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Ask Silicon Chip

Got a technical problem? Can't understand a piece of jargon or some technical principle? Drop us a line and we'll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097; or send an email to silchip@siliconchip.com.au

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, Vic).

Provided the 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., Kurrajong, NSW).

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) 9476 5854.

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, NSW).

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 power.

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).

All switch-mode 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 email).

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 output.

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 board.

Have you encountered this problem before. (D. D., Cheltenham Spa, UK).

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 – all unmarked!

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 attempt.

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 that reads:

.equ LOCK_ON_TIME =10

The maximum possible "on" time is 16 seconds – ie, change 10 to 32.

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 eight metres.

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 Australia.

For a do-it-yourself solution, check out the "Video & Pulse Distribution Amplifier" described in the December 1997 edition of "Electronics Australia".

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 box!

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 email).

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 open-circuit.

WARNING!

SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be 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 articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws.

Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable.

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