USB interface for PICs
I would like to know a good starting point for interfacing a
PIC-based project to a USB port? I understand how to do the serial port with a
PIC as there are only a few lines and the protocol isn’t too tricky. Is there
some snazzy chip that will do most of the protocol with a little bit of set-up
from the PIC? (M. S., via email).
If all you need is a
high-speed USB serial connection, the FT245BM and FT232BM devices from FTDI are
the way to go. Check out their web site at www.ftdichip.com for more information.
To make life even easier, these chips are available mounted on
small plug-in modules that include all the necessary support logic, including
the USB socket. You’ll find examples at www.elexol.com.au/USB_Modules and www.dontronics.com, who also stock the FDTI chips.
If your needs are more specific, then Microchip offer the
PIC16C745 & PIC16C765 with in-built USB ports. Alternatively, you could opt
to interface your micro to one of many single-chip USB peripherals. A good place
to start looking is www.beyondlogic.org
Headlight as WiFi reflector
I have a question regarding the article about "WiFry: Cooking
Up 2.4 GHz Antennas" in the September 2004 issue. You say to use a parabolic
shaped cooking scoop. Could a car headlight, with the glass removed do the same
job? I’m pretty sure they are the correct shape and you could just replace the
globe with the USB "dongle" and that would be it.
I’m not sure if the reflective material in the headlight is
enough but I guess it could be lined with foil? (D. L., Rye,
headlamp reflector is circular, it is a fair bet that it will be a paraboloid
and therefore suitable for a WiFi antenna. The headlight metallisation should
also work as a satisfactory reflector at 2.4GHz. On the other hand, most
irregularly shaped headlight reflectors result in asymmetric light beams so they
might not work as well with WiFi.
Transformer for 6-channel amplifier
I wish to build six 50W amplifier modules (SILICON
CHIP, January & February 2003) for use in a home theatre system. These
would be coupled with the 6-channel volume controller from the March & April
2002 issues. I am looking to build only one power supply for the six modules but
the question arises, what capacity should my power transformer be? (W. N.,
You need a single
transformer with a rating of at least 300VA. Unfortunately, the closest readily
available transformer is the Altronics M-5530 300V 30V-0-30V. This could be
reduced by using a 12V 2A transformer (Altronics M-256L) in auto-transformer
reduce mode, to reduce the input voltage to the 300VA transformer. The method is
explained in this month’s Circuit Notebook item on page 86. Alternatively, you
could get one custom wound to 28V-0-28V from Harbuch Electronics, Phone (02)
Reconnecting an LCD to a PC board
I have a talking clock radio that I rather like. Unfortunately,
when I recently dismantled it to clean the switch contacts, the LCD detached
itself from the circuit board and the attaching rubber strip of
micro-conductors. This has caused me great distress considering how I will
repair the connection.
Is my only solution to get out the microscope and some
conductive epoxy glue and try to repair it? Or am I able to purchase a
replacement LCD mounted on a board that I can solder? (D. V., Newcastle,
You should be able
to sandwich the elastomeric strip back between the PC board and the LCD and it
should all go again.
Controller for 10-channel remote
I have brought and made up a 10-channel IR remote receiver kit
from the February 2002 issue. I called Jaycar to get a kit to build a controller
for it but they said they don’t have one. I would like to know if there is such
a kit or the wiring diagram, etc.(G. M., Moura, Qld).
There is no kit for
the controller. As outlined in the article, the circuit is designed to work with
just about any pre-programmed remote that can control a satellite receiver. Have
another look at the article.
Class-H amplifiers switch supply rails
It has come to my attention that some audio push/pull
amplifiers have an extra switching transistor next to the main output
transistors. Now having built a few amplifiers and studied electronics, I simply
do not know what this extra transistor does. After looking at the old ETI-480
100W amplifier and a couple of other different amplifier modules, I cannot see
how a switching transistor could possibly be used. Is it a different way of
biasing the output transistors?
You are probably
referring to Class-H amplifiers (originally developed by Hitachi) which switch
the output stages to higher supply rails to enable much higher short-term
Have a look at the Mighty Midget power amplifier in the March
2002 issue. This used class-H.
Courtesy light delay for cars
I have built and tested the Courtesy Light Delay kit as
described in the June 2004 edition of SILICON CHIP. I have
installed it into my 1989 Mitsubishi Magna Station Sedan (12V, negative earth).
This model is fitted with a small light on the dashboard to show when the doors
have not been properly closed.
The kit passes all tests when the motor is off; ie, the
interior lights switch off after a delay of about 35 seconds or when the car
lights are activated. My problem is that after the delay period upon entering
the car, the interior lights then come on and dim to a lesser degree
continuously while the motor is running and I am driving. Switching on the
parking lights of course solves the problem. When the car is parked and the
motor is off, all is OK.
I have disconnected the tail-light connections to the kit but
the problem is still there. By disconnecting all four wires to the kit the
interior lights work perfectly, as was always the case.
This has me confused and I would greatly appreciate any help
you can offer. (R. C., via email).
The circuit should
not be triggered while the door switches are open. Capacitor C1 needs to be
discharged fully via a closed door switch before the circuit can be triggered
when the switch opens.
To solve your problem, you could connect the "to tail lights"
terminals to the ignition supply. In this way, the courtesy lights would be held
off via the optocoupler pulling the gate of Q1 to the source terminal.
Switching whine from speed controller
I am using a 12V Motor Speed Control, as described in the June
1997 issue of SILICON CHIP, on a vehicle windscreen
wiper motor. I get a high pitched noise from the motor (and others) when power
is supplied through the control unit. This does not occur if 12V power is
supplied direct to the motor. Any ideas on what is causing the noise and how to
get rid of it? (G. O., via email).
speed controls cause motor whine. If your car has electric windows, you will
probably hear some whine just as the motors come to a stop. The noise is caused
by the high-frequency switching signal which applies DC to the motor. You may be
able to reduce the noise by altering the frequency a little. Try replacing the
10kΩ resistor at pin 6 of IC1 with a 20kΩ trimpot in series with a 4.7kΩ
resistor. Then adjust the trimpot for the least noise from the motor.
Studio 350 fried resistors
I have built a Studio 350 amplifier module (SILICON
CHIP, January & February 2004) and upon powering it up, I found that the
voltage readings across the speaker terminals were nearly equal to the output
voltage of the power supply. I have checked all the board parts and connections
three times and can find no faults with the construction.
Later, as I was trying to adjust the voltages to zero as
described, the resistors in the area of the audio input (Q2, Q3) fried, damaging
the board surface somewhat. Can you suggest what to do next? (P. C., via
Our guess is that
you have swapped a pair of transistors or you have an open-circuit solder
connection somewhere. Replace the fried resistors and power up the board again,
with the resistors across the fuseholders, and check each transistor for a
base-emitter voltage of about 0.7V. An incorrect reading indicates a fault in
the transistor or its associated components.
Dr Video has dark rectangle
I have just finished building the Dr Video kit (SILICON
CHIP, June 2004) and I have a dark rectangle block in the top lefthand corner
of the screen. Any ideas?(S. J., via email).
It sounds as if your
vertical blanking pulse circuit (around IC6b, IC6c and IC5b) is generating a
pulse longer than the correct 1.1ms. This is probably due to within-tolerance
component variations, so we suggest you try replacing the 8.2nF capacitor with
one of 6.8nF or 5.6nF. This should remove the "dark rectangle".
Volume control for valve preamp
I have a question regarding the Valve Preamp For Hifi
(SILICON CHIP, February 2004). In the
article you place the volume pot after the preamp. I was wondering if it could
be placed before the preamp and would this change performance in any way?(R.
D., Doncaster, Vic).
There are arguments
for and against putting the volume control in front of a preamp. Putting it in
front means you reduce the chance of overload but it also means the signal to
noise ratio of the final signal may not be as good.
Energy Meter can measure to 15A
Why did you limit your Energy Meter (SILICON
CHIP, July & August 2004) to only 10A? I want to measure my air
conditioner. How could I modify your design to go to (at least) 15A, please? (P.
B, Turramurra, NSW).
The Energy Meter was
limited to 10A because this is the maximum rating of a general purpose mains
outlet (GPO). You can still use the meter to measure 15A if your GPO, the fuse
and power cords are rated for this. It would be wise to bypass the relay for 15A
measurements, to prevent damaging the relay contacts. Otherwise, the meter can
operate at 15A (3600W) without any software or hardware changes.
Micromitter’s filter is faulty
The last time I was in Australia, I purchased an FM Micromitter
kit (SILICON CHIP, December 2002) in Brisbane.
I have now built the kit and have a problem – there is very little RF
I looked at the Rohm website and found an Application Circuit
for the BH1417F chip employed in the kit. It shows a 1nF DC blocking capacitor
between the chip output and the GFWB3 filter. The data indicates that there
should be typically (Vcc - 1.9) volts on pin 11.
In my version there was no voltage on pin 11 because pin 1 of
the filter has DC continuity to ground. I have fitted a 1nF capacitor and the
unit now works correctly, with about 3.1V on pin 11. However, I wondered
afterwards if the filter input is actually pin 3, which does not have DC
continuity to ground and perhaps I could have just reversed the filter on the
Have you encountered this problem before. (D. D., Cheltenham
There should be no
need to have a DC blocking capacitor before the filter as the filter is
capacitive. Perhaps your filter has a fault, causing DC to flow to ground.
No secret code for LED marking
Like many readers I have been experimenting with a variety of
LEDs. Given the state of my workbench, I now have quite a random collection –
Is there any way of finding out LED characteristics from
scratch? And is there any logical reason why manufacturers refuse to mark LEDs
with type numbers (or have I missed some secret inscription)? After all, if
transistors were unmarked where would we be? (J. B., Dalton, NSW)
Short of testing all
your LEDs with a low-voltage DC source, there is no way of knowing their
characteristics. And if you can find the "secret way" of LED marking, please let
us know and we will pass it on to the world at large!
240V halogens still have UV output
I know you never were too keen on 12V down-lighting with
inefficient hot transformers but have you seen the new 240V halogens that K-mart
and other chains now sell? They are glass encased, so no UV radiation and are
rated at 50W.
I am wondering if these kinds of halogens are OK to use with
normal light dimmers. Mine dim OK but I am worried if I am reducing the life of
the halogens that may need to run hot like other halogens.
240V halogens have
been available for some time but we are not aware that the types you refer to
have zero ultra-violet output. Given the very high filament temperatures, that
seems highly unlikely. In any case, using a dimmer with any halogen has the
effect of reducing their efficiency as well as reducing life. Also 240V halogens
tend not to last as long as 12V types because their much higher resistance
filament is nowhere near as rugged.
Fridge causes TV interference
I have a new GE fridge which creates TV interference. There are
waves across the screen which vary depending on the fridge motor speed. How can
this be fixed? (B. C., via email).
First, you must
determine the mechanism of the interference. Is the interference visible on all
channels? If so, it is possible that the motor’s magnetic field is directly
affecting the picture tube. Is the fridge close to the TV? If so, the cure is to
move the TV away from the fridge.
On the other hand, if the interference is present only on one
channel then the source is possibly some electronic circuit within the fridge.
If so, it may well be a fault since consumer equipment is supposed to meet EMC
standards. We would then make a complaint to the distributor, or in the first
instance, to the retailer.
RFID module: installation security concerns
I have recently purchased the RFID module (SILICON CHIP,
June 2003) and have successfully assembled and tested it. This is an excellent
device, with many possible uses and was easy to construct and use.
However, there are areas where it may be improved. First, the
Reader Module could be closer to the outer panel (of whatever container is used)
to maximise the operational range. At present, this is limited by the height of
the output connectors.
Second, in the case of an external installation, the security
of the door strike function is considerably reduced by the exposure of the
connector terminals. It would be a simple matter for anyone who is "tech savvy"
and attempting entry to jumper terminals until the strike was activated. The
fact that an alarm is triggered (if present) may not deter a serious
The time allocated for door opening is about five seconds,
which seems too short. It could be doubled to about 10 seconds without
decreasing security very much.
In my situation I only need the door strike function, but the
unit needs to be installed externally. I intend removing the connectors and
hard-wiring the outputs to in-line insulated bullet connectors, which will
connect to the external cabling behind the PC board.
The Reader Module will be placed as close as possible to the
outer panel by lowering the higher components. The initialising can be done
pre-installation (on the bench).(K. M., via email).
We can’t see how security of the door
strike is reduced by the exposure of the connector terminals. Even if the module
was completely encapsulated, the wiring would still be open to tampering.
The door strike "on" time is indeed set to five seconds. If you
are knowledgeable about AVR microcontrollers and have access to a programmer,
you can increase this time by altering the relevant parameter in the source code
(RFID.ASM) and reassembling it.
Look under the heading "CONSTANTS". There you’ll find a line
.equ LOCK_ON_TIME =10
The maximum possible "on" time is 16 seconds – ie, change 10 to
Extending a video monitor connection
I wish to locate my computer screen an increased distance from
the computer chassis. The required cable length is of the order of seven to
Can the average video card drive the signals this far? If not,
can multiplexers/repeaters be used for the faster signals; ie, the R, G and B
lines. Also, what type of cable would be required and are there any other issues
involved with the increased distance?
Alternatively, can the keyboard and mouse be extended by same
distance? (E. R., Rye, Vic).
We’re not aware of the maximum cable
length for such a connection but believe that it would vary considerably from
manufacturer to manufacturer.
We’d suggest initially trying the hookup using one or two good
quality monitor extension cables (available from most computer resellers). If
the results are unsatisfactory, then you have a couple of options.
You could purchase a purpose-built SVGA video extender, such as
the "Belkin OmniView". These are available from various computer resellers in
For a do-it-yourself solution, check out the "Video & Pulse
Distribution Amplifier" described in the December 1997 edition of "Electronics
Playmaster 300W amplifier hums & thumps
I’m hoping someone can shed some light on a couple of problems
I have with a power amplifier for a 500W subwoofer. It’s from the April 1995
issue of "Electronics Australia".
Ever since I built it six years ago, it has had a weird quirk:
about one minute after switching power off, the attached speaker starts
thumping. It starts off slowly (around two thumps per second) and loudly (about
10mm speaker excursion) and over the course of five minutes the thumping speeds
up (maybe six thumps per second) and dies off. Somewhere in the middle, the
thumping seems to switch to double-time.
The amplifier also has a "clipping" LED on the front panel,
which is meant to show if you’re driving the amplifier too hard. During its
thumping routine, the LED flashes in time with the thumping.
Yes, I have checked for animals and small people trapped in the
The other quirk may be related – loud humming. Not when a
source is plugged in but only when you touch the signal terminal of the input
RCA lead with a finger. I don’t know what frequency the hum is (no oscilloscope)
but it sounds low, like a truck horn. It might be 50Hz but I don’t really know
what 50Hz sounds like.
The humming is quieter if you touch the shielding of the RCA
lead with the same finger or if you touch the amplifier case with your other
hand. Is this expected/normal? As I said, it only happens under these
conditions. The amplifier operates noiselessly and as expected when plugged into
my preamp and playing music. Thanks very much for any insight! (C. C., via
The hum is probably due to instability in
the Mosfet output stages; they’re probably oscillating at 100MHz or more (you
can check that with an FM radio). Check all the Mosfet gate capacitors.
The slow oscillation could be related to the above; ie,
motorboating. Perhaps some of the bypass capacitors on the supply rails are
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considered dangerous or even lethal if not used safely. Readers are warned that
high voltage wiring should be carried out according to the instructions in the
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you are not confident about working with projects employing mains voltages or
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