the appropriate modes, range,
and pulse characteristics. For
the transmitter (as with the 555
implementation), the output
period can be controlled by a
photoresistor as one leg of a
voltage divider.
However, with this circuit,
you can just as easily drive the
off-time input from a voltage
source. For instance, you could
use a photocell whose output
voltage varies with light
intensity.
Schematic 12A is an
example of the transmitter using
a PIC 555 replacement and a
photocell. The circuit is
essentially a VCO with a 50%
duty cycle. The placement of
the photocell may look a little
strange, but keep in mind that
lower voltages on the control
inputs correspond to lower
period values, i.e., higher
frequencies.
Putting the positive terminal
of the cell on the +5V line will
cause the control voltage to
decrease with higher light
intensity which will then yield
higher frequencies.
Schematic 12B shows a
similar circuit using the NCO
feature of a PIC18313. (See Part
3 of this series for sample
programs.) In this case,
however, higher voltages on the
control input cause higher
frequencies on the output.
In both cases, you’ll want to
make sure that the maximum
output voltage of the photocell
is less than 5V. Resistors R5 and
R8 are there to limit the current
to the inputs in case of
excessive photocell voltage.
For those of you who like a
programming challenge, you
can edit MSMVx2.asm and modify it
so that one section is an astable
multivibrator and use it to drive the
IR transmitter. The other section
would stay as a monostable to be
used as the receiver.
Mims Circuit 28
The last circuit in the book
(Schematic 13) is a DC-DC
converter. It uses a 555 in its astable
configuration at about 2. 9 kHz with a
low pulse width of about 7 µs. This is
very close to a 98% duty cycle.
In my experimenting with the
circuit, I found that I could not even
get close to Mr. Mims results using
the values and configuration in his
schematic. I had to make some
changes to get nearly the same
results.
I used a different transformer
which would have affected my
results. Refer to Schematic 14 for my
final 555 circuit.
The first problem I found with
the original circuit was that the 555
was getting quite hot. I believe this
was due to the very high duty cycle
64 July/August 2018
SCHEMATIC 12. PIC analog lightwave
transmitters.