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Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that Silicon Chip Publications Pty Ltd may edit and has the right to reproduce in electronic form and communicate these letters. This also applies to submissions to "Ask SILICON CHIP" and "Circuit Notebook".

Standby power in HD set top boxes

I bought a HD set-top box (Digicrystal) in Adelaide. As with the Tevion TEV8200 reviewed in the March 2008 issue, it also had a large standby wattage.

It appears that in standby mode it merely removes the program content from the video signal, so the box "appears" to start up almost instantly. All well and good, except my video set-up has an automatic switch box (Jonsa Ellies) to direct the most recently switched-on video signal (DVD, SD box/recorder, HD box) automatically to the amplifier and TV. As the video signal is always present, the auto switch box ignored the HD box after the first switch-on.

I returned the Digicrystal HD box and have since bought a Topfield TF7010HT HD set top box (also the same price – $198) which does switch to full standby and works perfectly.

I always find SILICON CHIP interesting each month and have built many kits since issue No.1.

David Lawrence,
Victor Harbor, SA.

Micro projects need more software explanation

Your February 2008 editorial raised some interesting points regarding magazine content. Firstly, as you have said, your magazine has many microprocessor-based designs and I dare say John Clarke must know just about everything there is to know about using the PIC processor considering the number of designs he has come up with. Do I think there are too many micro based projects? On balance, I would say no.

If I look back at my "pre-micro" magazines (I collected EA, ETI & AEM when they were published and of course, SILICON CHIP, for more years than I’d like to admit to), the range of projects was far more limited. I much prefer the diverse range of projects you continue to cover. I think it’s a credit to you that you continue to dream them up!

As an electronic enthusiast for over 40 years, I prefer to try my hand at new things. In fact I now put together more practical projects, most using the mighty PICAXE processor, than I ever did in the past.

I often use parts of a circuit you have published and modify them for my own purpose. If a micro can save dozens of logic ICs I’m all for it and I don’t consider it diminishes the excitement I get making something work.

The one criticism I do have is the lack of detailed software explanation. You seemed to put more effort into this side of your projects a few years back but I don’t see it as much any more.

I use the PICAXE chip as it’s very easy to program and you cover this quite well in your articles. If some of your simpler PIC designs gave a more thorough description of the software program and or some more tutorials on programming, it would prompt me to be more adventurous and maybe then I can try to modify your programs and hardware for my own projects.

As for your second comment on projects and technical articles, yes, I would like to see a few more articles, particularly around theory. ETI had a great series called "Lab Notes" for a few years. Even EA’s "Let’s Buy An Argument" series had lots of interesting technical topics.

Notwithstanding these comments, your current format with its emphasis on well-designed projects has outlived all of your competitors. Please, fine-tune as you like but keep the basic format as it is.

Finally, as I congratulate you on your 20 years of publication, I do wonder how you will eventually change
over to a new guard. Most of the SILICON CHIP staff appear to go back many years and I do worry that the next generation will not have the same fascination, commitment and resourcefulness currently shown. Or are there young adults out there that share the same passion? I only hope there are.

Clive Allan,
Glen Waverley, Vic.

Comment on microcontroller projects

It has been impossible to ignore the proliferation of PIC-based projects in SILICON CHIP over the last few years. And while I agree with your reasons for basing projects around microcontrollers, there are some negative spin-offs. I don’t see how PIC-based projects will offer the same opportunities to learn about circuit design, the effect of individual components on the whole or the ability to add and delete functions.

I am not suggesting that microcontroller projects should banned from SILICON CHIP! They provide a learning opportunity of a different kind, as well as projects which, as you correctly observe, would not be feasible with any other design approach. But I hope you will continue to develop and publish circuits built around conventional discrete components.

John Keitley,
Vermont, Vic.

RF projects wanted

Your Publisher’s Letter in the February edition has prompted me to write. First, congratulations on reaching 20 years. I thoroughly enjoy every issue and appreciate all the hard work that goes into designing the projects as well as the rest of the magazine.

I do like microcontroller projects as I have the software and can program PICs but it would be good to balance it out with projects that use normal components too as this helps beginners learn electronics building blocks.

I am not a radio amateur but I am interested in RF stuff, as follows. How about a project for an RF oscillator for aligning AM and FM radios? Many readers have asked for a TV field strength meter. Maybe a set-top box could be modified to do it?

Furthermore, how about a demodulator probe for tracking a video signal through the IF sections of a TV set and a piezoelectric clamp that attaches to the injector pipe of older diesel engines to fire a timing light?

Geoff Coppa,
Elanora, Qld.

Circuit Notebook: a few observations

I have a couple of minor observations in respect of two items published in the Circuit Notebook section of SILICON CHIP for March 2008:

(1) The PC cooling fan driver on page 71 has no hysteresis. I think that the fan could hunt around the set-point whenever the +5V rail fluctuates, as it does during variations in CPU load.

(2) The alternator controller on page 68 suffers from two potential problems. The first is that the battery will discharge into the field winding at a maximum rate of ~4A if the motor stops for any reason. To prevent this, I propose that the field section be powered from the alternator’s exciter diodes. The initial self-excitation current could be provided by a momentary start switch as in this example at:

The second problem is that a flat battery will be subject to the full output of the alternator which could be 40A or more. I propose current limiting the output as in the previous example (suitably adapted for a high-side field winding).

Franc Zabkar,
Barrack Heights, NSW.

PC boards still use
Imperial measurements

I read with a touch of amusement some of the readers’ comments about being aghast at the magazine expressing some parameters in the old Imperial units and not in current trendy metric.

Domestic TV sets are expressed in "cm" (diagonal screen size) but an odd thing is that if you take the back cover off the TV set, the actual size of the TV monitor may have, embedded in raised glass (as poured from the factory), the size expressed in inches.

All computer monitors are proudly in inches and of course, 99.99% of all integrated circuits have 0.1-inch pin spacing.

Industrial electronics remains in Imperial, as does aviation which still uses feet above sea level (and pounds of fuel). The Navy use fathoms below the keel and I note that the Bathurst 500 (or whatever) motor race now refers to pounds of fuel and the competitors are not permitted to use litres any more.

A large circuit board company here in Sydney had to close not long ago because they accepted a contract for a large order from a European Telco with all dimensions of the circuit board expressed in metric, with the result that their CNC drilling machines could not drill the boards within the contract time, due to the slowness of a 0.1mm XY axis movement (rather than the normal 25-thou hop, step and jump). 99.99% of all CAD-designed circuit boards are SI’d in Imperial not metric.

So do not be so quick to discard the Imperial measurements when there is really nothing equivalent in metric.

Bob Barnes,
RCS Radio Pty Ltd,
Chester Hill, NSW.

Hardware still comes
in Imperial sizes

I was bemused by the gripes of your correspondents, T. Robinson and Ray Smith, regarding the triviality of a few Imperial measurements in your February 2008 issue. Mr Robinson, in particular, was rather childish in my opinion, by threatening to stop buying the magazine if he sees Imperial references. I bet he wouldn’t!

Personally, I find "litres per hundred kilometres" to be useless. We buy fuel by the litre, so why not a more useful "kilometres per litre"? I shouldn’t have to fool around dividing figures.

Mr Robinson asks who still uses Imperial units. Bunnings hardware stores are full of them! Buy a length of plywood and you can get 1800mm. Why not 1.5 or 2 metres? Because 1800mm is approximately 6 feet. And it used to be 1840mm incidentally, which is almost precisely 6 feet.

Or you can buy 2400mm (rather than the more logical 2.5 metres) which is the old 8-foot length. Better yet, buy it by the lineal metre, which is 3.66 metres or dare I say it, exactly 12 feet; an astonishing coincidence!

Having a Stratco catalog in front of me, I see roofing sheets with a width of 762mm – down to a measly two millimetres? The weird thing is that 762mm is exactly 30 inches or 2.5 feet! Wall siding at 1220mm is exactly 4 feet. Why are ladders 1.8m or 2.4m instead of 2m or 2.5m? Because they used to be 6 feet or 8 feet and basically, they still are! There are lots of odd metric sizes with some equalling precise Imperial equivalents, and the rest being near as hardly matters. We still buy Imperial sized items; they just give us metric equivalent dimensions.

Mr Smith could go to an auto parts store where he can buy a 2000 lb winch or view many other products having Imperial references – not equivalents but delineations. Read a car magazine or watch Top Gear and it’s nearly all MPG, cubic inches and foot-pounds of torque. Mr. Smith should also check the diameter of his car’s wheels! And might I add, it wasn’t all that long ago that TV screens were still quoted in inches, with metrics more of an addendum. Many rulers and tape measures are still marked for both systems and I expect forever will be.

I love metrics. I’d much rather measure down to the millimetre than 13/64ths any day. It is so beautifully simple: a litre of water weighs one kilogram and one thousand of them will fit into one cubic metre and weigh one tonne. And there is the simplicity of the millimetre and its decadic multiples. But nobody should be so blind as to think that the Imperial system is nowhere to be found. Look around, it’s everywhere, and it won’t go away any time soon.

Paul Carson,
Westmead, NSW.

Interesting circuit doesn’t work

I had occasion to build an expanded scale voltmeter recently. I remembered you had already published a very clever version of this, in November 2006, on page 7. This was a reprint from the "Circuit Notebook" June 1995, which I didn’t have.

I built it in about 10 minutes and it didn’t work! I took some measurements and pondered for a while; it’s a bit of a brain-teaser. The problem is that the 7805/7905 devices are designed to source current to the load, and in this case the current is flowing the other way back into both the regulators. This causes the regulation to fail and the output terminal voltage to rise (with respect to its GND pin).

The 7905 was much worse than the 7805. With only 1mA flowing backwards into its output pin, the voltage rises to over 6V. I tried a number of regulators from different manufacturers but they were all the same.

To explain the problem, if we assume 12V is applied to the input, the output pin of the 7905 will be sitting at +7V (ie 12V-5V) with respect to the input minus. This +7V has a current path via the meter to the 5V output of the 7805, thus tending to pull the 7805 to a voltage above 5V. The same happens to the other regulator.

In order to fix it, you need to ensure current is always flowing out of each regulator. This requires a load resistor from each output pin to the respective GND pin. I would suggest that, to keep it stable, about five times the current should flow via this resistor, ie, 5mA through the resistor and 1mA through the meter.

I am surprised that Wal Douglas claims to have used it successfully for years.

Bruce Boardman,
Telstra – Wireless Engineering & Operations,
Sydney, NSW.

Comment: well you have uncovered a mess. We did not spot the problem in 1995 and we didn’t spot it again in 2006!

Comment on Prius battery life

I can’t believe you published Gorton Drennan’s letter without a warning about its accuracy. His assertion that the Prius’ battery has to be periodically replaced is not true. Toyota claims the battery will last the life of the vehicle. They achieve this by ensuring the battery is never fully charged or discharged.

Mr Drennan has confused the expression "limited capacity" with "limited life". The limited capacity (4Ah) and extra weight of the Prius means that when it is used for any purpose other than city driving, it will not achieve better mileage than many conventional petrol vehicles.

Paul Smith,
Port Macquarie, NSW.

Comment: in fact, the battery does not last forever. We recently had a correspondent who purchased a grey-market import (ex-Japan) 1999 Prius in which the battery has failed. Toyota apparently will not help because the car was not purchased from a Toyota dealer.

Non-metric standards

I was about 10 when we went metric, so became conversant in both metric and Imperial systems. I became familiar with SI units through physics and can happily work in several different number bases.

However, I have never been able to get familiar with kilo-parsecs per 100 litres (or whatever) or hecta/megapascals. MPG and PSI still hold a lot of meaning to me, as well as for a large number of other people. Pray you never need to work with old English equipment, when it seemed a new thread was invented for every application!

If seeing MPG on a magazine cover is so distressing to
T. Robinson, I hope the editorial staff at SILICON CHIP never show a valve project on the cover again. Presumably, medical assistance would be required!

Brett Cupitt,
Ashfield, NSW.

Comment: you should realise that the metric thought police will be around to your place shortly.

DDS VFO project feedback

I have recently acquired an RCA 89P WW2 (1942/3 build) 2MHz - 20MHz AM crystal-controlled transmitter. This was used on WW2 warships and was built like a rock, weighing in at a shade under 350kg. This is a long-term restoration project and part of the idea was to convert the crystal oscillator to a VFO.

The DDS VFO article in the March 2008 issue was exactly what I was looking for, so I have embarked on acquiring the parts. The display interested me as I have a few other projects that could use this approach, so I did a search using a popular search engine.

I read around 30 or 40 hits, finding prices ranging from $9 to $61. The $61 was in the first few hits, the $9 in one of the ones much further down. (Tip: when using a search engine, look beyond the first page of results, especially if you want to find the good prices.)

I was pleased to find an Australian supplier of the display (Peakhour at I ordered a few for the various projects, paid online and they were delivered in two working days. One of the units was damaged in transit and I emailed Peakhour with a photo and they dispatched a new unit on the same day. Ah . . . customer service as it used to be still exists.

Matt Howard,
Melbourne, Vic.

Video on soldering SMDs

Following your article on soldering SMDs in the March 2008 issue, there is an excellent video on this topic at

Robert Ellis,
South Oakleigh, Vic.

Comment: thanks for the link although we have to say that we would rather hear the Americans refer to solder as "solder" instead of "sodder"!

Watermark logos
are useful

On page 4 of Mailbag in the April 2008 issue, the writer is bothered by television stations identifying their transmission. In fact, I actually find this handy.

Very few modern television sets have channel identification numbers on the front panel. The last TV I owned which showed which channel had been tuned was a 1998 Philips. Newer TVs need a press of the remote control to reveal the station you’re on whereas the watermark allows me to see the channel at a glance.

I never find the logos intrusive, as they are "translucent". One can still see the picture behind it. As for sticking a grey paper over the watermark, what does that achieve – only a worse distraction?

The watermark is handy when the program memory location numbers don’t equal the channel numbers. For example, my mother has been in hospital three times in the last few months. The TVs there don’t display actual channels, only program memory numbers. When I subscribed to cable TV, watermarks were used frequently – including when viewing a recorded program; you knew where you’d recorded it very easily.

P. Smith,
Albert Park, Vic.

PICAXE circuits are not
the only winners

I have noticed a big trend towards PIC-based projects. More worrying is the appearance that the only circuits that have any chance of winning the Circuit Notebook prize are those that include a PICAXE. I have been put off submitting my ideas and circuits because of this. I wonder how many of your readers are in the same boat.

I have built a lot of projects in my day and have found that micro-controlled projects tend to do what they are claimed to and have greater reliability. Is this a good thing? I’m not really sure.

I remember building the 240VAC light chaser some years ago. It was a big kit with some 300-odd components and it took a couple of days to build. What I remember most is what I learned about fault-finding in getting the thing to work. It was most frustrating but in the end rewarding and I did it myself!

Maybe we need a mix of projects and some more "back to basics" articles on microcontrollers for us old guys out there.

The other thing I have noticed with PIC kits is that their price is generally between $70 and $100. I sometimes find it hard to see the value in kits like the "PIC-Based Battery Voltage Monitor" (SILICON CHIP. May 2006) for cars which retails for around $70 when you can buy an LCD module for around $20 that will do the same job. Hobby electronics is no longer cheap and some people will find it hard to justify the growing cost of kits. So maybe some low-cost projects could be added for these readers too.

Paul Dawson,
Waurn Ponds, Vic.

Comment: with regard to the matter about Circuit Notebook items only being winners if they use a PICAXE or PIC, we can see how you could easily get that impression since there have been so many winners along those lines in the last 12 months. However, that is not intentional and we have had non-PIC winners in the March, April, May, August & December 2007 issues and in the March & April 2008 issue.

In judging the winner each month, we are looking for ingenuity and if the circuit involves a PIC/PICAXE and is ingenious, then so be it. But we would love to have a larger variety of circuits – if you have some, please send them in. Having said that, there is no guarantee that they will win or even be published (we can be hard to please . . .).

As far as kit prices are concerned, if you look at them in real dollar terms, they are still good value for money compared to kit prices of 20 or more years ago.

Impedance bridge does not measure impedance

I must comment on the Impedance Bridge in Circuit Notebook on page 70 of the March issue. I realise that this is not your circuit but one look at the actual bridge circuit shows that it can only be used for resistance measurements (much simpler to do with an ohmmeter). If the unknown is in any way reactive, the bridge will not balance, or be correct.

For instance, if the unknown is a pure reactance of half the value of the multiplier (ie, an impedance ratio of 2:1), the minimum balance reading (which will be very broad and some 40% of the input voltage) will occur at a 4:1 ratio of the range switch.

As is well-known, impedance bridges must have a compensating reactive component in one of the branches. Even for resistance measurements there is a problem as, with the values shown, the maximum resistance that can be measured is 10kW! And of course, no need for choice of frequency. Sorry to spoil the effort.

Charles Borger,
Pascoe Vale, Vic.

Old inverters had lots of interference

In the February 2008 issue, Rodney Champness had an interesting article on old inverters. I grew up on a property in northern NSW with a 32V lighting plant. The important thing to remember about these installations is that they were just that: "lighting plants".

Owners often forced them to do more than just illuminate, eg, operate power tools, etc. Most could not oblige unless you ran the generator as you worked.

We had one of the little Ferris inverters as illustrated in the article. Having a 33kV line running right past our home, we lived in hope of "Town Power" and so had some small 240VAC appliances. The inverter was bought to operate 240VAC electric razors and a battery/mains portable radio. We soon found the interference Mr Champness mentioned.

On enquiry to Ferris we were told that these inverters were not intended to operate radios. Their attitude seemed to be that if one wished to use radios, it would be better to buy an inverter or genemotor that operated at 50Hz. These were available but were very expensive and inefficient.

Of course, at a vibrator frequency of 100Hz, the rather rudimentary power supply filtering in battery/mains radio sets would not have been very effective. But like Mr Champness, I think that the RF filtering could have been better.

Bruce Bowman,
Ainslie, ACT

Microcontroller articles are appreciated

As a regular purchaser of SILICON CHIP, I am responding to your February 2008 editorial where you ask for comments re future articles. I am a farmer in East Gippsland, Victoria. I expect that I would be classed as something of an experimenter in electronics but I do take that further to a point where I build helpful and reliable devices to assist around our home and our farm.

The trend towards microcontrollers: I really appreciate these articles, so keep them coming. It would be a retrograde step for you to ignore their existence and to replace their function with complicated circuitry. One uses an 8-pin 555 timer without a second thought so why single out, say, the 8-pin PICAXE or PIC as being too complicated?

If you set out to ignore them, you might as well close the book on solid-state and go back to having everything operating with thermionic valves. I consider that PICs are incredibly useful and foresee utilising them in various projects in the years to come.

However, I am continually surprised at how many people in the electronics business know almost nothing about the PIC programmers. And I have to admit, were it not for Stan Swan’s series of easy-to-read and understand articles on them, I would have had a lot of difficulty in coming to terms with them.

Even with his articles, I was sure that I was going to seriously damage my computer with the serial port link to the device but that didn’t happen. The BASIC language they use is not hard; in fact quite fascinating to create, to program into the device and to see whether it does what you want.

In looking for a direction for the future, I think that you should, no, you must publish very elementary projects like Stan’s on a regular basis. There are a lot of people to be brought into the PIC fold and the more complicated projects will not entice them into it.

Emphasis on electronic projects as against articles about new developments: I see your magazine as a constructional one and that your present approach is the best. I recall ceasing buying Electronics Australia because it had too much on new developments and nowhere near enough comparatively simple constructional projects.

Instructional detail: if one decides to undertake a particular project, you just cannot have enough detail. Your writers do an excellent job and it would be a shame to see that reduced in any way. The consequence of too brief a description is failure of the project.

Internet: I live in an area where rural telephone wires have made ground access to the internet pretty much out of the question. Our only solution would lie with satellite coverage but the cost has been a problem for us. So, with no access, when an article suggests that part of the project information, say for example, a PIC program, should be obtained via the internet, we are in trouble.

Still, I expect that we are the exception and there will be an ever-increasing reliance upon it for such references. Perhaps you could give some consideration to those folk who are for whatever reason, without internet access.

Keith Traill,

Buchan, Vic.

Comment: we can make software available on CD-ROM for $9.50 including postage.

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