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but what is exactly a dummy battery. Please explain in more detail please. (G. M., South Morang, Vic). • The expression “dummy battery” is explained in the fourth paragraph of the article on this circuit. As it says, the dummy battery replaces all the cells in the device under test and a variable voltage supply provides the power for the device. In the simplest application, the dummy battery could be a 9V battery snap connector. Sound meter wanted for PA installations You may care to consider doing a project which is able to measure in a dynamic fashion the sound level in a PA installa­tion. I help out at our local church with operating their PA system and with the varied number of people using the PA system, it can become awkward to ensure that the level of sound heard by the audience is constant. In most fixed PA installations, unless the controller is located within the listening area, it can become tedious and inconvenient to have others relay messages to the controller on whether the volume levels are set correctly. Even if you can see the audience and the speaker, often the sound heard by the opera­ tor is different since he is a little remote from the audience. What would be useful is a sound level meter which could be mounted in the control room with a microphone located in the listening area. The meter could be switchable between RMS and peak levels to determine the optimum volume level. While there are a number of sound level units available from many electronics outlets, these are typically portable units which require the operator to be in the listening area and do not Better reception from two antennas? I have a 4X4 and travel to many places in the Outback. I like to listen to the car radio and have noticed that with an aerial mounted on the right rear quarter of the vehicle, the radio has a much better reception if the left front of the vehicle is aimed at the transmitter. This presents a problem because roads are not always correctly orientated. Is it possible to use two antennas and couple them via matching stubs of coax or a resistor network? The plan is to use two equal antennas, vertical, on the mudguards, just forward of the windscreen to get a more circular reception pattern. generally lend themselves to being run remotely. In any event, the displays are typically quite small. What would be more useful is a jumbo size display compris­ing LEDs which could be mounted on a wall in the control room to allow easy viewing by the operator. (N. A., Lyneham, ACT). • You may want to consider our Sound Level Meter Adaptor for DMMs published in the December 1996 issue. You could install several of these around the auditorium and then switch the DC output signals to a DMM with large display at the control desk. We can supply the December 1996 issue for $7.70 including post­age. Digital thermostat needed for a PC I am currently studying electronics at school for my Tas­manian Certificate of Education. One of the assessment Do you know of the correct way to couple antennas to work on both FM and AM? (B. W., Curtin, ACT). • It is theoretically true that two antennas can be connected together with a suitable phasing system to improve reception – for example, phased TV antennas are often used in low signal areas. And for years, truckies have used twin CB antennas mounted on their mirrors, again connected via a phasing harness. However, because of the differences in mounting positions and vehicle types, it is rather difficult to forecast the directional pattern you will achieve using two antennas. The best receiving antenna (as far as uniformity of direction is concerned) would be mounted right in the middle of the vehicle roof. things I need to do is a project by the end of the year. I have been looking into building a digital thermostat to be placed inside the PC case. I would like this to control a fan so if the temper­ ature exceeds a particular level, the fan switches on. I would also like it to run off the PC power supply. (C. A., Devonport, Tas). • We published a thermostat fan control as part of a speaker protection circuit for the Ultra-LD amplifier, in the August 2000 issue. You could just SC build that part of the circuit. Notes & Errata 5A Motor Speed Controller, October 2002: the PC board wiring diagram on page 17 shows a 100nF capacitor next to diode D2. This should be 47nF, to agree with the circuit on page 17 and the parts list. 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. www.siliconchip.com.au November 2002  91