PICAXE control for 1W Luxeon driver

I want to make a PICAXE control the dimming of a Luxeon Star 1W LED. How can I modify your 1W Star Experimenter’s Power Supply circuit (featured the December 2003 issue) to enable the PICAXE to dim the Luxeon using its PWM function? Is it as simple as connecting the PICAXE to the out of the 555? (E. P, via email).

Although we haven’t tried it, we can’t see why you couldn’t feed the PWM output from a PICAXE into the base of Q1 (via the 3.3kW resistor), replacing the 555 and associated circuitry.

Nail detector wanted

I know you have done a number of metal detector projects, however they have been sizeable devices designed to detect objects deep below most surfaces. How about a handheld device?

I re-use a lot of old building material and nails are sometimes difficult to find until you hit them with a power saw! The device I have in mind would be about the same size as the ones used at airports, hopefully a lot cheaper and be able to detect nails and/or staples up to about 25mm below the surface of timber. Hope you can help. (I. B. Monash, ACT).

None of our metal locator projects would be suitable. You would be better off using a stud locator, available from hardware stores for about $30. Some of these are quite good at locating nails or screws in walls, so they would probably work for your application.

PIC programmer question

I was wondering if the Parallel Port PIC Programmer and Checkerboard, described in March 2001, can program a PIC16C71 chip? Do I also need a MAX232 to transmit data to a PC from the PIC16C71? (D. V., via email).

The hardware can be used to program the PIC16C71, although the software described in the article is not suitable. You’ll probably need to update to one of these packages: (1). IC-Prog from www.ic-prog.com; or (2) PICALLW from www.picallw.com.

The details on using this software were published in the "Updating the PIC Programmer & Checkerboard" article on page 79 of the July 2003 issue of SILICON CHIP.

Note that if you’re developing a new project, we would recommend selecting one of the more recent "F" (FLASH memory) series microcontrollers, which can be erased and reprogrammed in circuit.

As far as a PC serial port connection is concerned, you’ll need some kind of RS232 interface and the MAX232 is one of the easiest ways of building one. The PIC16C71 does not include a UART, so you’ll need to implement this yourself in firmware. An application note (AN555) is available from Microchip (see www.microchip.com) describing how this can be achieved.

Powered subwoofer has thump problem

I have just recently purchased a Surround Sound home entertainment centre comprising a Yamaha RX-V650 receiver with the following speaker configuration: B&W DM601 (Mains), B&W DM600 (Rears), B&W LCR600 (Centre) and B&W ASW300 (Subwoofer).

It is a very impressive system with one small problem. I am getting irregular "thumps" over the Subwoofer when the system is powered on. It would appear the thumps are being transmitted via the amplifier, as they disappear as soon as I disconnect the subwoofer to amplifier speaker cable.

I have tried all number of remedies, including a surge protection power board, disabling sleep mode on the subwoofer, isolation of mains circuits, relocation of the subwoofer, etc, but have made no impact.

I am reasonably confident most of the interference is being initiated by my fridge when it powers up/down but am unsure what to do next. Can you offer any suggestions as to what I might try to fix the problem.

The sound is not detectable on any speaker other than the subwoofer. I am running Monster Cable 16 gauge to all speakers (fronts all bi-wired), with a special interconnect cable from the subwoofer to the amplifier. At present, I am being forced to leave the subwoofer powered off, as the thumps are very annoying.(C. D., via email).

Our guess (and it is only a guess) is that the interference from your fridge is being picked up by the input signal lead to your powered ASW300 subwoofer. Keep this lead as far away from mains cables as you can (including cabling buried in the walls). If the input cable must go close to a mains cable, try to route it at right-angles, to minimise pickup.

Electronic blood pressure monitor

I received an electronic blood pressure monitor as a gift. The device is fully automatic and battery-powered only. I have connected it to a 6V DC lab power supply and it worked perfectly except for the valve which controls the air outlet.

When the device is turned on, the valve opens and stays open while the pump is pushing air in to the armband. As soon as the batteries are replaced, the monitor functions perfectly again. Can you solve the mystery and help? (B. K., via email).

Our guess is the that the bench power supply cannot deliver the peak current required for correct valve operation. Try connecting a 2200mF electrolytic capacitor across the output of the supply. This will reduce the output impedance of the supply and it should then be able to supply a higher peak current.

TV aid for hearing impaired

I am hard of hearing and use a set of IR phones while watching TV. This is fine at home but not so good when I visit friends or stay at a motel. The IR transmitter needs to be plugged into an audio output on the TV and of course, not all TVs have this. And even if they do, it will be tucked away right at the back of the set.

Would it be possible to create some kind of acoustic coupling between the transmitter and the TV set? It doesn’t have to be hifi – I only want to hear the program dialog.

Years ago, when phoning England was a once-a-year event, I used to record the phone conversation onto a cassette recorder using a little gadget with a rubber sucker. This was stuck onto the phone and did the job beautifully. So could it be done?(J. G., via email).

The easy solution may be to use a radio microphone, along the lines of that described in the November 1998 issue. Just place the microphone in front of the TV speakers. If you use a lapel microphone, you might be able to clip it to the front of the TV set. You could then pick up the signal in a small FM radio with earphones.

This will not be a hifi solution because there will be a tendency for the microphone circuit to pick up frame buzz from the TV’s CRT deflection circuitry.

Alternatively, if you can feed the signal from a lapel microphone into your IR headphone transmitter, you could achieve the same result.

Master volume control for PortaPal

The PortaPal project in the February and March 2003 issues is a great PA amplifier although it does not include a master volume control. Would it be feasible to place a potentiometer in the feedback of IC5a in place of the 15kW resistor to achieve this? (M. M., via email).

A volume control of 10kW would need to be placed at the output of op amp IC5a, with the wiper connecting to point "A" on the circuit, before the 1mF coupling capacitor and tone control stage IC6a. You will need a 1mF coupling capacitor at the output of IC5a.

Dimming the PicToc clock

Your July 2001 PicToc clock can be used in a car but is there an easy way to dim the display at night? I can only think of switching in resistors to RA0, RA1, RA2 & RA3 separately on the PIC chip, when the parking lights are on.

I also want to turn off the display when the ignition is off. In this case, I would have to use relays to disconnect the RA outputs completely. Is there an easier way. (H. G., Port Augusta, SA).

There is no easy way to dim the display except, as you say, to switch in different resistors for the segments. You could use reed relays to switch resistors in parallel with higher value resistors, so when the relays are closed, the LEDs will be brighter. Doing all that requires no less than eight reed relays. Messy!

VK Powermate modifications

I want to build the VK Powermate 25 (EA, February 1990) but want to modify it and need advice as to whether this is possible.

The plan is to have three stepped output voltages: 6V, 12V and 24V, using a 3-position rotary switch and presets in place of RV1. I suspect a pull down/up resistor on pin 4 of the LM723 would be a good idea as well, to stop the output going sky-high during movement of the rotary switch.

The crowbar circuitry will also have to be changed and I think the easiest way is to make it trip at anything higher than say 27-28V (for simplicity). I’m just not sure what has to be changed to do this. The transformer I have is rated at 28V AC @ 20A. (S. W., Auckland, NZ).

We would not recommend you upgrade this supply with your 28V transformer and then provide adjustable output. It would need massively larger heatsinks to cope with the greatly increased power dissipation at the two lower voltage settings

Conversion of AM car radio to FM reception

Can you please help with a kit or information on how to convert an old AM car radio to an FM unit internally (electronically) so as to keep the outside appearance.(K. M., via email).

What a question. We don’t have any information on how to do such a conversion and we would have to say that it would be practically impossible to do.

What sort of old car radio are you considering? A vibrator-powered valve unit, a push-button unit with permeability or maybe an early germanium transistor unit with transformer coupling between the audio stages? In each case, the only practical approach would be to scrap the entire internal circuitry and just end up with the metalwork and then try and adapt a modern circuit to the existing tuning dial.

If you have an old car fitted with an original radio, our advice would be to get it restored. It will be far more authentic that way.

Cable for PIC-powered rain gauge

I am interested in building the PIC-Powered Rain Gauge described in the June 2000 issue and have the following questions in relation to some aspects of the design.

I saw a commercial unit on the internet and this unit also used a 10m cable to connect the detector with the electronics. Is there a specific reason why the cable is 10m in length? If the situation required it, could the cable be longer or shorter, without affecting the operation of the rain gauge?

Does the cable have to be passed through a grommet and be soldered directly to the PC board or can it be terminated with a stereo 3.5mm phono plug with a suitable socket fitted to the box and terminated on the PC board, or perhaps even use an RJ12 plug and socket. (P. M., Karabar, NSW).

The wire length can be more than 10m and you can use a jack plug and socket.

Bridge mode for Mosfet amplifier modules

In August 2001, you described a 200W Mosfet amplifier module put out by Altronics. I built a pair of them into an old AT case as a disco amplifier for my son. He is upgrading his gear and wants to know if they can be run in bridge mode?

Can you help with any information on this and what sort of power into what load we may expect? Have you described a circuit or kit to enable the bridging? (T. C., via email).

The modules can be run in bridge mode but only into an 8-ohm load whereby they will deliver a total of around 400W.

We have not designed a bridge adaptor but you can refer to the design featured in the June 1985 issue of "Electronics Australia". We can supply a photocopy for $8.80 including postage within Australia.

Moving coil preamp for the LP Doctor

Have you ever published an RIAA preamp for moving coil cartridges. Alternatively, what modification do I need to make to the LP Doctor to match an MC cartridge to it? (P. S., via email).

The preamp in the LP Doctor cannot itself be modified to provide more gain but the inclusion of the following stage involving IC2 probably means that enough overall gain is available, provided the gain control (VR1) is well advanced. If more gain is required, change the 10kΩ feedback resistors between pins 1 & 2 and 6 & 7 of IC2 to 22kΩ.

Powerful horn project wanted for car

Having recently encountered a B-double petrol tanker trying to move into my lane while I was still in it, I found myself wondering if the driver could hear the puny horn of my car in his cab so far away, over the clatter of his own truck.

Car horns today are about 60mm in diameter and are wimpish tooters! This got me thinking on the possibility of making a tone generator and using a horn speaker to make a car horn that sounds like a car horn! This could be a fun and unusual project for those of us who want a better warning device.

The tone could simply emulate the sound of real horns like cars used to have when cars themselves were real. Is it worth considering as a magazine project? (P. C., via airmail).

It really is not worth doing as a magazine project. One of our staff member’s wives has a Toyota Avalon. It is a great car but comes with a single wimpy horn. That was easily fixed. He had the Toyota dealer fit a pair of two tone horns purchased from a car accessory outlet. The cost was less than $100 and now the horns are loud enough to wake the dead!

Lip-Sync Problem With Plasma Screen

With plasma screens increasingly more common, lip sync where the audio is in advance of the video is a common problem. This appears to be caused by video processor delays relative to the audio and can be quite variable, depending on the mode of viewing.

High-end audio amplifiers provide variable audio delay but for many readers, a low-cost digitally selectable delay in the order of 20-160ms would be an ideal project to correct an unexpected problem. Have you any projects that already fit the bill or would this be a new adventure? (P. D., via email).

There is no easy answer to this problem. As we understand it, depending on the particular mode of display and the particular plasma screen, the video picture can be delayed by almost half a second. The delay is caused by the considerable amount of processing required to turn the video signal into a huge pixel display. So even 160ms will not be enough. Nor would it be a simple project concept.

Our advice for anyone purchasing a plasma screen for use in a home theatre system is to thoroughly research this problem before you buy. If possible, try the screen with the proposed audio equipment it will be used with.

Audio Interference To Video Signals

I wondered if you had a project that could help with the following problem. Recently, our church installed a data projector set up and then the computer was moved to the rear of the building to make operation less distracting. The video cable length increased from three metres to 25 metres.

The system works well but we are now getting an interaction between the hearing aid induction loop and the video, where I presume the longer cable is acting as an inductor. The interference appears as dark horizontal bands which are in time with the audio and increase in width and visibility as the audio gets louder.

My impression is that low frequencies are the main contributor as it is most noticeable when there is a lot of bass but then that also usually means more volume! The problem disappears if I turn off the hearing aid loop or pull down the mixer feed to the hearing aid loop during loud audio.

When we ran only a short cable to the projector there was no problem but now we have a long cable the issue has arisen. I have tried two different high-quality video cables, with no difference between them.

The loop runs around the entire sanctuary at floor level or just above, so all cables are actually within the loop area and have to physically cross the loop cable at one point to reach the projector. I know the loop field is strong, as using inductance headphones, it is possible to hear some distance (up to 10 metres or so) outside the building.

The simplest solution would be to install some form of automatic limiting device at the audio mixer feed to the loop amplifier to replicate what we do manually, pull back the fader, like an AGC in reverse. Have you ever had such a project? I do not know if filtering the video would work but if so, we would need a filter at each proposed output point, as the final configuration has video distribution amplifiers. (G. C., via email).

We don’t think there is any easy way to keep the low-frequency audio signal out of the (unbalanced) video cable since you have such a long run. The only practical solution we can think of is to send the video signal via optical fibre if that is possible or send it via a 2.4GHz link.

Question On Charging Battery Packs

Many battery packs for drills and cameras have three terminals. Can you explain what the third terminal does?

I have pulled the battery pack for my Black & Decker drill apart and found that the third terminal is a tap half way up the battery chain. I can only assume that some part of the charger circuit is looking for less than the full voltage rail (ie, presumably the fastidious control circuitry needs less volts than the power control circuits) but it does seem like an appalling practice to deliberately run some of the cells down to a lower state of charge than their mates. (P. M., via email).

The tapping is probably there so that the batteries can be charged as two series strings. This is better than charging the whole set of cells in series and closer to the ideal of charging each cell individually.

Alternatively, on some battery packs, the third terminal is actually a thermistor output. The thermistor is buried in the battery pack and acts as a temperature sensor. It effectively shuts down the charging circuit when the battery pack temperature rises at the end of the charging cycle to prevent overcharging and increase battery pack life.

Notes & Errata Shunt Regulator, June 2005: transistors Q3 & Q4 are shown incorrectly oriented on the overlay diagram and photos and must be rotated 180°. The source and drain markings for the SDP55N03L Mosfet shown at the foot of the circuit diagram are reversed and the 47kΩ resistor in the collector circuit of Q3 connects to 5V rather than V+ as shown. Depending on circuit tolerances, it may not be possible to adjust the trip voltage to 15V as described in the text. To improve the adjustment range, some changes to the resistive divider connected to pin 2 of the L4949 are required. For use with 12V systems, replace the 120kΩ resistor with a 100kΩ value, the 12kΩ with a 9.1kΩ and the 1kΩ resistor with a wire link. All resistors must be 1% tolerance types. Assuming a nominal reference of 1.23V, this will give an adjustment range of about 13.1V to 15.9V. For 24V systems, replace the 120kΩ resistor (connected to VR1) with a 24kΩ value, the 12kΩ with a 4.7kΩ value and the 1kW with a wire link. The120kΩ resistor connected to V+ must be replaced with a 15V, 0.5W zener diode. The effect of the zener is to reduce the hysteresis voltage so that it remains about the same as for the 12V system. Assuming nominal reference and zener voltages, the adjustment range after these changes will be about 25.9V to 30.5V. SMS Controller, October & November 2004: the supplier mentioned in the article (mobileextras.com.au) no longer stocks the data cable needed to connect the phone to the controller board. The project requires a Nokia DAU-9P or compatible data cable with an RS232 serial (not USB) connector. Cellink market a suitable cable, part number MDC887. You may be able to obtain one of these through your local mobile phone accessory store, or you can order one from Wagner Electronics Services, phone (02) 9798 9233 or on the net at www.wagner.net.au. Meter Probe Extensions, May 2005 (Circuit Notebook): the PC board pin sockets supplied by Jaycar (Cat. HP-1260) may vary from those described in the text. If the original items prove to be too small for your probe tips, then an alternative is to use standard female bullet connectors. These will need to be gently squeezed with pliers to close the contacts slightly for a snug fit over the probe tips.

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.