>>>READER-TO-READER QUESTIONS AND ANSWERS
want to get as much power as possible
without burning out the motor. In this
application, space is limited and there
is minimal air flow for cooling.
Is there a way to limit the
maximum current allowed to the
motor, even if stalled (a virtual short
circuit), while still allowing throttle
control up to that limit? The smallest
motors run 10 amps continuous at
7 to 12 volts and the big ones over 40
amps at 30 to 50 volts.
#1087 Dan Schwartz
Northbrook, IL
buttons which she can’t see or
remember which ones to press.
The functions she needs are TV
off/on, volume and channel up/down.
I want to build a simple remote with
giant buttons that are perceptually different for each of these two-dimensional
control functions in hopes that she will
intuitively feel the difference.
#1 I think I have found the perfect
remote for Stan's mother to use. The
TV remote that he described in his
question reminded me of one that we
had purchased for our kids to use. It is
called the "weemote." When I visited
the manufacturer's website to provide
Stan with the information, I found that
they now make an adult version called
the "weemote Sr. TV remote for
adults." More information can be
found at www.weemote.com.
Greg Cloutier
South Windsor, CT
#2 If you go to www.tauntek.com,
check out two chips: one called
IRMimic and another called IRMimic2.
>>> ANSWERS
[#10074 - October 2007]
I have been using the serial port of
my PC and languages such as
assembly, C++, Java, and VB to
communicate with custom designed
circuits. Nowadays, all PCs have
replaced the serial port with multiple
USB ports. I want to migrate my applications but have found working with
USB rather difficult. Could someone
please suggest a way to send/receive a
simple 1-0 digital signal using any programming language (preferably Java)?
[#10075 - October 2007]
I have an irrigation system with a
six zone controller. It has only four
wires plus a ground going to it and
the valve box. It would be impossible
to run new or additional wire as the
wiring goes under a concrete floor.
Four zones are being used. I
recently added a fifth battery-operated zone, but it’s a problem as
the batteries tend to wear out. I want
to use the controller for timing zone
five and would appreciate a circuit
that could do this.
I'm assuming that your current
Java implementation uses the javax.
comm package to communicate with
the serial port. If this is the case, by far
the easiest way to adapt your projects
for use with computers which have no
serial port is to use a USB-to-serial
converter. These are available from
many sources on the web for under
$20 and are very easy to use. See
http://sewelldirect.com/usbtoserial.
asp for a specific example. If you'd
rather have something that can be
built into your project, take a look
at www.pololu.com/products/pololu
/0391/
Let's take a simple, low-tech
approach to the problem. The zone
outputs are all AC. By using only the
positive half-cycles from one zone
and only the negative half-cycles of
the next, we can have two zones
share one control wire. We will
use the common ground as one
conductor (which will remain
grounded), and use one conductor
for unswitched 28 VAC. This leaves
three conductors for control, which
allows you to use all six zones.
Referring to the diagram, here's
how it works. When zone 1 is active,
28 VAC is rectified by D3 so only
positive half-cycles are sent down the
control wire. Out at the valve end,
D1 allows the positive half-cycles to
charge C1 through R1. C1 keeps the
12 volt DC relay from chattering
due to half-cycles being applied to it.
R1 drops the voltage down to
approximately 12 volts. Since D2 is
reverse-biased, RLY2 is not pulled in.
The same thing happens when zone
2 is active, only this time we are
sending negative half-cycles down
the control wire to pull in RLY2.
The drawing shows the circuit
for two zones. By duplicating the
circuit two more times, you will have
six zones available. All circuits
share the same ground and 28 VAC
conductors.
The typical RadioShack part
numbers are:
C1, C2:
D1 - D4:
R1, R2:
RLY1, RLY2:
272-1015
276-1003
271-1118
275-248
Rick Curl
Pinson, AL
Figure 1
Tom Dimock
Ithaca, NY
[#11072 - November 2007]
My mother has Alzheimer’s and is
unable to operate her television
anymore. We had a very simple remote
with about six large buttons which she
could use. That TV broke and our new
37 inch LCD is a good size for her,
but the remote has about 40 tiny
102 January 2008
