Digital Capacitance Meter with RS232 interface
Unlike almost all SMD resistors, the SMD ceramic capacitors are not marked with their values. To find the value of these capacitors we need a capacitance meter. The circuit shown below can measure capacitors between 1pF and 65uF on two areas which are switched automatically.
Circuit diagram of capacitance meter
Consumption of this assembly is very low, only 4 .. 5 milliamps so it can be powered from the RS232 serial port. The power supply is provided by RTS pin (#7) of the RS232 serial connector (#1 on J1) and it's regulated by Zener diode D3 to 5,1 volt. Q4,D1,C5,R9,R10,R11 provide an RS232 interface for serial communication to the PIC12F629. The negative voltage used for RS232 RxD pin (#2) logic level, it is taked from TxD pin (#3). To compensate the reverse biased diodes leakage used in PIC12F629 on I/O pins an LM358 as voltage follower is required.
Because this circuit is self calibrating it must powered with no capacitor connected on J12. The accuracy of this circuit it is better than two percent. However, statistically speaking, capacitors with tolerance less 5% are not commonly used.
RS232 capacitance meter
HyperTerminal or other RS232 serial port freeware utilities can be used to display the capacitor value. Settings are 9600 8N1 and no flow control with some exception. Rsterm terminal need to set hardware flow control. In Terminal v1.9b check RTS/CTS box from Handshaking window before to press connect button.
Hex and Orcad source files are available for download.