Prawnlite protection not required

A thought came to mind when looking at the schematic of the Prawnlite in the January issue. There is no reverse polarity diode in the power supply line. Fitting one might just keep the smoke inside the components when someone accidentally connects the power leads to the battery "about face" in the dark. Is there another form of reverse polarity protection in the circuit that I’m overlooking?

As an aside, my experience with white high-brightness LEDs has been less than favourable. Over time they tend to fade in brightness or just fail completely. They are far less uniform/reliable than single colour equivalents.

Brad Sheargold,
Collaroy, NSW.

Comment: there is no reverse protection diode in the circuit but if the supply is reversed, it is unlikely to do any damage to the components. In fact, we inadvertently reversed the supply during the night-time photo shoot – as you suggested might happen. No damage occurred.

Cheap electronics & TVs with digital tuners

I have read the Publisher’s Letter and the letter entitled "TVs should have digital tuners", in the January 2005 issue of SILICON CHIP.

Currently, there is legislation in North America requiring new TV sets over a certain size to have an integrated digital TV tuner. If the Australian authorities follow suit, this would certainly promote the awareness of digital TV. Also, I think publishers of TV guides should include indications of programs available in digital widescreen, high definition and those which have multiple camera angles. This would also further promote the availability of digital TV.

For the last few years, I have been seeing audio equipment with outrageous power output claims (particularly on eBay). My Pioneer C-2000 valve amplifier/tuner has an RMS power output of 11W, with an EIA music output of 34W, which is measured to an industry standard. I think all amplifiers should have music output power measured under EIA test procedures.

Also, I have begun to notice shifting trends in the composition of equipment dumped in landfill. On occasions, I buy some of it from a junkyard very cheaply and I then carry out a Build Quality Investigation (BQI) on them.

Do you think the low cost of such equipment could be a result of unfair trade practices? I think such poor quality equipment could be an environmental problem, because the chemicals used are often not identified, because the manufacturer could not be identified. My 20-year old Thorn TV set (Japanese made) is still working well and it never needed repair. I think such sets are very well-made.

Bryce Cherry,
via email.

Critical comment on the November issue

I have a number of questions and comments on things I have spotted in the November 2004 issue. In "Circuit Notebook" on page 81, Tarek Helland made a slight mistake: pin 12 is not +12V; it’s -12V (blue wire -12V, orange wire +12V).

Although this has nothing to do with the PS_ON wire on the ATX plug, it might cause a problem if someone used the diagram for another purpose.

In "Ask SILICON CHIP" on page 108, in the answer to the letter on the Playmaster 300W amplifier, touching the audio input will cause your body to act as an antenna for 50Hz mains hum. Also, touching an earthed item (RCA socket shield or amplifier case) will reduce this 50Hz voltage induced into the amplifier.

On a similar note, if touching the shield (cold side of an amplifier input) causes hum, there is either an open-circuit resistor between signal ground to main ground, or the mains earth wire is disconnected from the amplifier. This has come from my own experience.

Also in "Ask SILICON CHIP" on the same page, in the letter on the Micromitter’s faulty filter, the GWFB3 filter pins are wrongly marked; the dot is pin 1 which is the 1nF capacitor, so you have the filter orientated the wrong way around. I discovered this by measuring between pin 2 to pins 1 and 3 with an Ohmmeter. Of course, a capacitor will read open circuit and an inductor nearly a short circuit. Also the circuit of the GWFB3 is in the construction article, so it should not be hard to figure that out.

Finally, I have quick question on the 3-Step Charger for deep-cycle 12V batteries. Is there protection if the battery is too cold or even frozen? This is bad for the battery’s life but not necessary fatal as long as the case does not crack. A frozen battery should be left to thaw out before attempting to charge it, so the question is: "is there a low temperature cut out?" If not, it will be a simple matter to add to the code but what temperatures should be used?

I buy your magazine every month and I enjoy reading it. The only problem is when I finish reading it I’m after more. Keep up the good work.

Danny Rawlins,
Portland Vic.

Comment: Tarek Heiland did not make the mistake – it was a drawing error on our part.

You are right about the induction of hum into the input of amplifiers but we still think that the Pro Series is quite prone to oscillation at around 100MHz, leading to the motor-boating symptoms described.

As far as the Micromitter is concerned, our prototype was never fitted with the CFWB3 filter even though we made provision for it. We were not able to source it during the development. Having said that, hindsight says that reversing the in-line 3-pin device could cause the fault – we did not think of it when the answer was prepared. Thanks for the tip.

We did not make provision for prevention of charging of frozen batteries and it would not be easy to do so with the specified thermistor. In any case, how many people would attempt to charge a frozen battery and how likely is it to occur in this country?

To be frozen, the battery would have be very deeply discharged and extremely cold. We agree that there may be people "out there" who would attempt to do it, in which case they could certainly cause damage to the battery.

Glad you like the magazine.

Battery refinements for Energy Meter

I’ve recently built your Appliance Energy Meter (July & August 2004) from an Altronics kit and have one concern and one suggestion.

I’ve included a rechargeable battery and note that the battery runs the full electronics when the mains is disconnected which flattens the battery during storage. I suggest that a switch is required to isolate the battery – to save having to unscrew the lid each time. This is even more important if one is using a primary battery.

I’ve also come up with an easier way to secure the battery within its steel clip that saves having any holes in the side of the case. Aside from the two Nylon spacers mounted through the lower PC board at either end of the battery, I’ve added a 15mm Nylon spacer on the end of the centre top screw in the display PC board. Into the end of this spacer I’ve screwed a Nylon M3 bolt, leaving it protruding about 4mm. This just touches the battery when the lid is in place. Much neater!

Clive Woodward,
Perth WA.

Comment: thanks Clive. These are worthwhile improvements.

Confusion about instantaneous power

The Appliance Energy Meter article in the July 2004 issue has mistakes. Is John Clarke aware of them? The caption for Fig.1 says this is an in-phase example. It’s not; it’s a 90° current lag example. Looks like Figs 1 & 3 need swapping. This really "threw" me.

I understand that the required power to drive a purely reactive load is very high but your 90° V/I case says that the power is lower than the 0° and 45° cases. Maybe this is the power dissipated by the load; ie, perfect inductors or capacitors do not dissipate power. However, the power company would have a problem if all loads were purely reactive since the power required is huge.

Nice little box he built.

Mike Peters,
Colorado Springs, Colorado, USA.

Comment: the Fig.1 and Fig.3 current and phase diagrams are correct. There is no error. Fig.1 shows both current and voltage in phase. Fig.3 shows the current lagging the voltage by 90°.

Instantaneous power (green curve) has the same excursion in all three cases (Fig.1 to Fig.3) but average power is different in each case due to the power factor. The 90° lag example in Fig.3 shows the average power as zero and this is correct.

It is true that high power factor loads do cause serious problems to the distribution system but this is not a problem for the consumer.

About DVD background music

I have just a short note regarding a problem a gentleman wrote in about in the January 2005 issue (page 98). He had problems with the balance between the background music and
dialog on DVD films. This may solve his problem. The sound effects and music on the film sound tracks is encoded on the front left and right channels behind the screen, while the dialog is encoded on the centre channel behind the screen.

If he hooks up one channel of his stereo amplifier to the centre channel of his DVD player and the other channel of his amplifier to either the left or right channel (or both) outputs of the DVD player, he should then be able to balance the dialog and music with his amplifier balance control. I have not tried this but as a long-time pro-jectionist, I sometimes have problems with the mix on the soundtracks.

Unfortunately, in the cinema we cannot adjust the balance in this manner but it should work in a domestic environment. The result will not be stereo but it should give good wide bandwidth sound for the hard of hearing. With the advent of cheap DVD players with 5.1 decoders built in, this option can be had even if a new DVD player has to be purchased.

Graham Johnston,
Port Pirie, SA.

Radiator fan running after engine turn-off

I too have my radiator fan wired to come on at will as does F. W. of Airport West, Vic, "Fan cooling does not work" (page 97, January 2005) and I believe that it is beneficial (especially as the vehicle isn’t young) to help dissipate the heat with the aid of the thermo-syphoning effect and the thermostat being open.

B. G.,
Kempsey, NSW.

Comment: we have doubts about whether there will be much thermo-syphoning in modern cars, with their quite shallow radiators, the tortuous water path through the stationary water pump and hoses and the very high setting of today’s car thermostats.

On the other hand, if you can hear the radiator fans running when you are about to turn the engine off, it is probably good practice to wait until they stop running before switching off.

Listening to TV channel audio

This is to comment on the question and answer on page 97 in "Ask SILICONCHIP", January 2005.

The easiest way to listen to analog TV audio is to get a VHF/UHF scanner with a wideband FM demodulator. I use an Icom IC-Q7A. The TV audio carrier frequencies are in memory.

The least expensive way is to get an inexpensive FM radio (almost anything goes) and build a simple pre-converter. That is one dual-gate MOSFET and one crystal oscillator module. The oscillator can be anything from around 7.5MHz up to 20MHz. Oscillator harmonics cover all TV frequencies. An 8MHz oscillator puts channels 1MHz apart on the FM radio tuning dial but any module from a discarded computer would be OK.

Some calculations are needed, so that the converted TV sound would not overlap with a strong local FM station nor other TV sound channel. A simple LC filter is needed to select proper harmonics of the crystal oscillator module. The crystal oscillator module needs 5V but works happily with a 6V battery and two voltage dropping diodes, 1N4148 or similar. For better sensitivity, a one-transistor RF amplifier could be used between the oscillator and the mixer.

Common FM radio chips won’t go over 200MHz but it is not too difficult to build the whole receiver using a MOSFET mixer and an FM radio chip (TDA7000) as the IF strip, operating with an intermediate frequency of around 70MHz.

Sakari Mattila, VK2XIN,
Canberra University,
Bruce, ACT.

Current transformers can be dangerous

Please contact the writer of the letter entitled "Current Transformer for Appliance Measurement" on page 98 of the January 2005 issue, before someone gets killed. For example, a thousand turns on the secondary winding of a current transformer makes a 1000:1 transformer. This can and will generate lethal voltages.

Bought devices include a shorting device built into the secondary winding to prevent this. It usually consists of a relatively low-value calibration or shunt resistor. Its value is chosen so that when it is connected in parallel with the meter, the voltage generated by the secondary current gives a correct reading on the meter.

Incidentally, passing the wire through the core represents not half a turn but one turn precisely. Certainly that turn goes via the appliance and the transformer outside somewhere. It is still precisely one turn.

I have built a current transformer like the one this person seems to require. It was about 25 years ago using an old TV horizontal line transformer. From memory, I used something like 470 turns on the secondary and a 4.7W shunt. The simple maths tells you that this gave 10mV per ampere with a high impedance meter or scope connected to it.

One ampere of primary current will produce 1/470 of an ampere of secondary current. 4.7W is the secondary voltage per ampere of secondary current. Thus 4.7/470 = 0.01V/A. Incidentally, the insertion loss is approximately 0.01/470 = 0.0000213 ohms. As you can see, there is no need for an empirical calibration.

David Millist,
Toowoomba, Qld.

Comment: we fail to see how a current transformer can generate lethal voltages unless it has a significant voltage across its primary winding. In a typical current monitoring application, this is not possible because virtually all the voltage is across the load being monitored. That accords with your figure for insertion loss.

Nor can we agree that a wire passing through a toroid constitutes a full turn. Considering the magneto-motive force (ampere.turns) generated by the wire, it can only be a half turn, at best.

Tsunami warning system via email not workable

Your editorial in the February 2005 issue on the recent Tsunami is clearly noble in intent, though I believe flawed in a few points. Specifically, the suggestions on how to warn people of impending disaster – to expect those most affected by such a disaster, those living a subsistence life on the ocean’s edge, to have access to phones of any sort (land-line or mobile), let alone the Internet, is a fallible view.

The concept of using the Internet (web, email) is, alas, impossible. As you know, little on the Internet can be trusted and it is all too easy for a vandal or miscreant to impersonate a trusted entity. Basically, if email were an official channel for advising of disasters, guess what every second spam message’s subject line would read?

Even for the recent tsunami, a number of low-lifes tried to capitalise via spam or fake "donation" websites (http://google.com/search?q=tsunami%20spam%20site).

Of course, in a world that has equal access to technology and a secure infrastructure (eg, http://en.wikipedia.org/wiki/S/MIME), it would be a great disaster advisory channel.

Ben Low,
Perth, WA.

Solar hot-water controller

P. B. writes from Queensland (page 98, January 2005) asking about a DIY solar HWS. 17 years or so ago I designed and built a controller for the circulator pump on a 450-litre HWS. The controller included a circuit that turns on the pump if the collector temperature approaches zero, as it occasionally does, here in the Adelaide Hills. The HWS has a copper, gravity-feed tank and I made my own collector panels.

My installation required the panels to be at the same level as the tank so a pump was required. The pump and controller have been very reliable, not having needed any attention in the 17 years unlike most of the stuff you buy off-the-shelf these days.

Keith Gooley,
Adelaide, SA.

Implantable RFID tags have many drawbacks

The editorial on RFID tags in the December 2004 issue got me thinking and I would like to offer my thoughts on the whole idea of chipping. Lest I be considered a Luddite, I should preface my remarks by saying that I work in the IT industry so I appreciate technology. Oh, and I like gadgets.

I don’t think that the introduction of implantable RFID chips is the solution it claims to be. Rather, it raises more questions than it answers, the first being: are they really necessary? While they may allow patients to be identified in a hospital, that necessitates some sort of central database which contains a record of every patient to whom a chip has been issued and that every hospital has a connection to that database and permission to view the patient records.

Would this be feasible for small country hospitals? What about when you are travelling overseas? Would there be a worldwide database or would each country maintain its own? If a worldwide one, what language would the patient records be stored in? What safeguards would there be to ensure privacy of information? Who would have permission to update the records? How could its security and confidentiality be assured? And so on.

I would argue that there already is sufficient technology for identifying patients for treatment in an emergency. For example, patients could elect to have medical information attached to their Medicare number. This is subject to many of the questions I raised above, however it is a system that is already in place.

Alternatively, they could carry a card or wear a bracelet, etc which was coded in human readable format (colours, letters, digits, etc) to indicate allergies to drug families. While this would not indicate the full patient history, it would prevent a medical practitioner administering penicillin, for example, to someone who was allergic to it.

Also, do we have any statistics on the number of people who are unable to advise doctors of their sensitivities in an emergency? Of these people, do none of them have spouses or family members who could be called on?

If the chipping is to be an elective option when one is admitted to hospital, what is wrong with having a bar-coded bracelet instead? This could provide access to the same information that would be available by a chip but would remove the need to be exposed to yet another RF device. And I believe the chips don’t react favourably to NMR/NMI scanners so even in a hospital they could only be used in limited ways.

Would it assist identification of people? No more so than existing techniques. Each of us already has a number of unique identifiers, the most obvious being fingerprints. Rather than implant a chip in a newborn baby, why not fingerprint it? This is less invasive, much less painful and just as effective. Sure, the search technology for fingerprints is more processor intensive but it is not an insurmountable problem (and surely its resolution would encourage our different law enforcement bodies to work more closely together).

I don’t believe that an implanted chip would prevent or reduce identity fraud, even assuming that a "clean" database of identities could be established and maintained. Most people are basically honest and aren’t out to defraud anyone. Those who are dishonest will always find ways to do so.

Let’s consider a few ways in which this can be done. If everyone has a chip implanted, what is to stop another person obtaining an RFID scanner, scanning one or more selected individuals and having chips made with these IDs? The chips do not even have to be the same type as the "official" ones – all they have to do is return the desired code when scanned.

These fake chips do not even have to be implanted. There is nothing to stop someone carrying any suitable ID when opening a bank account. I can’t imagine a bank teller requiring a customer to roll up the sleeve of his suit in order to verify that the scan is from an implanted chip – but even then it’s not guaranteed.

What happens when the chips fail? The expected life is 20 years, more or less. If identity is determined by a chip, when it fails would a person have access to any of his assets or bank accounts? How would he prove who he was?

There is a lot more to discuss but I suspect I’ve used up my quota of words (and I haven’t even touched on the social engineering aspect). I wonder whether perhaps we are becoming too reliant on technology to solve all our problems?

Michael Byrne,
Morayfield, Qld.

AWA Radiola model number

The text adjacent to the photo of the Radiola 120 on page 96 of the February 2005 issue refers to the table model companion as a model 120; this should be model 110. Looks like the typing gremlins sneaked in somewhere along the line.

Warwick Woods,
President, Historical Radio
Society of Australia, Inc
www.hrsa.asn.au