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EDN, May 26, 2011

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designideas At this point, the output of the sense , changes from logic zero to logic one. When this event happens, the threshold of the gate passes the unknown voltage. You can estimate the unknown voltage using a graph of threshold voltage versus supply voltage, gate, IC1A 7 6 ESTIMATE THE UNKNOWN VOLTAGE USING A GRAPH OF THRESHOLD VERSUS SUPPLY VOLTAGE. such as the one in Figure 2. By fitting a parabola or a polynomial to the experi- mentally obtained points—say, some 20 points lying in the supply-voltage range of 2 to 15V—you can estimate the threshold voltage, VT 5 * * THRESHOLD VOLTAGE (V) 4 3 * 2 1 0 , for any sup- ply voltage. This circuit has been built and tested. The online version of this Design Idea includes Octave/Matlab code that you can view at www.edn. com/110526dib.EDN * * * 24 68 10 12 SUPPLY VOLTAGE (V) 14 16 * * * * EXPERIMENTAL FIT * * Figure 2 The gate's threshold voltage is nearly linear with respect to the power-sup- ply voltage. You can download this plot at www.edn.com/110526dib. EDN 110512DI5141 FIGURE 2 DIANE Measure resistance and temperature with a sound card Zoltan Gingl and Peter Kocsis, University of Szeged, Szeged, Hungary ↘ Unless you add a measurement instrument to your computer, you have only the sound card as an analog I/O port. You can use the sound card to digitize ac analog voltages but only within a limited range. You can, however, add some signal processing and measure a wider variety of signals, even those that produce dc or low- frequency outputs. For example, you can directly connect a thermistor to make a sound-card thermometer to monitor or record the temperature on PCBs (printed-circuit boards), circuits, heat sinks, and more. Thermistors are popular tempera- ture sensors because they allow easy detection of changes in resistance. 58 EDN | MAY 26, 2011 EDN 110526DI5148 EQUATION DIANE T= TO 1 where RT and TO 1 + β ln RT , RO You can find the value of β in a thermis- tor's data sheet. Figure 1 shows the easiest way to interface a thermistor to a sound card. The microphone input has an internal bias resistor, R, with a typical value of 2 to 5 kΩ. A dc bias voltage drives this resistor. The bias resistor connects the thermistor between the line or the which the thermistor's resistance is RO is the thermistor resistance is the temperature in Kelvins at . Once you measure a thermistor's resis- tance, you can apply the following equation to find the temperature: 1 headphone output and the microphone input, which forms a voltage divider with the internal bias resistor. Those components are all the circuit needs. Note that some microphone inputs may have different internal connec- tions, so check yours before use. You also need a sinusoidal signal because the sound card's inputs are ac- coupled. The sound card's audio out- put can produce an ac voltage at the microphone input, whose amplitude is proportional to R/(R+RT ). You can do as 0 and 10 kΩ. A sound card's measurement accu- racy is worse than what you could achieve using a commercial data- acquisition card, but this ratiometric arrangement and calibration keep errors to approximately 1% for resistor values of 1 to 100 kΩ. Even without a simple calibration to find the output signal's amplitude and the value of R by replacing RT with known values, such

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