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Notes & Errata Burglar alarm extensions, June 1999: on the circuit on page 83, IC5a adjacent to IC4a should be labelled IC5c. IC6b’s output should be pin 8. The “ZONE 3” label on the line to pin 5 of IC8 should actually be on the line to pin 7 of IC8. In addition, the output of IC8 driving the Zone 1 relay should be pin 12. The second last paragraph of page 84 should read “Pins 4, 10 and 12 of IC3 are connected to the roller door switches”. Audio-Video Transmitter, July 1999: on the overlay diagram on page 38, the .001µF capacitor shown connecting pin 3 of IC1 quite a lot of standards conversion required in that the horizontal and vertical sweep rates for PAL are quite a lot slower than typical VGA. To do the standards conversion, the circuit would require a line or frame store. If you need one, the best approach would be to buy it. Two of our advertisers, Microgram Computers and Namlea Data Systems, should be able to help you. Faulty sensor on speed alarm Back in your December 1997 issue you published a project for a car speed alarm. I am having trouble with this kit. I cannot get the sender to register any speed. It will show a speed when I switch it on and it will show different speeds when the buttons are pushed but it will not show any difference in speeds when the sensor is put into operation. Also, is there any way that I can make the alarm sound louder to ground should be .01µF. The circuit diagram is correct. Daytime Lights for Cars, August 1999: on the overlay dia­gram on page 33, the 470Ω and 1kΩ resistors shown below diode D3 should be 4.7kΩ and 470Ω respectively. The circuit is correct. Line Dancer Robot, May 1999: in the circuit diagram on page 18, the 4.7kΩ collector resistor for Q1 should be labelled R5 instead of R3, while the 1.5MΩ base resistor for Q1 should be labelled R6 not R5. On the PC overlay diagram on page 19, the 470Ω resistor below R14 (270Ω) can be replaced with a link (it is in series with R5). as I am a bit hard of hearing? (K. M., North Booval, Qld). •  We are not sure that you do have a problem with your speed alarm. The display section of the project is operating correctly since it does display the alarm speed and can be changed using the buttons. It will not and is not supposed to show the dif­ference in speed between the set and actual speeds as the car is driven. However, if you are saying that the alarm does not sound when the preset speed is exceeded, then you may have a problem with the sensor circuit. Check that the magnets and sender are located close together when they pass each other. You may find that the sensor operates correctly if the magnet is flipped over so the opposite pole is facing out. Check all wiring from the Hall Effect board to the main PC board. The output level of the speed alarm can be varied by adjusting VR2. This alters the frequency driving the piezo transducer and if adjusted carefully you will find a position where the sound is loudest. This is the resonance point for the transducer. An alternative method of increasing the sound level is to use a powered piezo transducer which will operate at 5V. This can be driven between the collector of transistor Q3 and 0V. Note that for this connection you will always have the siren sounding above the alarm speed setting. A suitable siren could be the Dick Smith Cat L-7024 or Jaycar Cat AB-3462. Transistor brands for class-A amplifier How important to the final performance of the Class-A amplifier July & August 1998) is it to use the speci­fied Philips/Motorola BC547s, 557s, 327s and 337s? I look forward to building it. (S. H., via email). •  In a high performance amplifier such as this, it really is important to go for the “brand-name” transistors. Other transis­tors will certainly work but there is no guarantee that the finished amplifier will have really low distortion. SLA charging information wanted In an issue of SILICON CHIP I read an article on an SLA charger. That article had quite a bit of information on SLA batteries. I am chasing after information on charging techniques and more importantly discharge curves for SLA batteries. Can you pinpoint the issue please? (A. H., via email). •  It seems likely that the article you are referring to was one featuring the UC3906 SLA battery charger IC, published in the March 1990 issue. We can supply this issue for $7 including SC postage. 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. September 1999  93