DUAL_TIMER_MEASURES_C

Published:2009/7/10 21:37:00 Author:May | From:SeekIC

One section of U1 (two 555s in single package) is connected as oscillator that serves as trigger for other section (U1B). Ratio of R1 and R2 determines length of pulse generated during each oscillation cycle, while C1 and same resistors set frequency at about 500 Hz. U1B produces predetermined-duration output pulse for each start pulse regard-less of starting pulse length. Pulse duration is set by R4-R7 and external capacitor being measured. Smaller capacitor in given range produces shorter output pulse from U1B mono MVBR. Average pulse power increases with pulse length and increases meter reading linearly so capacitance value is indicated directly.Values shown give ranges of 1000 pF, 0.01 μF, 0.1μF, and 1 μF full-scale. R10 serves as calibration resistor for all three higher scales. D1 is 50-PIV or higher silicon power-type diode, and D2 is 1N914 or equivalent.-D. A. Blakeslee, An inexpensive Capacitance Meter, OST, Sept.1978, p 11-14 and 37.



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