Electronic Design

Sense Small Capacitance Changes Over A Wide Range

With this simple circuit connected to a serial port, small capacitance deviations on the order of 0.01 pF can be easily sensed over a wide range of several hundred picofarads (see the figure). Using an RC oscillator such as a TLC555 and an eight-pin Atmel ATtiny12-8 microprocessor, the period of multiple oscillator cycles--which is directly proportional to capacitance--can be sensed. Applications include displacement measurement and liquid-level sensing.

The 555 oscillator's frequency is given as F = 1.44/(R1 + 2R2)C, where C is the sum of the IC's internal capacitance (or its equivalent), C1, and the CX to be sensed. The RISC-based microprocessor operates at an instruction rate of 8 MIPS and counts the number of crystal-oscillator clocks between a fixed number of 555 output pulses. This count is formatted and transmitted as three serial 8-bit binary bytes at each RS-232 DTR transition. The three-byte count is given as:

Co = 0.693NM(R1 + 2R2)C

where:

N = fixed number of 555 clock pulses (synchronized to the negative edge)

M = MCU crystal-oscillator frequency

R1 = resistance from VCC to pin 7 of the TLC555

R2 = resistance from pin 7 to pin 6 and 2 of the TLC555

and

C = CX + C1 + CI

where:

CX = capacitance to be sensed

C1 = fixed value

CI = TLC555 internal equivalent capacitance

The circuit shown allows up to 14 measurements per second for CX values of 0 to 300 pF and can indicate changes of 0.01 pF throughout this range. Due to the circuit's low current requirement, it can be powered by unused RS-232 signals as shown.

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