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Why have extra holes in PC board?

I am currently working on the LP Doctor project described in the January 2001 issue and am enjoying doing it. This is a fairly complex project for me so I am proceeding very slowly and carefully. I need to ask one question however: Can you tell me what the extra drill holes are for under trimpots VR2, VR3 & VR5-VR8? There are three holes for the trimpot legs but there is another hole in the middle. They seem to be there for a purpose (particularly VR6 which connects to the 100kΩ resistor to its right) but I cannot work out their function (if any). I'm sure I have all the components in the right place.

(M. P,. via email).

The extra holes are there so that different trimpots can be fitted - just ignore them.

Display turn-off for rain gauge I am about to build two Rain Gauges as in your June 2000 issue. One will be run on mains power while the other will be run on a 12V battery. The battery will be charged by solar power. Would it be possible to switch off the display to save power? If so, what needs to be done? (G. J., via email). You can switch off the supply to the display by switching the +5V supply to the emitters of Q1-Q4. You will need to cut the relevant copper track on the PC board.

High energy ignition for a 1968 Porsche

I have built the High Energy Ignition described in June 1998 using a Jaycar kit. I installed it on a 1968 Porsche 911S using the original distributor with points. These cars have a very marginal ignition system originally and have frequent plug fouling problems, particularly if the dwell is not set to the maximum.

The HEI module has transformed the car, making it much easier to start, allowing it to idle (ie, keep running) below 1000 RPM and most importantly, has eliminated the ignition miss above 6500 RPM (redline at 7300). The platinum plugs now last almost indefinitely even in competition use whereas a few thousand km was all I could previously expect.

Needless to say, I am extremely happy with the unit, except that I am having trouble driving the tacho. As suggested in the article, I have driven the tacho from the coil (not the Q3 output) and this works fine up to about 5500 RPM where the tacho becomes erratic (fluctuates). Given that the car is used in competition and is an extremely peaky engine with maximum power at about 6700 RPM, I frequently operate it in the 6000-7300 RPM range. It does have a mechanical (rotor button) ignition cutout but this is not totally reliable. Bottom line is that I need a reliable tacho. The tacho in question is the original VDO impulse tacho based on 1950s, or at the latest, early 1960s technology.

I have now built the Fig.8 auxiliary circuit as recommended but this does not drive the tacho at all. I am fairly confident that I have built it correctly and have tested it statically with a multimeter up to the base of Q2. I do not have a CRO and so I have not been able to test the output effectively but I have done a range of tests using an analog multimeter to measure AC voltage output.

Overall, I think that the problem is probably related to the shape of the waveform being generated by the output with the overall voltage contributing to some extent. Please help me with my next move. Some of the things I am considering are as follows:

(1) Use the Coil output from the High Energy Module but include a capacitor in the circuit (tacho/coil output to earth) to limit the coil voltage and modify the wave shape. However, I am concerned that this might also modify the ignition output.

(2) Use the Auxiliary circuit with a a different capacitor (not sure what) or a larger capacity coil to increase the voltage output.

(3) As a variation of the Auxiliary circuit above, use an additional coil just to drive the tacho. (This does seem overkill but should work).

(4) As another variation of the Auxiliary circuit, use the transformer as a step-up transformer to get sufficient voltage and then limit this with a capacitor and series resistors.

A second, but less important issue, is the use of a GT40 coil. I have used one for some time to get everything I can out of my ignition and I am reluctant to give away any performance unless I need to. The Jaycar notes recommend against this as it suggests that they may seriously overheat. My interpretation is that this would only occur when 12V was applied during start (ballast out of circuit) and should not be a significant problem except for prolonged cranking.

Furthermore, it should be possible to modify the various resistances (in the current limiting circuitry) to provide appropriate current limiting for the GT40 coil. Is this correct? If the only problem is a reduced life of the coil, I really don't care, as the car gets limited use and the replacement of a coil occasionally is not a problem.

(B. P., via email).

The Fig.8 circuit to drive the tacho may work if the .033μF capacitor at Q2's collector is reduced in value. Try .01μF 630V instead.

Alternatively, a 1mH RF choke may work better in place of the transformer. You could try the Jaycar LF-1546 (page 91 in their 2002 catalog).

You may also obtain a better tachometer result above 5500RPM if the zener voltage across Q1 is increased by another 75V. Try adding an extra 75V zener in series with the ZD1-ZD4 string.

You can use a high output ignition coil if you wish since reliability is not your concern.

Voltage divider for sound card output I am trying to find a hi-low converter that will allow me to plug the speaker output of a PC sound card into the microphone input of a voice recorder without frying either of them. I need to attenuate the signal and am unable to find any product or project that will do this. It seems to be a simple and obvious thing so I'm assuming it has been done before. (R. C., via email). All you need is a simple resistive voltage divider to drop the audio level from the sound card to a level suitable for the microphone. If we assume a dynamic microphone with a nominal level of 5mV and that the sound card has a nominal level of 1V (say), then you need a 200:1 voltage divider from the sound card. Use a 22kΩ resistor from the sound card and then a 100Ω resistor across the microphone input. If the resulting signal level is inadequate, increase the value of the 100Ω resistor.

Increasing reluctor sensitivity for ignition

I have completed assembly of the Universal High Energy Ignition as described in the June 1998 issue of SILICON CHIP but I cannot get the system to trigger at cranking speed. The system triggers when spun by hand at slightly higher speed. The circuit is built for a reluctor to suit a 1984 Toyota Corolla. As a further test, I have tried a Mitsubishi Sigma distributor which also has the same problem.

The air gaps and trigger polarities have been altered without success. As a last resort, I modified another kit which has been running in an early Commodore on a points circuit but still no luck! Any suggestions would be much appreciated.

(D. H., via email).

You can increase sensitivity of the reluctor circuit by changing the 47kΩ resistor connecting to the cathode of zener diode ZD5. Making this value larger will improve sensitivity. Try a value between 47k&Omega and 100kΩ or use a trimpot (say 200kΩ) and adjust it until the circuit works. Then select a fixed resistor that is close to the trimpot resistance.

Mighty Midget needs low resistance cables

I constructed the Mighty Midget power amplifier from the March 2002 issue. It operates as it should except it has a slight problem.

When the Bass control is turned into the boost region, the speaker cones (both channels) exhibit a large excursion once every three to four seconds; the current drawn at this point is about 2.5A. This was observed using both a 10A power supply and a car battery. The speakers used were 4-inch, 4-ohm dual cone speakers. All component values and connections have been checked. Seeking enlightenment.

(P. L., via email).

The problem is "motor-boating" and is caused by inadequate power supply leads. You need lower resistance power supply cables. Use 4mm auto wire or thicker; the more copper the better.

Increasing Woofer Stopper output

I have just completed a Woofer Stopper Mk2 (February 1996) successfully but now I want to increase output of this project. I have only one piezo tweeter (KSN 1005A) connected at present but the effect on the dogs doesn't seem to be enough. Should I be considering the KSN 1177A TD?

The output from the Woofer Stopper is very dependent on the piezo drive. So the KSN1177 twin tweeter which produces 99dB for 2.83V compared to the 1005A at 94dB for 2.83V in will produce much more sound. Paralleling up a few will also increase sound levels.

Feedback on Midi-Mate interface I built the Midi-Mate Interface for PCs described in February 2001 but found that it didn't completely work. MIDI OUT was fine but MIDI IN was not working at all. Strangely, if I plugged my MIDI keyboard into the MIDI in and my synth into the MIDI THRU port, MIDI was being transmitted correctly from keyboard to the synth. At this stage, I guessed that the MIDI IN was driving MIDI THRU correctly but was not able to drive the computer due to the Darlington output of the optocoupler. To fix this I added an inverter after the inverter connected to the opto output (ie, double inversion). This fixed the problem and I now have working MIDI IN, THRU and OUT. Maybe you should consider this as a modification to the circuit? (J. E., via email). There's no mistake with this project. We can only assume that you have a non-standard game/MIDI port input in your PC or the 6N128 optocoupler in the circuit is borderline in terms of specs. It certainly shouldn't have been necessary to add an extra inverter.

Problems with the MP3 jukebox

I'm having trouble with my MP3 Jukebox project.I have a Duron 750 with 768MB RAM, 20GB HDD, running Win98 and Winamp 2.75. The Irremote program loads the playlist but only loads the first song and not the others.

I only have 40 songs in the playlist and the playlist is in the same directory as my MP3. If I add songs after the playlist loads, the title info stays on the display but the song length changes. Can help me with this problem?

(W. A., via email).

Only one track is ever displayed in Winamp's playlist - the track currently loaded by IR Remote (and displayed on the LCD). This is as we intended. Remember, the Jukebox software was designed to be used without the Windows graphical interface.

However, you should be able to move to any track in your playlist using your remote and the instructions detailed in the article. If not, then examine the information displayed in IR Remote's status window (use the UP arrow to scroll back) for possible problems loading/scanning the playlist file.

It's not possible to manually add tracks to Winamp's list while IR Remote is running. It is also important not to click on the "Shuffle" or "Repeat" buttons in Winamp, as this will confuse IR Remote.

Notes & Errata

Sooper Snooper, September 2001: depending on whether the Snooper circuit is built for electret microphone, dynamic microphone or RF pickup, the 4.7kΩ resistor should be included or omitted, as indicated in the article. However, if the 4.7kΩ resistor is included, the 1ΩF capacitor should have its negative side connected to the base of Q1.

If the 4.7kΩ resistor is omitted, the 1ΩF capacitor should have its positive electrode connected to the base of Q1, as shown on circuit but incorrectly shown on the wiring diagram. Alternatively, fit a non-polarised 1ΩF capacitor instead.

Audio/Video Distribution Amplifier, November 2001: as presented, the audio stages have a gain of two which will result in excessive audio level with some CDs and DVDs. To restore the gain to unity, remove the 100kΩfeedback resistor from pins 2 & 6 to the 0V line. This makes the op amps in IC2 operate as voltage followers, with unity gain.

LP Doctor, January & February 2001: in the text on page 28 of the January issue, the final sentence in the second last paragraph refers to IC5a providing a gentle treble cut at 2dB/octave above 10kHz. Instead it should refer to IC5b (and IC7b).

The overlay diagram on page 78 of the February issue shows two trimpots numbered VR8. VR8 shown near IC14 should be VR7. The test procedure (3) on page 82 should read "Monitor Test Point TP4 and adjust VR7 for a 0mV reading." (Not VR8).

Table 3 on page 80 of the February issue should have the heading "How To Set Different Delays For IC3 and IC7 using Linking on IC20" (not delays for IC2 using IC8).

Stepper Motor Controller, May 2002: on the circuit diagram on page 77 most of the earth symbols and one resistor failed to print. The "hole" alongside VR1, labelled 10kW, should have a resistor occupying it, while all nine of the vertical lines which end with nothing should go to earth (GND).

Mighty Midget 70W Amplifier Module, March 2002: this amplifier is very sensitive to dips in the supply voltage and will mute if it goes below about 7V. This may not seem likely but peak currents can be as high as 9A and with thin supply cables, the amplifier will repeatedly mute which can sound like motor-boating.

The cure is to use heavy-duty cable. We suggest 4mm auto cable as a minimum.

6-Channel IR Remote Volume Control, March & April 2002: the 33Ω 5W resistor in the power supply should be 330Ω 5W. This can be seen in the photos on page 64 of the March issue and page 72 of the April issue.

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.

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