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Got a technical problem? Can't understand a piece of jargon or some technical principle? Drop us a line and we'll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097; or send us an email.

Playing 78s with the LP Doctor

I have built the LP Doctor described in the January 2001 issue and may I say that your efforts on this project have been tremendous. I present nostalgic music by playing 78 RPM recordings which are broadcast to air on our community radio station. I still have a problem getting rid of surface noise on 78s. The LP Doctor does a great job on clicks and pops but as you suggest in your article on the subject, it does little on surface noise.

I have done a bit of playing around with a filter published in ETI of September 1980. I managed to achieve fairly reasonable results by using the filter set to 5kHz and incorporating the LPD for dealing with the clicks and pops. I feed the turntable audio into the LPD and I feed the filter by paralleling the LPD's output (ie, by bringing the L + R positives together and so the commons together).

One assumes by doing that this I have created a reasonably effective common mode rejection filter. It does make a difference in regard to the 78s' surface noise.

On a related subject, I have constructed several RIAA phono preamps (from the April 1994 issue) and they are terrific, the low noise factor in particular. My problem is that though they work splendidly playing vinyls, they don't like the audio from 78s. I assume that the 78s cause the cartridge to produce a higher voltage than vinyl records. This tends to create clipping/distortion especially on the highs of a 78 recording (eg, a tenor hitting a high note).

At first I thought I had a faulty cartridge but not so. I have tried several magnetic cartridges including high quality Stanton and Shure types, which produced little difference. I was once told by an old broadcast engineer that the equalisation required for 78s is different to that required for LPs/vinyls. I managed to solve the problem by installing a pad on the tagstrip under the turntable unit.

(K. J., Epping, Vic).

It is true that 78 RPM records will generate higher signal outputs from the cartridge and therefore there will be more likelihood of overload in the preamplifier. The solution to this is to reduce the gain of the preamplifier by increasing the value of the feedback resistor (R4) from 390Ω to 1kΩ or more. Do not reduce the output of the cartridge as you have because this will degrade the overall performance.

The equalisation was different for 78s; in fact there were quite a number of different equalisation curves in use before LPs came onto the scene and the RIAA curve became the standard. Each recording company had its own recording characteristic and therefore the required equalisation could be quite different from the RIAA curve. You can find more info on the this subject in the esteemed "Radiotron Designers Handbook" but the more you look into it, the more it becomes a can of worms.

To minimise surface noise from a stereo cartridge when playing 78s, the cartridge left and right channels should be paralleled and then fed to a single channel of the preamplifier. Paralleling the outputs of the LP Doctor is not recommended.

You can increase the treble filtering from the LP Doctor by changing 150pF capacitor associated with IC5b & IC7b to 330pF and the 560pF to .0012μF. This is a simpler and more effective approach than using the ETI filter.

Immobilising a V6 Commodore

My nephew has purchased an engine immobiliser & keypad kit to be fitted to his 1994 VR Commodore 1994. At present, we are unsure what wire is the output from the coil to connect to the immobiliser. He has purchased a manual however it only shows the active from the ignition switch. Have you fitted (or had fitted) a unit to a similar car? Could you please inform us what to do.

(G. R., via email).

As described in our feature article on the Commodore in the December 1988 issue, the Commodore V6 uses three double-ended ignition coils. Hence an immobiliser either has to kill all three coils or kill the signal from the Hall Effect pickup on the harmonic balancer. We think the safest (and most expensive approach) would be to use three high-voltage high-current diodes to kill the three coils with the one high voltage transistor in the immobiliser circuit.

With this approach, the anode of each diode would connect to the switched side of each coil. The cathodes would be connected together and connected to the collector of the immobiliser transistor.

Suggested diode type would be a BYT12P-1000 rated at 12A and 1000V and fast recovery in a TO-220 package. The diodes are available from Farnell, Cat no. 437-700 for $5.10 each plus GST and delivery. Phone 1300 361 005.

Light dimmer for halogen lamps

I wish to construct the remote controlled version of the lamp dimmer featured in the January and February 2002 of SILICON CHIP. However, I need to make some modifications to the set up for it to work in my particular application.

Firstly, I am controlling two 12V halogen lamps fed via two transformers fed from a standard domestic dimmer control. Can I simply replace the dimmer control with the remote controlled dimmer?

Secondly, I need to mount the infrared sensor off the board some 20 to 30mm away.Will this work satisfactorily? Also would I still need to be shield the infrared sensor and fit the 0.1μF capacitor between the sensor case and the PC board?

(T. B., Buderim, Qld).

The dimmer has not been designed for use with 12V Halogen lamps.
To operate successfully it needs a "snubber" network connected across the Triac so that it will turn off properly with the inductive load presented by the step-down transformers.

A suitable snubber network would be a 22Ω 1W resistor connected in series with a 0.1μF 250VAC (class X2) capacitor. This network would be connected between the A1 and A2 terminals of the Triac. Unfortunately, there is no space for these components on the existing PC board.

The IR sensor must not be placed away from the dimmer PC board since it is connected to the 240VAC mains and is therefore live (ie, at 240VAC).

Capacitance meter switch confusion

In the September 1999 issue, an electrolytic capacitance meter was featured which I am currently building in my spare time. I have almost finished and am ready to put it into its case along with all the switches and the rotary dial.

My question is this: in the parts list, a 2-pole 6-position rotary switch is required as a range selector. However, in the range selector in the front display only four positions are used. Is this a typo error, actually requiring a 4-position switch?

(A. A., via email).

The parts list is correct - you set the stop on the switch to set it to four positions. To do this, undo the mounting nut and remove the stop washer and then place it back on to the correct position to provide four positions.

Fuel mixture display sensor wanted

I've built the Fuel Mixture Display kit (September & October 2000) which I brought from Dick Smith Electronics in New Zealand. The kit is going to be used on my hotrod. However, I'm having problems locating the Bosch EGO probe you listed in the kit as being matched to the unit. The local Bosch agent said the part number is incorrect (LSM11 , 0258104002). Is the number correct or was there a mistake?

Even if you can tell me what type of car the above probe is from it would be helpful.

(J. B., Stratford, NZ).

The Bosch type number is correct. It is a sensor generally used for sensing exhaust smoke stacks, not necessarily in the automotive industry.

The sensor can be purchased from Farnell (NZ 649 357 0646) but it is cheaper to get an EGO sensor from a wreckers such as used in Ford and Holden 6-cylinder cars. The Fuel Mixture Display operates successfully with most automotive sensors.

How to bridge a stereo amplifier

I recently bought a 185W/channel kit stereo amplifier from Jaycar Electronics and I am wondering whether it is bridgeable? If so, how is it connected to the speakers and input and what is the output?

(J. S., via email).

You need an op amp adaptor circuit to bridge the two power amplifiers. We published a suitable circuit, although with no PC board, in the February 1988 issue. We can supply a photostat copy of the article for $7.70 including postage.

Ceiling fans run too fast

Many of the houses I have lived/live in have the toilet ceiling fan connected to the toilet light. The wiring to the switch is often inaccessible so alterations are difficult. In the ceiling, the fan plugs into a socket which is wired to the ceiling light.

There are several problems with this arrangement. The fan is not always required for a toilet visit. When the fan is required, it should continue running for a delay time after the visit. Most ceiling fans run too fast/noisy for the toilet and require some form of speed control.

This is what I propose: the original switch still controls the fan/light circuit but is left on if the fan is required. The light turns on/off immediately with the switch. If the light is turned on, there is a delay of about two minutes before the fan operates (this allows a short toilet visit with no fan).

If the switch is turned off, both the light and fan turn off. If the switch is left on, both the light and fan stay on for a delay of about 15 minutes (the fan/light will run for a period after the visit). If the fan/light have cut out after the time delay, the circuit is reset by turning the switch off then on. The fan has preset speed control. The components would be mounted in a box in the ceiling.

(A. D., Erskine, WA).

Unfortunately, your fan control concept involves control of both the light and fan, as well as speed control for the fan. While it is certainly feasible as an electronic circuit, the simplest approach would be to add in a fan switch (ie, separate circuit to the fan) with inbuilt time delay and add a resistor or capacitor in series with the fan to reduce its speed. In this way, you could turn on the fan when required and its noise would be reduced anyway because of the reduction in speed.

LEDs flashing on mixture meter

I have constructed the Fuel Mixture Display kit described in the "EFI Tech Special" The kit is not functioning how it should with lights buzzing left and right on idle at normal temperature. The red light stays on all the time. If I adjust the trimpot, the yellow light shows. What could be the fault in this situation?

(E. B., via email).

There isn't too much that can go wrong with this kit. It seems that the IC is driving the LEDs from one extreme to the other as the yellow (rich) and red (lean) ones light with variation of VR1.

Check that there are no shorts between tracks on the PC board, by scraping between tracks with a sharp knife. Also check that there are no solder bridges between pins on IC1 by comparing the published pattern with the underside of your board.

It is possible that the input at pin 5 has been damaged. It can be protected by connecting a .01μF capacitor between pins 5 & 4 and using a 100kΩ resistor in series with the input from the oxygen sensor.

Simple train controller wanted

I was wondering whether you knew of a circuit that would work as a speed controller on my train motor. The motor is from one of those old electric walk-behind lawn mowers with a roller to drive it and tubular blades. I have the train running from the smallest car battery you can get and that runs it for about 50 to 60 minutes before it starts to go flat. (J. P., via email).

Have a look at the Li'l Pulser train control in the February 2001 issue. Depending on how much current you need to supply, you may have to use a bigger FET, bigger relay and a bigger heatsink.

Speed Alarm won't limit car speed

Could the PIC-based Speed Alarm described in the November & December 1999 issues be used to limit a car's speed in a similar way to the PIC Tachometer described in April 2000, which limits the revs via an immobiliser?

I guess my question really is "Can the Speed Alarm be interfaced with the immobiliser circuit with only software revision?" Your help would be appreciated.

(S. A., via email).

The Speed Alarm is not suitable to actually control the speed of the car. For this you need a cruise control as it requires a means to manipulate the air flow to the engine via the carburettor or throttle body in a fuel injected car.

Enhanced plugpack power supply

I remember a very useful circuit that I'm sure I saw in SILICON CHIP magazine but I've looked all though the circuit listings and can't find it. It was a simple circuit to reduce mains hum when you're using a "plugpack" power supply with any sort of audio device. I thought it was in the "Circuit Notebook" section.

(M. C., Eight Mile Plains, Qld).

The article was in the December 1998 issue, entitled: "A Regulated 12V DC plugpack".

Universal battery charger differences

I am trying to find out what the differences are between the original and Mk.2 versions of the Universal Battery Charger. Can you help?

(C. S., via email).

The main differences are that the Mk.2 version has facility to charge Lithium-Ion batteries and there are more voltage ranges available for charging Nicad and NiMH batteries. Also the tendency for the Mk.1 charger to prematurely terminate charging for older batteries has been corrected.

You can upgrade the Mk.1 version to the Mk.2 version by transferring the components from the old board to the new PC board. This PC board is coded 14302982 and is available from RCS Radio Pty Ltd. Phone (02) 9738 0330.

Hardware items such as the case, the transformer, mains and battery connection wiring, heatsink and rectifier are unchanged. The front panel is changed slightly to accommodate the extra battery type and ranges. Of course, it is not necessary to include all the extra voltage ranges provided by the Mk.2 version or include the Li-Ion selection.

Main parts changes are the addition of a 2-pole 4-position rotary switch in place of the DPDT toggle switch used for S3 and some resistor changes.

Video mixer circuit wanted

I would like a circuit that would allow the mixing of two separate video signals into one (a video fader in the same manner as the familiar audio fader). I realise there is a problem with the synchronising of the signals and there would in all probability be a varying black horizontal and/or vertical bar as a result if the same circuitry as for audio were used. Could this be overcome with some sort of auto delay on one of the signals?

Please put on your thinking caps and provide me, and many others interested, I'm sure, with a practical answer.

(J. S., Woodville West, SA).

As you are aware, the two video signals must be locked together. In practice, the only way of doing this is to feed one of the video signals into a frame store so that it can be fed out in sync with the second video signal.

Frame stores are a feature of the "picture-in-picture" chipsets used in upmarket TV sets. However, while a PIP chipset could form the heart of a practical video mixer, we have not produced such a design and the chipsets are not readily available.

Peak hold for tachometer

I would like to know whether a peak hold function could be added to the tachometer circuit featured in the April 2000 issue.

I would like to use the unit in a Formula 500 Speedway car which is powered by a single-cylinder 2-stroke engine. Different tracks require different gearing and this feature would be ideal for checking for peak RPM - it becomes very difficult to keep an eye on the tacho when peak revs come at the end of the straight, right when you're sliding into corners and trying to avoid cars in front of you, while getting mud thrown at your visor.

(J. H., Perth, WA).

Unfortunately, the entire memory capacity of the microcontroller used in the tachometer circuit has been used to provide all the features. Without extensive rewriting of the code, there is simply no space to include a peak hold feature.

The accuracy of the peak hold would also be in doubt since many race engines simply change RPM too quickly for a reliable measurement, particularly over the 0.6 second update time for your single cylinder 2-stroke engine.

One suggestion would be to set the bargraph to operate over a narrow range of RPM so that peak RPM can be seen as one of the lit LEDs. This will provide a guide as to RPM reached, within the limits set for the bargraph.

Single phase & 3-phase - what do they mean?

Could please explain single and 3-phase? I have been informed during my search that 3-phase is only a name used by industry to obtain dollar energy discounts; at home, it is less costly to run an electric stove from a power point than its designated circuit attachments; and you use less power with an arc welder if you use high amp settings!

Clearly, these claims are ridiculous and I found it hard to keep a straight face. Give me something nice and technical to really make me think.

(R. L., Coolbellup, WA).

Many books have been written on this subject and you should find a few in your local library. In brief, all power stations around the world generate electricity from alternators which produce 3-phase power. The power comes out of the alternator in three lines (conductors), each of which is a sinewave at 50Hz or 60Hz. The difference between successive phases is 120°; three phases make up 360°.

Electricity is distributed all over the country as high-voltage 3-phase. That is why all high voltage towers always have three power lines. The same system is used in your street and typically each house is connected between one of the phases (ie, 240VAC) and neutral. Only when a house has a heavy power appliance such as an instantaneous water heater or pool heater is it normal for three phases to be connected. In those cases, you will find that the house has four power lines; ie, three phase lines (each at 240V) and neutral.

Many people (including electricians) are confused about 3-phase electricity and cannot understand how there can be 415VAC between each phase line but only 240V between each phase and neutral. The only way to understand the topic is to delve into the textbooks. If sufficient other readers are interested, we may do a short series of articles on the subject.

Notes & Errata

PC-Controlled Mains Switch, September 2001: to avoid the possibility of electric shock from contact with the power plug's pins when it is disconnected, a 100kΩ 0.5W resistor should be connected across the Varistor. This will discharge the 0.1μF 250VAC capacitor.

Also, to improve the voltage isolation of the PC tracks around the optocoupler, it is recommended that neutral cure silicone caulking compound be applied to pins 4-6 of OPTO1 and the nearby component pads.

Pardy Lites, December 2001: the resistor following D1 should be 820Ω instead of 4.7kΩ . Both the circuit on page 68 and the PC board on page 69 have this error.

Audio/Video Distribution Amplifier, November 2001: there is an error in the underside copper pattern for the PC board which causes both audio outputs from the fourth socket pair from the right-hand end (looking from the rear) to deliver the R channel output signal.

The problem can be fixed fairly easily. First, remove the PC board assembly from the case and turn it upside down with the output connectors on the top. Then locate the fourth audio output pair from the left and verify that the pads at the lower ends of the two output series resistors (originally 47kΩ , now 1kΩ ) both have tracks connecting them to the upper 'R' signal line track - unlike all the other output pairs. Cut the track on the right and, using a short length of tinned copper wire, connect the resistor pad to the lower 'L' signal line track instead.

Solar Power Battery Charger, March 2002: the MJE2955 labelling for Q2 and Q3 on the overlay diagram on page 85 is incorrect. They should be labelled MTP2955. (Note that an MTP2955 is a P-channel Mosfet while a MJE2955 is a bipolar power transistor). The circuit and parts list are correct.

In addition, the parts list incorrectly specifies a 4011 for IC1; it should in fact be a 4093 quad Schmitt trigger, as shown on the parts overlay diagram.


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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