Our text wrongly implies that we used the LM317 & LM337 adjustable regulators purely for the ability to decouple the ADJ pin. We neglected to mention that the adjustable devices have a tenfold performance benefit (according to National Semiconductor) over the fixed devices in line and load performance. They also feature improved overload protection.

Additionally, we envisioned that these supplies would be useful in a variety of projects, hence the benefit of adjustable outputs.

Regarding the reverse-connected diodes, it is indeed possible for one rail to be driven to the opposite polarity due to a failure in the powered device (not in the regulators, as stated). In this case, the diodes protect the regulators by clamping their outputs to one diode drop below (or above) ground.

Capacitors for valve preamplifier

I am currently replacing the capacitors in the valve preamp for hifi use (SILICON CHIP, February 2004) but I don’t seem to be able to find a 680nF 630V capacitor from either Jaycar or Altronics. Would a 470nF 630V unit be OK to use instead? (R. C., Doncaster, Vic).

If this does worry you, you could connect a 220nF 630V capacitor in parallel with the 470nF unit, to give a total of 690nF.

Killing that sound blast from TV commercials

Have you ever published a circuit to limit the surge in sound when TV programs go to commercials? If not, do you have any suggestions? (S. K., via email

Transformer for 110V iron

Some time ago while in Canada I bought a 110V iron to use for ski waxing. The iron is about 1kΩ and a transformer that would be suitable would be quite expensive. Have you ever published a project which would allow such a high-powered appliance to safely operate from 240VAC? (I. L., via email).

The only alternative approach is to use an auto-transformer or a fully isolating step-down transformer. Both will be relatively expensive. They are available from Harbuch Electronics in Hornsby (Sydney). Phone (02) 9476 5854.

Remote switching with the SMS controller

I recently built the SMS Controller which was published in SILICON CHIP in 2004. I also built several Remote Relay Switches, published in May 2006. I intend to use the controller to switch lights as well as other appliances on and off on a property on the Gold Coast.

I have tested the Remote Relay Switch with the SMS controller for short periods of time and it works quite well. I propose to connect three Relay Switches to the controller. The Relay Switches may have to be energised for extended periods of time (several days).

Do you see any problems with excessive loads on the switching integrated circuit? I have soldered it directly to the PC board. Or would you advise me to use the relay switch published in the November 2006 issue instead? (C. N., via email).

The DC Relay Switch described in the November 2006 issue is not suitable, as it cannot be used to switch 240VAC.

Can Radar Speed Gun measure cricket balls?

My three grandsons, being extremely keen cricketers, want me to build the recent Radar Speed Gun described in the November 2006 issue. Could you proffer an opinion if it could be used by their cricket club trainer to measure their bowling speed? (D. C., Bribie Island, Qld).

Controlling audio level when burning CDs

I burn LPs onto CD using my hifi record player and using the output signal from the headphone jack. This works well. However I cannot keep the volume the same between copies as my hifi volume control is by pushbutton and not by potentiometer.

Can I use the kit in the March 2007 issue, called the Universal Infrared Remote Volume Control & Preamp, to do the job? I would like to choose my volume and see exactly how high or low the headphone output is. (T. Z., via email).

An easier method would be to monitor and control the signal level using Windows-based recording software. Additionally, you can make use of the special LP noise filters built into many of these packages.

Check out our feature on transferring LPs to CDs in the September 2006 issue for details.

Power supply for Mighty Midget amplifier

I have built the Mighty Midget amplifier (SILICON CHIP, March 2002) and need some advice. Instead of lugging the family car battery with me, what sort of power supply would I require? (B. W., Toukley, NSW).

Electronic bell circuit wanted

I am looking for an electronic bell circuit. The one I have in mind is similar to the types used in school amplifiers for end-of-period, etc. I have hunted the "web" without finding anything remotely suitable. Has SILICON CHIP ever done such a project? (R. J., Kangaroo Flat, Vic).

Fuel Mixture Display shows overload

I purchased a Fuel Mixture Display from Jaycar. I have put the kit together and have one problem that I cannot figure out. I cannot set the span. It always goes to OL.

Can you provide some guidance? (B. A., Oregon, Illinois, USA).

The span problem could be because the offset is not set or VR2 is the incorrect value. Make sure the other resistors on the PC board are the correct values, in particular the 180kΩ & 100kΩ values associated with IC2a.

Porsche Takes Off In Second Gear

I am the happy (almost) owner of a 1985 928 Porsche. This vehicle is fitted with a Mercedes Benz 4-speed auto transmission and it is normal for this transmission to take off from rest in second gear (a strange Teutonic quirk). The only way to make it take off in 1st gear is to mash your foot to the firewall. This closes the kick down switch and powers up a solenoid. Unfortunately, this results in much protestation from the rear tyres.

In the interests of fuel consumption and driveability, I would like it to take off in 1st gear. I have fitted a manual switch in parallel across the kick-down switch but this has become tiresome in traffic. I want to have the kick-down switch automatically close after the car comes to a complete stop and to open after the car has moved away but with the opening point (speed) variable according to the throttle opening.

Is there a kit or some other way I can achieve this? The vehicle is standard with an electronic speedo and fuel injection so picking up a road speed pulse and variable voltage from the throttle position sensor would not be a problem. The auto transmission is a fully hydraulic unit apart from the electric kick down system. (R. A., via email).

Hesitation In High-Energy Ignition

I built one of your High Energy Ignition systems, as featured in the December 2005 & January 2006 issues. It has been installed in a 1996 6-cylinder Falcon.

I built the engine management version but rather than take a signal from the distributor output before any conditioning by the vehicle’s management system, I chose to take the signal from the coil primary connection, so that any timing changes made by the ECU will still have the same effect but will also be processed through the High-Energy Ignition.

I fitted the 100Ω pull-up resistor as per the points version of the ignition and the vehicle runs very well. The benefit is immediately noticeable. However, at the initial moment the accelerator pedal is pressed, there is a split-second hesitation – the motor stutters then away it goes. Other than this moment of indecision, the motor runs wonderfully.

I was wondering if the value of the pull-up resistor is causing my problem. The coil’s primary resistance is around 0.5W so the car’s management is not seeing the normal load. Do you think this might be the reason I am seeing the hesitation or could you suggest an alternative approach?

Also, if I were to fit a resistor of around 0.5Ω in place of the 100Ω unit, would the current drain have any impact on the High-Energy Ignition itself? (G. M., Endeavour Hills, Vic).

What we think is happening is that at idle the dwell period for the coil is correct, as the engine is running at a relatively constant RPM. But as soon as you put your foot down on the throttle, the engine load changes and the firing point (or timing) is changed.

This change can reduce the dwell period momentarily as the microcontroller readjusts its calculations. As far as the microcontroller is concerned, the engine has had a sudden RPM change.

This loss in dwell can prevent the coil from delivering its spark and so the engine hesitates.

Making The Active Crossover A 2-Way System

I was most interested in the article "An Active 3-Way Crossover For Loudspeaker Systems" which was published in the January 2003 issue of SILICON CHIP. I subsequently purchased the kit from Jaycar and it sat around for a year or two before I began work on the project.

Although I had initially planned for a 3-way system, I’ve since settled on a 2-way loudspeaker system and am therefore keen to know whether it would be easy to modify the kit or bypass part of the circuitry to allow the crossover to be used on a 2-way system.(L. D., Perth, W.A.).

The unused bandpass filter circuitry includes op amps IC3c, IC3b, IC5d & IC5c in the left channel and op amps IC4c, IC4b, IC6d & IC6cd in the right channel.

We recommend that you install the components for these op amps (use the default value 47nF capacitors and the 10kΩ and 20kOmega; resistors) to prevent them from oscillating.