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This circuit was originally designed to measure the volume of the fluid inside a 10cc syringe. It used two copper foil strips attached to the outside barrel of the syringe. The fluid between the two
copper strips increases the capacitance. As the fluid volume decreased, the capacitance also decreased. With the circuit shown below, it is possible to calibrate the circuit, so the voltage produced is proportional to the fluid volume inside the syringe. With the values shown,
the circuit produces a 5v DC output, when the 10cc syringe is full. It is possible to measure the fluid volume to a resolution of 0.01cc. The circuit can also be adjusted to produce a 5v DC output voltage for a capacitance of 10 picofarads to a solution of 0.01 picofarads.
The circuit measures the unknown capacitance by applying a clean 20KHz 5v peak to peak triangle signal to the capacitor. The current routed through the capacitance is sent to a current to voltage converter.
The voltage that emerges is therefore a 20KHz square wave signal, whose amplitude is proportional to the capacitance. The signal’s peak amplitude is rectified and filtered to produce a DC voltage. Additional op amps provide a means to adjust the zero and span of the circuit, so a fixed
change of capacitance will produce a known voltage change. A fixed 9v DC supply powers the complete circuit. |
