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Tuesday, 24 February 2009

Frequency and Capacitance meter

n [...] idea is to use the 555 as a monostable, and trigger it with a fixed frequency clock. Duty cycle will be proportional to capacitance.

Circuit will look something like:

+5 ---+---------------+---+
| | |
R +----------+
|(see below) | 8 4 |
+---+------+-----|7 3|------/\/\/\---+------- Vout
: | | LMC555 | |
C to test +-----|6 | -----
: | | -----
ground +-----|2 5|----+ |
| | 1 | | ground
| +----------+ 0.1 uF
| | |
Clock ----+ ground ground
^
(see | next section - CMOS Oscillator)

The ON time for the monostable is about 1.1RC, so component values that should work would be a 50 Hz clock, say a 1 Hz low-pass filter on the output, and R = 9.09K, 1%. That combination will give an output of one volt per microfarad. Switch R in decades for smaller capacitors. Trim R for calibration.

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