Nuts and Volts - May 2010

READER-TO-READER TECHFORUM

near the fixed one.

Mike Hardwick

Decade Engineering Turner, OR

not indicate what brand they were. If they are the no-name generics, they would be more prone to external noise.

[#1104 - January 2010]

Speaker Hum

I have a problem with my speakers humming on my desktop PC. I think this may be wall wart related but not 100% sure.

Is there a way to fix this problem or do I need to buy a new set of speakers?

Ralph J. Kurtz Old Forge, PA

[#1105 - January 2010]

Automotive Speed Sensor

I recently replaced the trans in my Ford van. Somewhere between 00 and 02 model years, Ford changed the speed sensor output to the PCM, but I have an analog gear driven sensor. The newer sensor is digital (variable reluctance). I need to change the signal back to analog so the speedometer, ABS, and cruise will work. Is there a device to do it, or is it even possible?

(see the National Semiconductor and Linear Technology sites), giving further insights on capacitor selection and much more. Alternatives are the LM331 (±0.06% non-linearity); Linear Technology has some precision circuits going into the PPM region.

Walter Heissenberger Hancock, NH

You could look at several areas to solve the speaker hum with your PC.

First, it could be a poorly filtered wall wart and you might want to replace it as the filtering that it has is just not enough to overcome hum in the system. You could also check/ replace the cable that goes between the sound card in the PC and the speakers. Could be that the shield is cracked and this is where the hum is being introduced into the system. Also, try to use as short a cable as possible between the two as the longer the cable, the more likely it will act as an antenna picking up any electronic noise anywhere near the system. Try moving the speakers to another location a short distance away from the PC and see if that improves the sound quality. If the sound card is external to the motherboard, try removing and reseating it as there may be some contact issues between the card and the motherboard slot that the card plugs into. Lastly, trash the speakers you have and go with a better quality speaker system. You did

[#1106 - January 2010]

Design/Test Software

I've had electronic engineering schooling back in the ‘80s and have tinkered a little since then. I've never had a computer until now and would like to know some free or reasonable circuit design/test software download sites to tinker with.

All you need is a Frequency-to-Voltage (F/V) converter with a Schmitt-trigger input. There are many published circuits available to pick from. Figure 2 shows an adaptation of a National Semiconductor circuit (AN-140). A comparator was used — instead of the CMOS-Schmitt inverter — to obtain better control of the input switching thresholds. R1 and R2 adjust the level, and R3 the hysteresis. A clean 50% square wave must be at the output of U1, swinging almost rail-to-rail (if necessary adjust R1, R2, and R3). Current flows on a negative output swing from the inverting node through D1 and C1. The current is only a function of the frequency and size of C1, since the output swing of U1 is fairly constant. C2 and R6 provide a voltage output. There are many different variants of this circuit

Circuit Simulators: LTspice IV (free, no limitations, excellent; Linear Technology) and Tina-TI (free scaled-down version; Texas Instruments).

CAD/Layout: ExpressPCB (free) and ExpressSCH (free) are PCB layout and schematic software. Very consistent and easy to use. QuickCAD mechanical CAD software is from Autodesk.

Miscellaneous: AppCAD (free; Agilent) calculates transmission lines, S-parameters, microwave designs, amplifiers, biasing, reflections, noise, etc. MPLAB (free; Microchip Technology, but needs a plug-in for C or MBasic, and a programmer) software is used to program PIC microcontrollers. There’s Microsoft Office which includes Word, Excel, PowerPoint, and Access for general office type applications such as documents, spreadsheets, presentations, and databases. FilterPro (free, Texas Instruments) software is used to design active filters. Last but not least is SpectrumLab (free; uses sound card to digitize and analyze spectrum).