.
Dear Russell: Re: Figure 4, Solar Collector
Schematic, November ‘ 11, page 24.
I don’t see how the circuit in the diagram shown
can function. It would appear that any voltage
developed by the solar panel would be blocked by
the reverse biased Schottky diode D2. The circuit
would not work without the diode either, as the -
side of the solar panel would conduct to the - side
of the battery continuously, through the MOSFET
reverse diode. I know that mechanical contacts are
not in vogue anymore, but couldn’t Q1 and D2 be
■ FIGURE A.
easily replaced with an appropriately selected
— Bob Hibbard
Response: You are right. Oh, my, what was I
thinking? I was treating the solar panel as a load
instead of a source, and the diode makes no sense.
I guess I should publish a corrected circuit with a
P type MOSFET on the positive side of the solar
panel (see Figure A). Thanks for writing.
555 AND CD4047
QI’m weaning myself from relay circuitry and am curious about the pros and cons of a clock
oscillator based on the 555 circuit
and the one shown on the CD4047
I imagine the two different ICs
are similarly priced, have much the
same power consumption, and are
more or less identical in size, so what
would influence the oscillator
designer towards one or the other of
these two ICs? How come the
CD4047 has such a bewildering
array of input controls — what do
they all do?
You’ve explained that the 555
can be halted by controlling the
voltage on the reset pin. Do any
of the CD4047’s inputs halt and
restart oscillation like the 555’s reset
Relays sure are simpler. And they
26 February 2012
have nice big solder terminals.
— Stanley Froud
AI have not used the CD4047 but it is an interesting IC. This is how it works (I believe):
When the astable-bar (pin 4) is
low, it is in astable mode and it
doesn’t matter what state pins 5, 6,
and 8 are in.
When the astable-bar is high
AND astable is low, it is in
monostable mode and the +trigger
and -trigger can be active. Looking
at the logic diagram, I think that if
you are using + (rising edge)
trigger, the -trigger should be held
low; if you are using the -(falling
edge) trigger, the +trigger should be
The oscillator out (pin 13) is a
pulse — not 50% duty cycle — and
the frequency is double the q and
not-q outputs, which are 50% duty
The reset only sets the q output
low and the not-q output high, but
does not stop the counter. The
retrigger input is intended to be used
in the monostable mode to make the
output time longer; I don’t know
what it will do in the astable mode,
but it may be possible to make one
oscillator a slave to the other.
The choice between the 555 and
the CD4047 rests on:
• Do you want 50% duty cycle
output? The CD4047 does that but
the 555 has limited capability.
• Do you want to pulse width
modulate (PWM) the output? The
555 can do that, the CDE4047
• Size might be an issue. The 555 is
eight-pin; the CD4047 is 14-pin.