## Frequency and Capacitance meter

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• Frequency and Capacitance meter From: kenny@m.cs.uiuc.edu An [...] idea is to use the 555 as a monostable, and trigger it with a fixed frequency clock. Duty
Message 1 of 2 , May 29, 2001

# Frequency and Capacitance meter

From: kenny@...

An [...] 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.

• Frequency and Capacitance meter From: kenny@m.cs.uiuc.edu An [...] idea is to use the 555 as a monostable, and trigger it with a fixed frequency clock. Duty
Message 2 of 2 , Oct 30, 2001

# Frequency and Capacitance meter

From: kenny@...

An [...] 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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