Help with LED bargraph ammeter

Can I ask for your assistance with the Bargraph Ammeter published the January 1999 issue? This meter measures 25-0-25A. How can it be altered to read around 10-0-10A as I want to use it in my caravan to keep a check on the battery which will be connected to an 8A regulated charger. The only analog meters seem to be 60-0-60A which would be useless. (N. G., via email).

There are two things which affect the circuit sensitivity: the shunt resistance and the gain of IC1a. Provided you have a suitable shunt, you simply have to calibrate the circuit for 10A-0-10A by adjusting trimpot VR1.

Can Mighty Midget run from 24V?

I was reading about the Mighty Midget power amplifier in the March 2002 issue and I was wondering if it can be adapted for 24V for my off-grid house. I could use a 24V to 12V step-down inverter for every amplifier but this is inefficient and wish to use a 24V amplifier. The TDA1562Q chip in your design has an 18V (max) rating. Is there a substitute IC that has a higher rating? (G. M., via email).

Sorry. No can do. The best we can suggest is the 25W module using the National LM1875T featured in the December 1993 issue. Running from a 24V (nominal) supply, it should deliver around 14W into a 4-ohm load.

High energy ignition current limit

I have recently constructed one of your High Energy Ignition kits and came across a small problem. Once I had fitted the kit to the vehicle and made all of the required connections, I went about setting the current limiter. I set trimpot VR1 all the way clockwise, got a 12V supply and turned the ignition on, all as described in the instructions although the lowest reading I was able to get across the 0.1Ω resistor was 0.28V. This went up to over 0.34V. However, 0.25V was specified by the instructions and I'm not to sure how this came about.

It should not be a problem though should it? This just means that the coil might get a little more current than if the pot was set to 0.25V? The car seemed to run really well and the coil did not generate to much heat. (J. B., via email).

The 280mV limit is OK. This sets the maximum coil current to 5.6A. Since the coil doesn't run hot it will be fine.

PIC-based Speed Alert uses speedo signal

I built the PIC-based Speedometer Alert (November & December 1999) project a while ago for a Ford Falcon XF. I have mounted the unit above the steering wheel column and it works fantastic. I was looking through the factory manual of the car and realised it has an electronic setup for the speedo that's the same as your design, using the magnet and pickup coil.

I successfully attached the factory pulse wire to your PIC speedo without any resistors between them. So now the two speedos share the same pulse wire. In doing this, it disables the factory speedo in the dash. As soon as I remove the PIC based speedo, the factory speedo works fine again.

Would you be so kind as to tell me how can I run both your PIC Speedo and the factory speedo using the same factory pulse wire? (A. P., via email).

The 1kΩ resistor at the pin 2 input to IC2a should be increased in value to 10kΩ to prevent it loading the ECU speedometer signal. Also, remove the 0.1μF capacitor at pin 2 of IC2a.

Windows-Based EPROM Programmer I'm having a bit of trouble with the Windows-Based EPROM Programmer software (SILICON CHIP, November/December 2002 & February 2003). It gives an error when I try to verify a write it has just done - it returns a run-time error 6 "overflow". I'm using the latest version of software from your website and I'm trying to program an ST M27C512 chip using the packaged device configuration. The write to the chip is good and it works OK but I simply can't verify the data written. Do you have any suggestions? (A. S., via email). It seems that you may be getting "ringing" on the printer port interface, when the data is being read back during verify. You may need to change the printer port settings in BIOS or reduce the value of the pullup resistors on the programmer's data line inputs. Another thought: are you using a good quality shielded DB25-DB25 data cable? If you're not, all kinds of weird problems can be experienced with an unshielded ribbon type cable. This type of problem is very similar to the problems caused by a poor quality unshielded cable.

Electronic delay for tweeter

Could you please advise whether the 20ms Digital Delay kit from the February 1996 issue is modifiable to allow different delay times, possibly as short as 1ms and how accurately? In addition, would the circuit have any effect on the signal?

I'm considering another project where this will be used to delay the signal to a tweeter but it has to be as transparent as possible to the sound, as it'll be part of a high-sensitivity horn system, which is very intolerant of distortion. (P. S., via email).

The 20ms delay project uses an M65830P delay IC. This can be set for delays between 1ms and beyond 30ms using codes sent to the REQ, SCK and Data inputs. Accuracy is set using a crystal timebase. Note that filtering at the input and output of the delay IC will introduce phase delays.

Total harmonic distortion through the delay is 0.3% at 1V and 1kHz and 3% at 10kHz.

We have used this IC for a stereo simulator (June 1996), a digital reverb (December 2000 & January 2001) and the LP Doctor (January @ February 2001). These each used a circuit to set the time delay using standard ICs. A microcontrolled unit was used in a Dolby Prologic Decoder (November & December 1995).

Fast clock for model railways

I would like to build a "fast" clock for my model railway. Have you ever featured one? I would like to use big digit displays. (K. S., via email).

We described a Fast Clock for Model Railways in the December 1996 issue. However, it was based on a standard 32kHz crystal-controlled clock movement so you can make an analog fast clock as small or as large as you want. Some people have even used a crystal watch movement and built it into a clock tower on their layout!

We have not described a fast clock circuit using 7-segment displays. We can supply the December 1996 issue for $8.80 including postage.

Winding inductor for battery charger

I have a quick question regarding the winding of the inductor in the Fast Universal Battery Charger Mk II (June & July 2001). It states "20 turns bifilar wound" which I understand. However the former only holds about nine turns when wound bifilar. Should I wind left to right, bring the wire back to the left and continue in this fashion until I reach 20 turns or should I wind left to right, then right to left, etc until I reach 20 turns? (A. L., via email).

You wind on the first layer and then continue winding with the turns returning back to the start end. Always continue winding in the same direction and do not bring the wires back to the start after each layer is made.

Tachometer for machine tools I'm looking to build a tacho for machine tool use; ie, lathes, milling machines, etc. However, this does not rule out an automotive tacho for the above purpose. The spindle speeds range from 10 - 10,000 RPM. I would like 10 RPM minimum resolution/increment or better still, actual real time RPM. I have searched the SILICON CHIP and EA websites and have found possibly nine tachos. The question is: which tacho is the right one to build? (S. D., via email). The 5-Digit Tachometer published in the October 1997 of SILICON CHIP is the only one which suits your application. It updates at 0.25s intervals (four times per second), has 1 RPM resolution and operates from 1 - 60,000 RPM. However, it only has a 100:1 range so if you want a 10,000 RPM reading, the lowest reading would be 100 RPM. If you set it at 10 RPM minimum, then the maximum would be 1000 RPM. You could incorporate a switch which selected the range for measurement (ie, 1-1000 RPM in position 1 and 100-10,000 RPM for position 2). This switch would change the capacitor on the phase lock loop oscillator.

Log and linear pots explained

I'm fairly new to electronics and this may be a silly question but could you please tell me the difference between logarithmic and linear potentiometers? Where are they best used in applications? (A. D., via email).

That's not a silly question. A linear pot has a linear increase in resistance between wiper and one end terminal as you turn the shaft. At half travel, you should have roughly equal resistance between the wiper and the end terminals. You can check this for yourself using your multimeter. A log pot has a logarithmic increase in resistance as you turn the shaft.

Log pots are often used in amplifier volume controls where their response is more suitable for matching the logarithmic response of human ears.

Hall sensor trigger for strobe light

Could you please tell me how to connect a Hall Effect sensor to trigger the Strobe Light described in the August 1998 issue of SILICON CHIP? I want to trigger it in sync with a rotating shaft. The Hall Effect sensor will be positioned to within 0.030-inch of the projections (bolts) on the rotating shaft and these projections will trigger a pulse in the Hall sensor. (P. G., via email).

The triggering will depend on the Hall sensor and what it gives as an output. If as you say it gives a pulse, presumably high when activated, then there is no reason why it cannot be used to drive a transistor in a similar manner to Q1 in the circuit. The collector of the extra transistor would connect to pin 2 of IC2.

If the Hall sensor provides a low output when activated and an open circuit output when not activated (normally pulled high with a resistor), then the Hall output could be connected to the anode side of diode D2.

In either case, the ground or negative supply for the Hall sensor would connect to the ground of the strobe circuit.

Multiple neons for the sound modulator

I would like to modify the neon tube Sound Modulator kit (SILICON CHIP, November 2001) so that it can power up to perhaps 10 30cm neon tubes. Which components would I have to change and what would their values be? (M. K., via email).

These neon tubes typically draw 250mA or 400mA at 12V so you could power 10 or 15 of the 250mA devices off the one neon modulator and you could probably run more than that if you fitted the Mosfets with a suitable heatsink. No other circuit changes would be required.

How to charge 11V Lithium batteries

Is it possible to modify the Multi-Purpose Fast Battery Charger (SILICON CHIP, June & July 2001) so it can charge the latest Sanyo Lithium Polymer battery packs which have a nominal output of 11.1V? (M. W., via email).

The 12V position for switch S5 needs changing to cope with 11.1V batteries. This can be easily done by removing the 150kΩ resistor in parallel with the 12kΩ resistor. Re-label this position as 11.1V. Note that it will no longer be suitable for charging 12V Nicad or NiMH battery packs.

Erratic results from current clamp adaptor I have completed the Current Clamp from the September 2003 issue and on first test in my car with headlights switched on and off, the indication was OK. However, attempts to repeat this procedure produced erratic results. I could no longer null the meter reading with VR3 and the digital meter gave a range of false current readings. Having disassembled the 100-turn calibration coil, I could only readjust VR2 to obtain a zero on VR3. It seems to me that VR2 at 50kΩ is too big a range of resistance for such a small fixed pot. In fact, a 10-turn pot would have been better. The existing VR2 could change its resistance slightly if the device was accidentally knocked or due to vehicle vibration. The drifting readings of the clamp could be due to the sloppy operation of the battery clamp itself, poorly constructed for this role. Have you any suggestions? The idea is great and for a device that would be used infrequently, it could play a very useful role in trouble-shooting. I built it to check the charge rate on a friend's boat, when he uses a petrol generator to charge his batteries via a mains charger. There is no ammeter installed. (D. J., via email). You can use a multi-turn pot but this would add to the expense. We used a better clamp available from Dick Smith Electronics. Some battery clamps are very poor, even for use as a battery clamp. The actual zeroing can really only be set to within ±0.1mV. Some multimeters can show better resolution than this. As mentioned, the core can become magnetised due to DC current. This can produce an offset in the reading. The zeroing range may not cater for this and the core will need to be demagnetised by reversing the clamp over the current carrying wire. For more critical measurements, a much larger core should be used which will not become magnetised as readily. The clamp would then need to be considerably larger to accommodate the core. For fixed current measurements, the clamp can be dispensed with and the core held in position over the wire with tape or cable ties or glue.

Sidereal clock wanted

Some years ago, your magazine had two articles of interest to me. One was a sidereal clock and the other an astronomical clock, made by a person who sold the PC board EPROM and switches as a kit. I would like to know whether the person is still doing the kit or failing that, whether the sidereal clock kit and parts are still available. (R. M., via email).

We have described two sidereal clocks, in March 1993 and August 1993. The March design had an LCD but did not use a micro to drive it. All the parts and the PC board should still be available. The August design used a Z80C micro and EPROM to drive a double clock display (7-segment LEDs). It is unlikely that it is still available as a kit.

Problems with digital thermometer

I have built the Digital Thermometer/Thermostat (August 2002) to control an incubator but I have run into problems with the testing procedure and also the alarm adjustment side of things.

The testing procedure starts off correctly with both TP1 and TP3 being able to be adjusted to +2.49V and -2.49V respectively. The problems start when it comes to measuring the offset voltage. With Sensor 1's positive terminal, TP1 and TP4 shorted to ground, the multimeter reads -0.3mV to -0.2mV and counts up to 0.0mV where it stabilises.

Continuing on with the testing procedure and taking the offset voltage to be 0.0mV, everything tests perfectly until step 9. Instead of the same reading on the display and the multimeter (reference thermometer), the temperature displayed on the unit is 4-6°C higher than that indicated by the multimeter.

The other problem is that S2 doesn't function correctly to set the alarm temperature. First, VR7 has no effect on the alarm temperature at all. Also, every time S2 is pressed, the display has a different reading.

The most common reading it will give is to delete the decimal point, so if the unit is reading 27.3°C, pressing S2 causes it to read 273°C. If the unit is switched to the higher range, pushing S2 usually has no effect at all. This is the most usual outcome but it also just gives random readings and will sometimes begin to count up or down.

I have been over the construction no less than four times and am positive that all the components are in their correct places and orientated correctly.

The unit, although reading 4-6°C higher than it should, appears to display the temperature properly, with only a faint blow on the thermocouple causing it to increase temperature instantly. (M. H., Rylstone, NSW).

Most probably, switch S3 is incorrectly wired. Check the contacts of the switch and note that the (C) common terminals are not the centre pins but the outside pins.

If the switch is upside down to the shown orientation, the wiring will be incorrect.

One-way intercom for deaf driver

A friend of mine suffers from age-related deafness. He wears a hearing aid but it amplifies everything, including background noise. The problem is most noticeable when driving as he is unable to distinguish conversation from the ambient noise. Since he is now retired and wishes to spend much of his time travelling, this has become a significant problem.

I have looked through past projects in SILICON CHIP but none appear suitable. What I had in mind is some type of one way intercom which has a sound activated and noise cancelling microphone for the passenger and a headset for the driver. Do you have any suggestions? (I. C., Euroa, Vic).

Perhaps the most applicable project is the FM radio intercom for motorbikes, published in the October & November 1989 issues. This was an FM link and two of the chips used in the circuit are now superseded. However, it did feature a noise-cancelling microphone which you could still build and this could drive our Guitar Headphone Amplifier, as featured in the May 1995 issue.

We can supply these issues for $8.80 each, including postage.

Calibrating the reaction timer I recently assembled the reaction timer from the June 2003 issue but have never been capable of calibrating it as per the instruction schedule. The best I can get VR1 adjusted to is 608Hz (not 1kHz). Any suggestions? (K. It sounds as if your 40106/74C14 chip has somewhat different switching thresholds compared with the chips used in our prototype timers, and this is lowering the clock oscillator frequency. That's no big deal. To fix the problem, all you need to do is reduce the value of either the capacitor or the fixed resistor in oscillator IC1c, to allow the frequency to be raised to 1kHz. You could replace the 100nF capacitor with one of 56nF, for example, or replace the 15kΩ resistor with one of 10kΩ. You should also check the supply to IC4 at pin 16. This should be around 5.6V. Note that you should have some frequency output from pin 4 of IC4, even if it isn't locked in phase with input. In addition, check for a short at pin 4 or check if pin 5 is at ground. Also, are the components connecting to IC4 correct and are there any shorts to adjacent pads or tracks?

Fish tank heater for etchant

Do you know of a suitable heater for Jaycar's etching tank? I would like to be able to heat up ammonium persulphate to the required temperature. Would a fish tank heater be hot enough, or would I need something bigger? (A. H., via email).

A fish tank heater will work fine but you need to set the thermostat as high as possible.

Bike horn for bike alarm

I have a question about the bike alarm featured in the January 2002 issue. Is it possible to wire it up to the bike's horn instead of using a separate piezo horn? Or won't this chirp properly as the kit is designed to do? (S. F., via email).

As far as the Bike Alarm is concerned, it is doubtful that the MJE3055 could handle the current of the bike's standard horn. The problem is not the rating of the transistor itself; it is just not supplied with sufficient base current for it to handle high currents. Maybe it would work if you substituted a high-gain Darlington power transistor (eg, BD649) and fitted it with a reasonable heatsink but even then it will only handle a current of 5A or so (and the bike battery will flatten quickly!).

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.