TV signal strength meter wanted
I would like to know where I can buy a kit or a ready-made
meter to measure signal strength for TV channels. My neighbour used one from his
work to help me locate the best position for my TV antenna. It was quite a
whizz-bang gadget, preset for all the TV channels, and he could tell me
immediately the dB for all the channels.
Have you people had experience with a simplified version of one
of these? Also, where can I get the carrier frequency settings for all the TV
channels? (G. B., Melbourne, Vic).
We have not produced a TV signal strength
meter. Many installers find that a portable TV is more convenient. Not only does
it give an indication of signal strength but it also shows the presence of
ghosting and enables the antenna to be oriented for best reception.
TV station frequencies can be found as a PDF file at:
Power reduction to derated transformers
I want to ask if it is possible to use 40-0-40V 300VA
transformers for the Mosfet amplifier published in SILICON CHIP,
August 2001. This amplifier is essentially a revamp of the old EA Pro One
amplifier. In both articles (EA & SILICON CHIP), the design specifies a
45-0-45V 300VA transformer. I have an old EA Pro One and want to restore with it
40-0-40V 300VA transformers.
Can you estimate the output power with these transformers and
if the 0.8V quiescent current set-up would be the same? (E. Z., via
Power is a square law. Since you are
proposing to reduce the supply voltages by a factor of 12% (40/45 = 0.88), the
power output will be reduced by 21% (0.79). This will be a barely audible
difference of -1.02dB. The biasing will remain the same.
Sniffer probe avoids crystal loading
I have just successfully built the Low-Cost 50MHz Frequency
Meter from the October 2003 issue (from a Jaycar kit). May I suggest a follow up
article on using this meter, describing probes and servicing techniques for
In conversation with Bob Young (one of your contributors), I
learnt that Bob described the construction and use of a "sniffer" probe for use
around radio control transmitters that allows frequency measurements to be made
without connection to the circuit under test, avoiding loading circuits that
could give an invalid reading.
Another useful probe would be an "active" probe that would
increase both the input impedance and sensitivity, especially at the high end of
the unit’s range.
I am using a CRO probe with the Frequency Meter but have found
that on probe setting ‘x1’ it loads the crystal oscillator circuit in the radio
control receiver, causing it to cease operating while on ‘x10’ the reduced
sensitivity means the meter reads "Await Signal". (J. K., Broulee,
We showed how to make a sniffer coil on
page 27 of the December 2003 issue. Also you might like to look at the active
sniffer probe described in June & September 1988.
When checking crystal oscillator circuits with a CRO probe, try
connecting it via a 1kΩ or
2.2kΩ resistor. This can be
enough to stop the probe capacitance from loading the crystal, while still
letting enough signal through to let you make the measurement. It can be useful
when using a scope too.
Phono Preamp For Surround Sound Amplifier
I recently purchased a surround sound receiver amplifier and
using it to watch and listen to movies and music DVDs is a real pleasure. My
request relates to the missing phono input on the amplifier. I have a reasonable
collection of LPs that I would like to occasionally listen to, even if only to
amaze my two sons with the big black disks.
Do you have any plans to design a phono preamplifier for a
magnetic cartridge which would feed a line input on a hifi amplifier? For some
reason, many hifi equipment manufacturers no longer include this in their
designs and a project like this would be fun to make and extend the use of most
modern hifi equipment. (J. C., via email).
Three designs are relevant: the Universal
Preamplifier in April 1994; the RIAA Preamplifier in March 2002 (these are
essentially the same circuit but have different PC boards); and the LP Doctor
(includes click suppression) in January & February 2001.
Line level switcher wanted
Has your magazine ever described a Line Level Switcher? I have
purchased a mini stereo system and it only has one line input and no line
output. I have a VCR, DVD, CD recorder, tape deck and a Mini-Disc recorder; as
you can see, too many for one input.
I would like these devices to go to the mini stereo system and
also be able to record from one device to the other. (R. M., via
Have a look at the Video Switcher in the
June 1992 issue. It can handle A/V signals from three sources.
LEDs for an LCD video projector
I was intrigued by your article on human-powered LED torches in
the February 2004 issue. I have been contemplating building a LED-based home LCD
projector. Traditional projectors use very expensive and hot running lamps,
necessitating special power supplies and cooling. LEDs seem an ideal solution,
if they can be made bright and focused enough.
Using this technology, it should be possible to build a
projector for much less than current commercial units. Is this something that
SILICON CHIP magazine could investigate? (J. H., via
LEDs have a very long way to go before
they can replace the halogen lamps used in projectors. We doubt if they will
ever be bright enough for that application.
DIY humidity sensor for PICAXE datalogger
I was very interested to read your project on the PICAXE
Datalogger in the February 2004 issue. I have made enquiries about various types
of sensors from some companies but have found that they are expensive and high
Do you know of any publications for the enthusiast that explain
how to make your own sensors – eg, relative humidity, leaf wetness, etc? (M. H.,
We do not have any information on DIY
sensors although some clothes dryers have a humidity sensor based on horse hair.
It may be possible to use such a sensor although we are inclined to think that
it would be fairly imprecise in its action.
Crossover Wanted For 2-Way Loudspeaker
I have recently purchased a JV-80 loudspeaker kit from Jaycar
Electronics and would like to know if it is possible to make a 2-way speaker
using one of the tweeters (D26 NC-15-06) and one of the woofers (P22 WP- 01) in
a 35-litre enclosure.
If so, can you tell me if it's possible to use the crossover
network (CS-2580) supplied with the kit, with suitable modification for a one
tweeter and one woofer combination, or do I need to use a completely different
Also, would one of the ports recommended for the JV-80 kit (ie,
66 x 140mm) be suitable for a 35-litre enclosure? (L. J., via
It is certainly possible to
build a 35-litre system with one of the woofers and a tweeter. It would then be
a 4-ohm system, which may or may not be a problem for your amplifier.
However, you cannot use the existing crossover since it is
optimised for an 8-ohm system. As a stopgap, you could try connecting the woofer
right across the full signal but still connect the tweeter via the existing
Nor can you just use one of the ports as is. The enclosure
needs to be designed using a software program such as BassBox to calculate the
Bigger bass for school PA system
I have a question involving a 100V line PA system that is
installed in a school sports hall with speakers down one side. A team wants to
use them to play up-tempo music for warm-ups and maintaining training intensity.
As the speakers are evenly spaced up and down the wall on one side of the
courts, evenness of volume is OK but they lack the needed bass punch. So I would
like to install a subwoofer that can be taken away when not required.
Is it possible to insert a passive subwoofer crossover directly
into the 100V line, feed the high-pass signal to the existing speakers and have
the low-pass signal feed a 100V to 8-ohm transformer and onto the sub? For a
100W amplifier, what size transformer would you recommend for the sub tap? Will
different taps change the impedance and so the crossover frequency? Or do I need
two 100V to 8-ohm transformers; one stepping down from 100V to ‘speaker level’,
then through the crossover and, on the high-pass side, a second stepping it back
up to 100V?
Or is it best to have an electronic crossover at signal level
and bi-amp it – one feeding the existing speakers and the other feeding the sub?
Is there any problem with the sub amplifier output being stepped up to 100V and
then back again at the subwoofer speaker?
Are there any traps for someone who has enough knowledge to
think they know what they are doing but not enough to know they actually don’t?
Being a school, cost is going to be a factor so I’d like to go with the least
expensive option that works.
By the way, have you considered doing an article on 100V
line-distributed PA systems? Ideally the article would at least cover the
background of why 100V and why there are 70V and other voltage standards. (T.
H., Calwell, ACT).
We would not try any passive subwoofer
system. It would be better to take the 100V signal and feed it to a separate
powered subwoofer. This would be much easier to set up and disconnect. You might
like to have a look at the subwoofer controller project in the December 1995
Thanks for your suggestion for an article on PA standards, etc.
We have already briefly covered the subject in an article entitled Plastic Power
PA Amplifier in the March 1997 issue.
Bypass capacitors for Multi-Spark CDI
I have a problem with the Multi-Spark CDI system featured in
"Electronic Projects for Cars, Volume 2". Here in Thailand I cannot get the
10μF 63V MKT capacitors for
decoupling the DC supply to the transformer (see page 82, Fig.2). Is it OK to
change these to 1μF 63V MKT?
(P. A., Amper Muang, Thailand).
10μF capacitors must be used rather than 1μF. These are MKT types and are necessary
for correct bypassing of the high frequency switching artefacts on the DC
supply. Alternatively, low-ESR 10μF 25V electrolytics could be used.
Farnell Electronics sell these MKT capacitors, Cat. 814-155.
Refer to: www.premierfarnell.com
Dimmer for 900W incandescent lamp load
I have a 900W incandescent lamp load and I wish to place a
dimmer on the circuit. The existing switch is on a multi-gang plate so the bulky
commercial dimmers are not an option.
My local electronics retailer in Adelaide suggested using a
linear pot and a 15A Triac. I purchased these parts along with a heatsink but I
am confident that more components are needed. So I need a circuit diagram and
parts list to do the job. (S. H., via email).
A 15A Triac is not robust enough for such
a load as lamp failure will cause the Triac to blow. Have a look at the High
Power Dimmer in the August 1994 issue.
Increased rating for electronic load
In the 50W Electronic Load (SILICON CHIP,
September 2002), could the addition of an extra MOSFET in the output stage
increase the current capability to 20A? (B. P., Palmerston North, New
Increasing the power handling capability
of the electronic load (while maintaining reliability) is a little tricky. A
second MOSFET in parallel with the existing device will increase power handing.
However, it’s not easy to get the two devices to share the load equally.
In fact, we built a 100W prototype using multiple devices but
eventually had to scrap the idea, as very large (and expensive) source resistors
were required to force equal current sharing.
Having said that, you could try one of these ideas: (1) use a
much larger MOSFET (such as one of the "DICE" packaged devices); (2) try MOSFETS
specified for audio use. These are designed for use in their linear regions, so
are likely to work better in this application; and (3) use matched
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
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Publications Pty Ltd disclaims any liability for damages should anyone be killed
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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
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