Help with LED bargraph ammeterCan 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 limitI 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 signalI 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.
Electronic delay for tweeterCould 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 railwaysI 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 chargerI 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.
Log and linear pots explainedI'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 lightCould 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 modulatorI 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 batteriesIs 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.
Sidereal clock wantedSome 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 thermometerI 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 driverA 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.
Fish tank heater for etchantDo 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 alarmI 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!).
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