Garage door indicator wanted

Do you have a design for a garage door monitor which would transmit the state of the door to an indicator in the house? I have seen a commercial design which mounts a reflector on the door and then sends an infrared beam to that reflector. It has a radio link to the house-mounted indicator.

(K. A., Sale, Vic).

We do not have a suitable design although it would probably be possible to adapt the 4-channel UHF remote switch using latched outputs on the decoder. This was published in the July 2002 issue.

Recording LPs onto CDs

Have you ever published a circuit for a cheap phono preamp to connect to a computer using the computer’s 12V supply and maybe an LM833. This would enable you to record old records onto HDD and burn MP3 CDs – all for under $10.

I think a lot of people have old records that they would like to transfer to CD and a cheap preamp powered by the computer power supply would be an alternative.

(R. B., via email).

Have a look at our RIAA Preamplifier for Magnetic Cartridges in the March 2002 issue. This uses a total supply voltage of 30V (ie, +15V, -15V) to get good overload margin from the preamp. It is not possible to get a sufficient overload margin from an RIAA preamp using only a 12V supply.

Double-heading with the Li’l Pulser

I have seen part of the article on "Li’l Pulser Train controller" (SILICON CHIP, February 2001). It may do the job I want but I would like to know if it is capable of running more than one loco (up to three). Also, where can I get a kit if one is available or a PC board?

(R. H., via email).

The specified power supply is rated at 1A, so it depends on how much your locos draw. It should cope with at least two.

If you use a bigger power supply and mount the Mosfet on a bigger heatsink, the circuit can deliver a lot more power and so you could hook up more locos. However, it is generally advisable to use identical locos when "double-heading" otherwise one loco will try to do the lion’s share and will end up spinning its wheels. The kit is available from Jaycar (Cat. KC-5304).

High-power ultrasonic pigeon deterrent

We need to amplify ultrasonic signals to over 100dB. I plan on using a Stamp 2 module to generate the signals and feed them to your Studio 350 Amplifier. We want to generate these frequencies to repel seagulls and pigeons from factory roofs.

We have done our tests and believe we know what sequence of frequencies is required. They range from 15-30kHz. We need to connect up to 40 tweeters (Jaycar Cat. CT-1912), all with 100m of cable to a central box which includes the microcontroller and amplifier.

A salesperson at Jaycar Electronics said the Studio 350 will handle frequencies up to 60kHz so I presumed it would be good enough to amplify the frequencies generated from my microcontroller. What type of PA would you recommend I need to interface to the amplifier to achieve this?

((V. J., St Albans, NSW).

Unfortunately, your scheme won’t work. For a start, the Studio 350 is not suitable for high power delivery at supersonic frequencies. Its output filter will burn out and the output transistors will all overheat. It would need a substantial re-design to make it suitable.

Second, it could not drive 40 tweeters, or anything like that number, and that particularly applies if the tweeters you propose to use are piezoelectric. This is because piezo tweeters are capacitive and therefore their impedance is very low at supersonic frequencies.

We do not know of any amplifier design which will deliver its rated power at 30kHz, let alone 60kHz. While typical audio amplifiers may have a frequency response to 100kHz and beyond, their power response is typically not much beyond 20kHz. You would need an amplifier specially designed to drive piezo electric transducers, as in high-power ultrasonic cleaning baths.

Ultimately, you would probably be better off using a very high power frequency-modulated oscillator, with transformer coupling to the tweeters. We have used a similar approach in past projects to discourage dogs from barking: (1) Woofer Stopper Mk2, February 1996; (2) Dog Silencer, July 1999; and (3) Dog Silencer Mk2, April 2004.

We can supply these articles at $8.80 each, including postage. You could take the same approach but at a much higher power level. The circuits all use power Mosfets in the output so it would be possible to increase the power substantially by using more Mosfets but the gate drive circuits would also have to be considerably beefed up.

The transformer would be a specially wound job. We suggest you contact Harbuch Electronics. Phone (02) 9476 5854.

6V Operation For New Ignition System I have already purchased a kit for the new version of the Electronic Ignition, as featured in the December 2005 issue. I am pleased that you have achieved a reduction in the coil current (coil heating) because the application I have in mind (a motorcycle restoration) has "delicate" coil hardware and excess heating has resulted in dead coils. It is a 6V motorcycle and coils for these are getting hard to source. This brings me to my first question: could this project be used for a 6V application? Would this new version be more frugal in the watts department and would it function on a (kick-start) motorcycle with only a 6V supply? Finally, I was disappointed to note that there has been no reduction in the size of the case. There simply isn't room on a modern (post 1960) motorcycle for a large diecast box! With the reduction achieved in coil current, couldn't some reduction in box size be achieved? (R. J., via email). The new ignition system will work on a 6V battery. And you could use a box smaller in height, provided the power transistor was mounted horizontally inside the case.

Where to obtain 240VAC-rated resistors

A number of your projects use 240VAC-rated resistors (eg, Philips VR25 1.2MΩ). I have not been able to find a supplier for these type of resistors. Could you advise of any known retailers, especially in the Canberra region?

(I. F., via email).

VR25 resistors (0.25W, 1150VAC rated) are available from Farnell, phone 1300 361 005 or browse to www.farnellinone.com.au. The stock number for a 1.2MΩ resistor is 341629 or 9477152, while a 1.5MΩ resistor as used in the Fan Timer (SILICON CHIP, October 2005) has a stock number of 341630 or 9477160.

Multiple message mode for voice recorder

I purchased the Voice Recorder kit (SILICON CHIP, May 2005) and it operates in single message mode with no faults. I have now programmed the module to run in random access mode for eight fixed messages, having installed the extra pushbuttons which now become the record/start play for each message. I have not been able to work out how to record the individual messages though.

Could you please advise me how to do this?

(M. I., via email).

In addition to adding the extra pushbutton switches to trigger each of the eight message areas individually for random-access operation, you also need to switch the RE-bar (record enable) input of the HK828 chip (pin 27) high for recording mode and low for playback mode. If you do this, you should find that it works correctly.

After-market chip for Land Rover

I am considering installing an after-market computer chip in a Land Rover Discovery. Can you tell me whether this is a safe modification and is a plug-in unit better than making changes to the settings in the ECU? (N. W., via email).

We are not in a position to make recommendations concerning ECU chips for cars. However, unless the chip has been specifically tailored for your vehicle, it is unlikely to provide any improvements to the engine.

If you want a tailored system, you will be best served by a workshop team who can dyno tune the engine and alter the code within the ECU. Any over-the-counter chips that simply plug into the ECU cannot be guaranteed to provide any improvements and may cause engine damage.

Mobile Phone Jammer Wanted Have you ever produced, intend to produce or would consider producing a mobile phone jammer (if this is possible)? I work in a customer service field and there is nothing more frustrating than the rude customer who comes up to the counter while talking on their mobile phone, or indeed even takes a call at the counter and expects you and all those behind them to wait while they complete their call. I see this happening with increasing regularity, not only at my place of work but also at the supermarket, bank, post office, and anywhere else I find I need to queue. It would ideally be a low-powered unit that could be concealed on your person, with a range of about two metres or so, and able to be left on all day so it affected all phones within range and prevented then from ringing in the first place, or caused them to drop out as they were brought into range. (T. L., via email). You want a "cone of silence", don't you? We doubt whether there is any low-power jamming device which would be effective on a mobile phone (and if there was, it would be illegal). As you know, two or more mobile phones can be used in fairly close proximity without interference to each other. People who behave like that are very annoying though!

Unmute for portable PA

I have built two PortaPAL PA kits (SILICON CHIP, February & March 2003) which work well and are good value for money. I would like to be able to bypass the "Mute" stage during a live performance, to prevent the truncation of the first few seconds of a song.

Is it possible to do this and if so, how can I go about it?

(D. R., via email).

Pin 4 of IC9 (TDA1562Q) should be tied to +12V to disable the muting. The connection to pin 7 of IC6b should be broken when doing this. A single-pole double-throw (SPDT) switch could be used with the wiper of the switch connecting to pin 4 of IC9. Connect +12V to one side of the switch and pin 7 of IC6b to the other side.

There is a link on the PC board between IC7 and IC8 that connects pin 4 of IC9 to pin 7 of IC6b. This can be cut to enable the switch connection, while +12V can be obtained at pin 7 of IC8.

What are impedance protected motors?

What does "Impedance Protected" mean? The statement "Impedance Protected" appears on a new in-line ventilation 240V fan I am using. I know it’s an induction motor type, so I guess it has something to do with protecting the motor’s windings. But I don’t know how, what or why.

I have asked my father this, as he’s a Visiting Professor Of Faculty Of Engineering. He doesn’t know either, which is strange. Perhaps you can answer this interesting question.

(A. B., via email).

Impedance protection prevents motor burnout if the fan is jammed. It is achieved by having high resistance windings, normally in a shaded-pole motor.

IR train detector needs pulsed signal

I have made several attempts to construct an IR train detector for my model train. I have found circuits in my model rail books dating back 10 years. I have also bought kits, some too expensive to duplicate and others where the parts are not clearly marked or the provider will not tell me the component values.

The circuit I have sent to you is the easiest one I have found but the LED is on all the time when I apply 12V filtered DC. I have tried changing the value of R1 downwards and have also tried D1 and Q1 from different sources to no avail.

Could you help me out here? I am a novice electronics user, although I have had previous success with other circuits.

(G. J., via email).

We are unable to assist with trouble-shooting of circuits not published by us. We have not published an IR train detector but would suggest that you could base one on the infrared light beam relay in the December 1991 issue or the Infrared Sentry in the April 1999 issue. Both circuits use a pulsed IR signal – any other approach is futile.

Projects suitable for diesel vehicles

Are the projects described in "Performance Electronics for Cars" suitable for diesel vehicles? I bought the book so that I could build the Intelligent Turbo Timer device for my vehicle which is a turbo diesel. What other projects in the book are applicable to diesels?

(G. B., via email).

Most of the projects are applicable to diesels. These include the timer, the frequency, temperature and voltage switches, speedo corrector and thermometer. The adjusters may or may not be applicable, depending on the type of engine management in the diesel vehicle.

Triac Failure in USB Powerboard I built two of USB-activated powerboards (SILICON CHIP, November 2004) and they worked fine for a while. The Triac in both units then failed short-circuit between pins 1 & 2 and powers the board at all times, even when the computer is off. I measured the worst-case current draw of my three peripherals - a small desk fan, scanner and printer - at 0.1A, 0.2A and 2.4A, respectively. This appears well within the 3A limit for the board. I replaced the Triac in one unit with a BT136 600E, which is the same as was supplied in the Jaycar kit. This too failed after four days, in the same manner as the original. I referred to the parts list in the Jaycar instructions and note that it calls for a BT136 6A 500V unit but the parts layout lists a BT136F. My perusal of the specifications of both online seem to show they are compatible. Should I be using a BT136F, a small heatsink or are my peripherals too close to the current limit? (F. W., via email). There is not much difference between the BT137F-600 Triac which we originally specified and the BT136F-600 device which we understand Jaycar have supplied in their kits. The main difference is that the BT137F has higher RMS and peak non-repetitive current ratings - 8A and 55A respectively versus the 4A and 25A ratings for the BT136F. As you suggest, these differences shouldn't normally make much difference in the USB Powerboard project, because the average load current should be below 3A anyway and the peak currents shouldn't even reach 25A. You don't advise in your email where you are operating your own unit but if it's in the country, your power lines may be subject to fairly severe "spikes" due to lightning strikes, etc. Perhaps these could have caused the damage. We suggest that you try replacing the damaged Triacs with BT137F-600 or even BT137F-800 devices if you can get them, to make the units more "rugged". It might also be a good idea to feed your computer and the USB powerboard through a power line filter and surge suppressor unit.