RF laboratory often requires capacitance meter for small capacitors in pF range. Such a device can easily be built by yourself. Here, a measurement converter for PC serial port is presented. The frequency of an oscillator is reduced by the target and measured on a PC. The appropriate conversion then allows the direct display of the capacity. The input uses a short and low capacitance probe tip. The opposite pole is clamped to ground cable with a crocodile.
The NE555 precision timer receives its operating voltage directly from the serial interface and produces a measurement object without C is a square wave with a frequency of 3.5 kHz. The signal is processed via the CTS input of the interface.
The VB program based on a frequency measurement program from the learning package electronics experiments on a PC. It uses the ELEXS.DLL to access the serial port. The measurement determines the capacity of the target C in relation to the reference Cref = 3300 pF. The meter can be calibrated by adjustment of Cref.
Const C ref = 3300
Private Sub Timer1_Timer ()
freq = 0
alt = CTS ()
While TIME READ <1000
new = (CTS)
If new> old Then freq = freq + 1
old = new
frequency = freq
If freq = 0 Then freq = 1
If zero * zero C = C ref / freq – Cref
C = Int (C)
Text1.Text = Str $ (C) + “pF”
Capacitance Measurement with the sound card
Especially for laptops without RS232 is convenient to use the sound card to measure. On his website you will find further applications of the sound card such as the measurement of inductance:
The capacitance meter now requires a different power supply. You can use a 9-volt battery or eg +5 V from USB of the computer. The NE555 provides a stable frequency that is largely independent of the operating voltage.
The program handles frequencies up to 10 kHz. Therefore, it is possible to use a smaller reference capacitor of eg 1 nF in the circuit. Thus, the resolution increases to about 0.1 pF. Also important is a level matching. The following circuit shows the fit obtained with a voltage divider. Here, the left channel of the line input is used.