READER-TO-READER TECHFORUM
almost no problems with it.
Ray
via email
#2 The least expensive system to
program a PIC16F876A is just about
free, if you have a reasonably well-stocked parts box and a computer
with an RS-232 serial port. The
programmer circuit in Figure 1 was
designed by Ralph Gable, WA2PUX,
and appeared in the August 2003
issue of QST Magazine. It is based on
the so-called "JDM" programmer.
However, the schematic in that article
contained several errors, so I have
redrawn it. One advantage of this
circuit is that it works with serial ports
that don't meet the RS-232 voltage
standards. The circuit requires a clean,
stable 13 volt supply from an
adjustable benchtop power supply, or
you can add an LM317 adjustable
regulator to the circuit to supply this
voltage. Connect the "flagged" wires in
the circuit to the appropriate pins on a
socket for the PIC and you're ready to
go. This programmer will work with
many different PICs, not just the '876.
On the software side, download the
free trial version of PICBASIC PRO
from www.melabs.com to write your
programs. To load the programs into
the PIC with this programmer circuit, I
use IC-Prog 1.05 which is a freeware
application available at www.ic-prog.
com. It has worked well for me in both
Windows 98SE and XP environments.
Dan Gravatt
Lawrence, KS
[#7101 - July 2010]
Measuring Frequency
I’m looking for a simple solution
that measures the frequency of a pulse
(. 5 Vss). It does not need to measure
higher than 1 kHz. Is there anything
out there off-the-shelf or a kit; a small
board with an LCD display?
If "discretion is the better part of
valor" then consider these ideas.
Obtain a digital voltmeter that
measures frequency, as well as the
usual parameters. Alternatively, obtain
a digital storage oscilloscope that is a
plug-in to a PC. Its math function
80 November 2010
usually includes the measurement
and display of the frequency of a
waveform. Either will be a welcome
addition to your electronic capability.
Start with a search of the advertisers in
this magazine. Jameco Electronics has
an impressive array of DVMs that
measure frequency, as well.
William A. Hanger
Churchville, VA
[#7103 - July 2010]
Servo Motors
I am building a kinetic sculpture. I
have a 5' wide plywood disk that
weighs 50 pounds. I need to be able to
spin the disk anywhere from between
60 to 500 rpm. Once the disk reaches
a given rpm, I need the disk to maintain it precisely. Using just a normal AC
motor doesn't work because the rpm
seems to be constantly fluctuating. I
am told that I need a . 25 HP servo
motor to achieve this. Any other ideas?
You left many important details
out of your question, like the supply
voltage you have available and exactly
what "precise rpm” means. For most
art projects, an adjustable frequency
drive controlling a simple AC
induction motor is entirely adequate.
For under $400, you can buy a 120
VAC, 1/2 HP drive from Grainger
(model 3XA39).
Couple this to a cheap three-phase motor (I cannot recommend
one since I don't know your mechanical requirements), and you can either
use the up/down pushbuttons on the
drive's front panel to control the rpm
or fit a remote speed potentiometer.
They have other models for 240 VAC
input, too.
Dan Danknick
Santa Ana, CA
[#7104 - July 2010]
PWM Signal Generator for
Camshaft-Pump Control
I am looking to do some development work of an injection pump in
conjunction with a camshaft signal. I
am looking to drive a coil with a
resistance of 1.6 ohms (at 20 C), and
an inductance at 1 kHz of 1.72 mH.
The signal needs to be a (frequency
4 kHz) PWM peak and hold type with
two parts: a V_boost and a V_hold (or
V_batt). V_boost needs to be
adjustable between 30 and 70V, and
the current adjustable from 4 to 12A;
V_hold should be adjustable between
6 and 18V. A zener diode should be
used to drain the current from the coil
when the driver is deactivated;
V_zener should be 28V. The signal out
sequence will be based on feedback
from a once-per-rev Hall-effect sensor;
conversion to TTL would be fine.
Before you start, please consider a
shaft pickup that has more resolution
than once per revolution. Unless the
engine is to be run at a nearly constant
speed or will be accelerated very
slowly, the coarse timing suggested
will not follow the run-up of a fast-accelerating engine that may double
its speed in one or two revs of the
camshaft. At least for developmental
purposes, use an absolute position
encoder or add a shaft digitizer with
multiples of 12 outputs along with the
once-per-rev pickup. From the higher
resolution encoders, you can decode
those desired outputs for the two,
three, and four lobes.
My experience with processor
control of timing of this sort is that the
processor ultimately gets busied up
with so much other mundane stuff
that it cannot accurately and timely do
the main job which must be done on
a tight schedule.
Other comments are: The peak
(pick) circuit can be common to all
drivers since their timing does not
overlap; and the coil discharge function should be back into the pick supply rather than into a zener.
At 4 kHz and four to 12 amps,
there is appreciable energy to waste
in the zener. Also, almost the same
energy is recovered in the discharge
function as is supplied in the pick
function (less IR and magnetic losses).
The driver circuit — when not active —
should keep the coil near ground or
at a negative voltage so as to avoid
electrolytic or Galvanic action to the
copper windings.
William A. Hanger
Churchville, VA
