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wire would upset the adjustment. I then “fixed” the turns with a dab of nail polish. L1 is very touchy; L2 takes maybe only 1/16 turn to reset. As I only need to “transmit” over about three metres I used the single wire antenna but even so it works well over 20 metres. (K. C., Balgownie, NSW). • The drift in frequency of transmission is probably due to the capacitance change of the tuning capacitors with temperature. Make sure that the 47pF capacitors across L1 and L2, the 15pF capacitors on pins 9 and 10 and the 4.7pF capacitor at pin 10 are NPO types. These will be labelled “NPO” or with a black dot. NPO stands for “Negative Positive Zero”. This means that the tempera­ ture coefficient of capacitance is zero for the normal operating temperature range. You can expect to affect the tuning when the antenna is held with your finger and thumb. This is because it alters the capacitance of the tuned circuit. Using a transducer with the speed control I am interested in using the 12-24V Motor Speed Controller featured in the June 1997 issue of SILICON CHIP. However, I wish to use the output from a MAP sensor in a car to control the speed of a 12V fan. The MAP sensor has an output range of zero to 5V and I wish to obtain full fan speed when the MAP sensor output is 5V. Furthermore, I would like to introduce an adjustable offset, so that the MAP sensor might produce a voltage of say, 1.5V, before the fan operated. Can this wish list be achieved or is it all a dream? (G. B., Unley, SA). • While we did not make provision for this sort of application it turns out to be relatively easy to do and just by adding one resistor. By connecting a Don’t overdrive the Bass Barrel I intend to construct the Bass Barrel Subwoofer from the August 1997 issue but I need further information. Firstly, what is the power requirement to drive this unit? Also what is the maximum (safe) power the unit can handle? Can you recommend a suitable amplifier kit? What passive low-pass filter could be used in place of the active one briefly alluded to in the arti­ cle? Thank you. (G. P., NT). • The specified woofers have a maximum rated power input of 75W and since they are in parallel, the Bass Barrel would have a nominal maximum input power of 150W. However, we do not think it would be safe to drive them at these high levels for anything more than a brief interval otherwise you run the risk 68kΩ resistor from your transducer to the pin 2 of IC1, the motor will have 12V applied to it, as set by VR1, for an input of 5V from the transducer. Furthermore, the initial offset will be 1.5V, so the effective input range is from 1.5V to 5V. By changing the resistor to 100kΩ, the offset can be re­duced to 0.3V and if you reduced the resistor to 47kΩ, the offset is increased to 2.5V. Notes & Errata Flexible Interface Card, July 1997; Stepper Motor Con­ troller, August 1997; and PC Card For Two Stepper Motors, Septem­ber 1997: in the circuit of each of these boards the resistor from pin 4 of IC1 is shown going to ground. This is how the PC boards of blowing them. In fact, we have heard of a number of instances where people have done just that. Note that this unit was originally designed for cars and is also suitable for small lounge rooms but if you use it in a large room and wind up the wick, you are bound to pay the penalty. A suitable amplifier module would be the 125/175W design featured in the April 1996 issue. This could be teamed with the subwoofer controller featured in the December 1995 issue of SILICON CHIP but remember our warning about winding up the wick. An alternative amplifier would be the 50W unit described in March 1994. Alternatively, if you intend using the Bass Barrel in a Dolby Surround system, you could use the Dolby subwoofer output to drive the Bass Barrel amplifier. will be supplied. The resistor should go to the +5V sup­ply, which is pin 16 of IC1. On each board it is quite simple to move the ground end of the resistor to the positive supply after drilling one hole. If you are only using a single board this may not be a problem, although we do suggest that you move the 10kΩ resistor so that it is wired between pins 4 and 16. The problem shows up when you are using several cards with different addresses on the same printer port and start changing these addresses. Low Dropout 5V Regulator, Circuit Notebook, October 1997: This circuit has the emitter and collector of Q1 reversed. The emitter should go to the +9V supply and the collector should SC connect to the 5V output. 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. November 1997  91