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Distortion in digital delay unit

Some time ago, I purchased two 20ms Digital Delay Units (SILICON CHIP, March 1996) from Jaycar for use in my hifi system. Since building them I have become dissatisfied as they are prone to distort on heavier passages of music. On high frequencies they sound very good but on lower frequencies they both distort.

The circuits are taking about 25mA as opposed to the 100mA stated on the instruction sheet and this current is drawn with an input signal of about 250mV. The chip in the circuit is the M65830BP CMOS delay IC.

Any light you can throw on this problem would be greatly appreciated. (C. L., Upper Hutt, NZ).

The 20ms delay circuit will distort if the input signal goes above about 300mV RMS. This would be more noticeable at the lower frequencies because the filtering and digital conversion allows the full signal level at these frequencies.

You can reduce the distortion by attenuating the input level applied to the delay circuit. This can comprise a volume control potentiometer. A 10kΩ log would be suitable, with one side of the pot connecting to ground. The wiper connects to the 20ms delay circuit input and the other side of the pot can connect to the signal source. Adjust the level for best noise performance but without distortion.

Photo-interrupter for PIC speed alarm

I have built the PIC Speed Alarm, as described in the November & December 1999 issues, using a kit purchased from Jaycar. A friend told me that the magnet/coil arrangement can be omitted and replaced with a photo interrupter.

Does this mean that the circuit involving IC2a and all the stuff coming off pin 6 of IC1 can be removed? I note that the PIC Rain Gauge descried in the June 2000 issue does use a photo interrupter sensor; ie, the 1kΩ and 100kΩ resistors and the 0.1μF capacitor connected to pin 6 of IC1. Can this serve as a pulse generator for speed input? (S. N., via email).

We used a magnetic pickup to detect shaft rotation because it allowed a fair movement between the magnet and pickup when their relative positions changed with suspension movement. Also the arrangement will work without concern for dirt or dust build up.

While you could use a photo interrupter as per the rain gauge circuit, there would be little allowance for any lateral movement of the driveshaft. Also the sensors would quickly become fouled with dirt. We would not recommend it.

Balanced input for 1-chip compressor

I recently constructed your audio compressor kit and found it works superbly, the only drawback being the unbalanced input. While this is fine for guitars, most quality microphones are balanced. The input buffer appears to be capable of a balanced input. Which would be better: (1) reconfigure the input (if so, how would you go about it?); or (2) add a switchable balanced input stage? (P. G., via email).

The compressor does not have facility for balanced inputs. You will need to make up a balanced amplifier and feed the output which is unbalanced into the compressor. The Analog Devices SSM2017P balanced microphone preamplifier IC is ideal. This device is used in the Altronics kit K-5531 (balanced input microphone preamplifier) which was published in April 1995.

The IC is available from Altronics (Cat Z-2800).

Jacobs ladder has no spark

We have recently built the Jacobs Ladder kit described in the September 1995 issue of SILICON CHIP but it did not produce a spark. We tested transistor Q1 and found it was producing a square wave at about 10-12V. Q2 also appears to be working. Can you offer any other advice on setting up the circuit? M. D., via email).

The first thing to check is the ignition coil. If it has an internal short it won’t work. You can check it easily by feeding 12V to it and switching it on and off manually.

If that is not the cause, the waveforms at Q1 should be checked on an oscilloscope. The base drive at pin 3 of IC1 should be an approximate square wave with a 3ms high time and 3ms low time. The collector of Q1 should be high (at 12V) for 3ms and off for the next 3ms.

Waveforms at the collector of Q2 need to be checked using a 10:1 probe. Set the vertical sensitivity at 5V to give an overall setting of 50V/division with the 10:1 probe. The collector volts should vary from 0V when switched on to about 225V DC when off. This is assuming that an ignition coil is connected.

The 225V is set by the zener diode voltage string ZD1-ZD3. Check that the zener diodes are connected with the correct orientation on the PC board.

Turbo timer for diesels

I was wondering if the Turbo Timer described in the November 1998 issue is compatible with diesel engines, specifically the Mitsubishi 4D56 2.5L Turbo Diesel? (E. B., via email).

The Turbo Timer keeps the engine running in a petrol engine by maintaining the ignition supply even though the ignition switch has been turned off. If your diesel engine uses an electrical switch to apply power to a fuel solenoid, then the Turbo Timer can be used in the same manner to maintain power to this after it has been switched off at the "ignition" switch.

Big ask for sound level meter

I want to know if it is possible to make a modification to the Sound Level Meter adapter published in the December 1996 issue so I can measure higher dB levels. I would prefer to be able to measure up to 180dB or as close to that as possible.

The reason I require this mod is because I am an amateur car stereo installer and would like to be able to prove the "bone-crushing dB" I can get out of the systems I install. (R. J., via email).

While we are sure you are producing very high levels, a sound pressure level of 180dB would disintegrate buildings, kill any animal, etc. Get the picture. In reality, somewhere around 125dB is the threshold of pain for most people and jet aircraft at close range produce around 140dB. In any case, electret microphones are only good for about 130dB, as far as we know.

You could also never get enough power to produce the levels you are talking about. If it took a 500W system to produce 120dB, to get to 130dB you would need ten times the power, ie, 5000W; to get 140dB you would need 50kW and so on.

Moisture sensor for clothes dryers

How about an add-on for your clothes dryer that gives it shut-off-when-clothes-are-dry capability? All you need is a (high temperature) moisture sensor you place at the dryer exhaust, some mains switching and a start-up delay to allow time for the exhaust air to become moist. The existing mechanical timer could be used as a backup to stop the machine. You could also monitor the mains input current and light a LED if the mechanical timer times out before the clothes are dry. (D. T., via email).

As we understand it, some dryers already have a moisture sensor. Some of these are based on horse hair while others are solid-state (made by Philips, etc). The only real problem we see with the idea is finding a position for the sensor in the exhaust where it will not be clogged by lint. We are not sure how commercial dryers solve that problem.

Excessive voltage from electric fence

I have built the Electric Fence project described in the April 1999 issue and it works well, apart from a terrific arc across the output transformer to the PC board. I have rewound the coil three times, each time as per the instructions. I even made up a couple of 2mm Lexan spacers to keep the turns away from the former edges and lifted the transformer about 6mm off the board but I still get arcing. Any suggestions? (P. R., via email).

You do not say where the arcing is occurring. Is it from within the transformer to the PC board or from the transformer output lead to the PC board? Also where does the arc go to on the PC board? Moving the transformer should prevent arcing to the PC board. Perhaps the PC board does have a fine unetched copper area under the transformer area. Alternatively, the DC-DC converter section may be producing a very high voltage. This should be checked out.

Dimming problem with speed alarm

I recently built the Speed Alarm described in the November & December 1999 issues and apart from stuffing up the location of the sensing unit and having the driveshaft kill it I am very pleased with the unit’s operation and the ease of changing settings. The sensitivity of the LED display at night in dim mode is irritating as street lights cause it to go from intense to dimmed too easily. Very bright sunlight also makes the unit difficult to read. Is there anything I can do to improve this situation.

Could the unit be adapted to work as a cruise control as this feature would be nice to have when driving on the highway? (M. D., Donvale, Vic).

The dimming is supposed to operate quickly so that the brightness of the display reacts to the prevailing light conditions, to make it easy to read. You can slow down the dimming reaction time by connecting a capacitor across the LDR. A 100μF 16VW electrolytic would be suitable but take care with the polarity.

There is little that can be done to make the specified LED displays visible in direct sunlight. They need to be shaded from sunlight. Alternatively, you can use sunlight-readable common anode displays such as the HDSPH151 types from Agilent Technologies. Farnell sell these under their catalog number 264-313. Phone 1300 361 005.

The unit is not suited to operating a cruise control.

FM transmitter for guitars

I am interested in the Minimitter described in the April 2001 issue. Would this type of transmitter work well in a guitar setup? Would the signal be of good quality and hold true to the guitar? Also what type of kit would you suggest for the receiver. I plan on connecting the receiver kit direct to my amp.

The Minimitter should work well with guitars. We have described a mono FM receiver in the November 1992 issue but we would be inclined to use a good quality AM/FM stereo tuner to drive your amplifier. Better still, since you are likely to moving around a stage, you may like to consider our Dual Diversity FM Tuner described in the August & September 1994 issues. This design has two inbuilt FM tuners and constantly switches between the two to give the best reproduction.

Locating interference to AM radio

Has SILICON CHIP ever published an article on how to locate interference to AM radio? I suspect a switched mode power supply from one of my neighbours is causing annoying interference on the main radio station I listen to – 612kHz (4QR) in Brisbane. The interference is there most of the time at night but it sometimes disappears.

The noise is worst on a mains-powered radio but I can hear it in my car radio as I pull into the driveway. Is it possible for my burglar alarm to be causing the problem? I have eliminated all the PCs in my own home by switching them off and the noise is still there. (A. J., Riverhills, Qld).

We have not published an article on locating interference to AM radio but the usual method is to use a sensitive AM portable radio with a long ferrite rod antenna. You will find that this is highly directional and can be used to pinpoint interference provided it is radiated directly from the source and not from surrounding mains wiring.

Other possible sources of interference are Pay-TV cables/decoders, telephones, light dimmers, compact fluorescent lamps, electronic dishwashers and washing machines (even when on standby).

TV boosting & conversion

I want to step down UHF TV (five channels) from a local country TV repeater to VHF for use in a small motel with only VHF sets (the UHF/VHF ones get pinched).

There are any number of dead video recorders available which can still function as a tuner only, providing a good AV output to play with.

In the August 1989 issue of "Electronics Australia" they featured a RGBI to PAL encoder/modulator which used an RF modulator in a tiny metal case for VHF Channel 1. In the July 1999 issue, SILICON CHIP featured a video transmitter with an LM3850 video modulator (from Jaycar). Are these only available with Channel 0 or 1 output; ie, can you get units for other VHF channels (with PAL D output)?

I need five channels. The video recorders can provide two (channels 0 & 1 VHF) but I need another three channels. There is lots of information and products shown on various US websites but most seem to be for UHF and/or NTSC systems.

I want to re-transmit the received UHF TV signals at reasonable strength on a ridge down about 500 metres into a valley in which there is next to no signal. I have in mind to put a 91-element receiving Yagi on one side of the ridge and connect through a 44dB UHF antenna booster (Kingray) into another Yagi pointed down into the valley.

Both yagis have a fair front-to back ratio and would be well-separated. Can you suggest any better way? I don’t think I can afford a full-blown translator or whatever is current for such a problem. (P. B., via email).

Your idea sounds feasible but we would be inclined to try modifying the inbuilt modulator in your defunct VCRs to shift up from Ch0 or Ch1 and get the extra VHF channel signals that way.

Possible ghost in amplifier

I just bought a Technics SU-V460 amplifier and when searching the net I came upon your page www.siliconchip.com.au/html/featserv.htm where I found the SU-v460 under the title "When The Ghost Comes Early". What does that mean? Have I bought a problem amplifier? (How about the sound? Is it garbage?

I had a really old Pioneer before and I think it sounded better; it had a warmer round sound. (M. J., via email).

"When the Ghost Comes Early" was the headline on a TV Serviceman story in our December 1991 issue and it also included a servicing story on the Technics amp. If you would like a copy we can supply the December 1992 issue for $7.70 including postage.

We have not had any experience with the SU-V460 but we would expect it to be fairly run-of-the-mill. Some, but not all, older amplifiers may sound better than some newer amplifiers (we are hedging a fair bit here) because most older amplifiers had discrete output stages whereas quite a few modern amplifiers are based on monolithic designs (ie, large ICs).

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.

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