This section presents supported sensors that are compatible with the evaluation board.
The astable multivibrator sensor performs capacitive proximity measurements in loading mode. Two electrodes can be connected, one sensing electrode and one shielding electrode. The sensor draws a supply current of 1mA at 5V when active and qualifies it for low-power proximity sensing applications. In the following a virtual capacitor denotes the capacitance between the sensing electrodes to environmental objects increases with closer distances.
The sensing circuit schematic is shown in Figure 1. It is based on a timer that controls the charging and discharging cycles of the virtual capacitor that is built by the sensing electrode and the surrounding environment. The timer toggles from charge to discharge at the time when a threshold voltage at the capacitor is reached. This results in an astable operation with succeeding charge/discharge cycles. When the capacitance of the virtual capacitor increases, the charging time will also increase and vice-versa.
Therefore, the capacitance is inversely proportional to the number of charging cycles in a given time span. In order to guard the sensor from measuring the capacitance to underlying objects, a shield electrode is placed directly underneath the measuring electrode. The shield is driven with the same potential as the sensing electrode, such that the capacitance between the two electrodes is negligible. Using this shielding method, the measured capacitance will only be slightly affected by the grounded underlying arm.