Philips humidity sensor for weather station

I am trying to build the "Three Function Weather Station "featured in the April 1993 edition of SILICON CHIP. My problem is that I cannot source the Philips 232269190001 humidity sensor. Would you know where I can obtain one? (J. E., via email)

Philips sensor is not readily obtainable now but a similar unit can be obtained from Farnell Components. Its order code is 414-669 but unfortunately its cost is $41.33 plus tax. Phone 1300 361 005.

Bridge amplifier for LM3786s

I am referring to the bridge amplifier published in "Cir-cuit Notebook" in the June 1996 issue of SILICON CHIP. Does it actually work OK? I see that you have used two LM3886 chips but I have two LM3876 instead. Will these work as direct replacements and what do you think the power output would be? (J. L., via email).

Of course it works. Would we have published otherwise? Yes, you can use LM3876s without changes but you will get more power from LM3886s because they are more suited to 4Ω loads. This is im-portant because if you are using an 8Ω speaker, each amplifier in the bridge will "see" 4Ω.

Speed control for electric outboard

I would like your advice on whether the motor speed control published in the June 1997 issue is suitable for my application. I wish to control the speed of my 12V electric outboard motor. I have tested its power consumption when in water, the amperage being around 19.7A. I understand that the motor will draw more current than this when under a heavier load (eg, when going though weeds).

Because of this, it has been suggested to me that the modi-fications for high current use, as outlined in the article, may not be sufficient. Ideally, I would like to run a more powerful motor from this circuitry so I would like it to handle up to 50A. I will be happy to have it run my existing motor, however.

If this circuit cannot be modified sufficiently, can you suggest an alternative? (D. D., via email).

The circuit as it stands will not work. You would need to use at least four Mosfets, each with their own 4.7Ω gate resis-tors and you would still need heatsinking and you would need to "beef up" the copper tracks to the Mosfets to make sure they can handle the current.

A better choice might be to use the 50A speed controller featured in the May 2000 issue but you will need a separate 5V rail to run the ZN409 and the accompanying pulse generator cir-cuit. We can supply the May 2000 issue for $7.70 including post-age.

Help wanted with DVD audio levels

I find the sound on DVDs is so dynamic that I frequently have to adjust the volume throughout the movie because of wide audio variations. I built a SILICON CHIP Surround Sound Dolby Decoder kit which works fine and I would like to know if there is a kit available to tame the audio dynamics down without affecting the surround sound decoding.

I am using a PC-based DVD and feeding the sound card output to the Dolby Surround Decoder. (C. B., via email).

Have a look at the CD Compressor we described in the June 2000 issue. It is designed for exactly this application.

Low-fuel indicator flashing, not steady

I recently purchased and built a Low Fuel Indicator, as described in the February 1993 issue of SILICON CHIP. While the kit works, it has some "features" that do not seem to be in accordance with the circuit description.

I have fitted the unit to a Datsun 1200 which has a bi-metal strip fuel gauge and a resistance wire sender unit. The gauge shows full when the sender unit wire is grounded.

When the ignition is first turned on and the fuel tank is less than half full, the indicator light flashes a few times and then extinguishes.

I have adjusted the unit so that the indicator lights when the gauge is about three quarters empty, with the car stationary. When the tank gets to about 7/8ths empty and the car is being driven, the unit starts to flash on and off, slowly at first (perhaps once a kilometre) and the flashing rate increases to about once per second as the tank approaches empty. It does not seem to ever come on permanently, despite almost running out of fuel and having less than two litres left.

I would appreciate any suggestions for a cure, although the unit is still useful and serves its purpose (stopping my son from running out of fuel, which he has done four times in one month). (G. P., via email).

It sounds as though the 220µF capacitor is open circuit or is the wrong value. Try changing it.

Speed control for a centrifuge

Can you please help with the following? I have been asked to repair an old centrifuge. The unit would not regulate the speed of the drive motor. The electronic control is unservice-able. It is a small pack similar to a light dimmer but rated at 700VA (watts). Standard light dimmers are normally 300VA. So I built the universal motor speed controller kit from the September 1992 issue.

The problem is that the controller will not reduce the speed to a low level. I would have thought that it would regulate from 0 RPM to full speed. The centrifuge needs to run from 0 to about 3500 RPM. The motor does not start until the control knob is about 1/3 advanced and jumps to about 1/4 full speed with the preset at zero ohms. Adjusting the preset starts the motor spin-ning at a lower setting of the main control but the speed is about the same.

I have added the 1kΩ resistor in place of the link and tried increasing the value of this resistor. I also tried in-creasing and decreasing the value of the 150kΩ resistor, all with no luck! I have checked all components and they seem OK. The original control says "700VA Triac control, 240V only" so a Triac control must be able to do the job. Can you suggest a cure or better mod? (R. M., via email).

The September 1992 design will not regulate from zero to full speed. Essentially, it applies about 170VAC (maximum) to the motor so at best it will run at about 75% maximum speed at no load. At the low end it may run at about 20% of maximum speed but not smoothly; ie, it will "cog".

For a wider range of speed control, you need our full wave design published in November 1997. This is rated at up to 10A. We are assuming, of course, that the motor is a universal type (ie, series wound with brushes).

On the other hand, if it is a small "shaded pole" motor it might work with a modified light dimmer circuit with a Triac rated at 6A or more. It will need a snubber circuit across the Triac to ensure that it will "commutate" properly with an induc-tive load. A circuit along these lines was published as the Speedi-Watt controller in the December 1987 issue of SILICON CHIP.

While the kit is no longer available you could modify a commercial light dimmer along the same lines.

Fluorescent starter makes buzzing noise

I built the Fluorescent Starter from the August 1996 issue. It works fine and is able to start a fluorescent light almost instantaneously. The problem is that it buzzes after it starts the fluorescent light. There is no RFI emitted, just the buzzing which makes it irritating. Is there a way to eliminate the buzz? Could I connect a capacitor or retrim the circuit RC network to reduce or increase the timing and thereby eliminate the buzz sound? (M. O., via email).

A buzzing sound can be produced by the Mosfet and is due to its rapid switching. The same effect can be heard with SCRs, Triacs and the power transistors in high power audio amplifiers when handling the higher frequencies.

Having said that, it should only buzz while starting and then any buzzing would be louder from the ballast as the starter is then out of circuit. If you are concerned with the buzzing during starting then the only way to stop it would be to fully encapsulate the unit in epoxy resin.

CD input for 50W module

Will your 50W amplifier module (March 1994) accept a CD input without pre-amplification and still produce full power? If not, can I increase the gain so that it will? If so, how? (C. C., via email).

Input sensitivity for full power is 1V so a CD player could overdrive with its 2V (max.) signal. Fit a 50kΩ log pot and 22µF bipolar capacitor from the wiper to the amplifier input to solve the problem.

Cheap headlight dimmer wanted

I am looking for a way to dim my headlights to act as park-ing lamps. Just wondering if there is a cheaper way than the $50 Jaycar kit for the Daytime Running Lights project featured in the August 1999 issue, to run them at about 50% brightness? (A. L., via email).

Have a look at the 10A Speed Controller kit we published in June 1997. It is available from Jaycar (Cat. KC-5225) for $21.05. You will need the dimmer modification featured in the October 1997 issue.

Electric fence controller for plasma display

I am about to build the Electric Fence Controller from the July 1995 edition of SILICON CHIP, mainly because I have all the parts at hand. I wish to use it as a small plasma display using a light globe. Is the 5kV output high enough to get a good display or is it possible to increase the output? (J. B., via email).

5kV is not enough for a plasma display; you need around 10kV to 20kV and the frequency of the sparks would have to be greatly increased to around 100Hz. To do this, you would have to modify the electric fence circuit so that it was the same as the Jacob’s Ladder circuit published in September 1995. The modified circuit pulls a lot more current and produces very hot sparks. Even so, it may not be enough for a really good plasma display.

You might want to have a look at the plasma display article published in the September 1998 issue. We also published articles on plasma displays in August & November 1988. We can supply back issues or photostat copies of these articles for $7.70 each, including postage.

3-55L loudspeakers blown up

Some years ago I built the Studio 3-55L 3-Way Loudspeaker System described in the September 1991 issue. I used to enjoy listening to them but have managed to blow them up a few times. How? I was using these speakers with an amplifier and preamplifier described in another magazine.

In the beginning, too much volume blew up the Magnavox 12WRs then I bought a new Samsung fridge that creates a pop when the compressor switches off. These pops helped to destroy the second set of 12WRs, so I replaced them with some better quality higher-rated speakers from Jaycar (Polycone CW-2130).

I also installed some varistors on the 240VAC input side of the amp-lifi-er and some polyswitches on the output side of the amplifier. But the pops didn’t stop. Later, high volume has blown up the new woofers and partly cooked the crossover networks.

I would like to know how to filter out the pops without having to rewire this house – I’m still only renting in the bush and I had to move the household pressure pump to another circuit for the same reason (induced pops when switching on and off).

I would also like to know how you think the 12-inch Re-sponse 200W Polycone (CW-2138) or Carbon Fibre (CW-2145) Woofers from Jaycar would go with a set of heavy-duty crossover networks in these boxes?

I have read every issue of your great magazine since you started in 1987 and find it very informative and useful. You have designed some great projects over the years. (C. L., via email).

First, the fridge should not cause loud pops in the amplifi-er. It suggests that the amplifier or pre-amp-lifier is very prone to EMI (electromagnetic interference). Possibly the amplifier is unstable. Try using a power filter on the input to the amplifier and make sure that the speaker and input leads are well away from the fridge. Second, run the fridge on a different power circuit if you can.

Really, the popping problem lies with the preamp/amplifier and not the speakers. Of course, if you regularly overdrive the amplifier it will tend to blow the speakers regardless of their power ratings.

Yes, you can use the suggested woofers in your sealed boxes but we really can’t say which ones would be better or even if they would give close to optimum performance. To get the best performance it is necessary to do a full redesign using the Thiele-Small parameters, etc.

It should not be necessary to change the crossovers. If you have blown them, it suggests that you are driving the speakers to really excessive levels.

Compact fluoro inverter for horse float

I would like to modify the Compact Fluorescent Lamp Driver kit (SILICON CHIP, July 2000) to operate off a 24VDC supply. We have a horse truck that has on-board 24VDC only, with ample cur-rent. We use this truck to transport horses to venues. We also sleep in the truck.

Some venues we attend don’t have 240V avail-able and we are forced to use torches and Tilly lamps at night. My electrician friend and I have thought about wiring up the truck using CFLs as a primary lighting source in the horse area and sleeping quarters. These are thoughts on modifying the above kit.

All the ICs would have to run from a 12V regulator and the centre tap of the primary of T1 would be taken to +24V via L1. The voltage rating of the 2 x 4700µF electros would be upgraded to say 35V. I also wonder if the primary windings of T1 have to be increased by a few turns? (B. B., Mittagong, NSW).

As you state, it will be necessary to run the ICs from a 12V regulator. The inverter transformer will need to be rewound with more turns on the primary; ie, 2 x 12 turns instead of 2 x 6. The 16V zeners should also be increased to 39V.

Jumbo LED clock has faulty displays

I am having problems with the Jumbo LED Clock in the March 1997 issue. I purchased the displays and most of the ICs from Jaycar. The hour display blinks at one-second intervals, the third display does not light at all and some segments in the fourth display do not light up. I have replaced most of the ICs. (D. B., Port Macquarie, NSW).

The display that does not light is likely to have an open circuit in the connections to the common cathode line, pins 3 & 8. The missing segments for the fourth display will be associated with open circuit connections to the relevant 390Ω resistors, the display pins or the interboard connections.

We do not think it should be necessary to replace any ICs as most of the problems are likely to be due to missed or bridged solder connections. You may need a magnifying glass to carefully inspect the PCboard for these faults.

Higher voltage for 500W amplifier

I have a question about the 500W amplifier published between August & October 1997. I have a pair of 65V 1kVA toroidal trans-formers. I got them at a very special price. What would happen if I used these instead of the 57V 800VA toroidal transformer recom-mended for the project? What modifications would I need to make to the components to accommodate the extra rail voltages?

I note that the amp is basically an up-rated version of the 175A amplifier of April 1996. This would seem to suggest that there is room for slight modifications to cope with the extra voltage. (P. O., via email)

Sorry but 65V is just too high. You would be increasing the overall amplifier dissipation by more than 30% and there is no way the design could cope with it.

Electric fence output voltage uncontrollable

I have built the Electric fence kit from April 1999 and when I try to set the capacitor charging voltage via VR1 (to 340V) I find that it shoots all over the place (starting from about 56V, up to 1000+ and off the scale of my meter) with very small changes in the pot position, and cannot be set. I’d be grateful if someone could suggest what might be wrong. (J. N., via email).

Check the components around VR1. Check that you have 4.7V across ZD1. Also check that you do not have high resistance in the wiper of VR1. Our tip is that the bottom leg of VR1 is open circuit.

Data logger for pH readings

I have made the pH meter for swimming pools described in the April 1988 issue and it all checked out OK. I would now like to connect the pH meter to an ADC so the readings could be timed and stored using a computer. I have searched the web for a cir-cuit which could read microamps using an ADC but with no success. (J. R., via email).

Have a look at the Mini-log, an 8-bit data logger published in the July 1996 issue. It has an ADC and is based on the Basic STAMP II.

Notes and Errata

Structured Cabling Systems (August 2000): in the Cat5 cable diagram on page 74, pair 1 (blue/blue-white) are shown reversed - blue should connect to pin 4 and blue-white to pin 5 of the RJ45 jack. Fortunately, this diagram would rarely be translated into practice because the vast majority of RJ45 jacks are colour-coded to show which wires go to which pins.

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.