Capacitive Touch Switch for Automotive Applications 3

The circuit consists of four components, with the discharge switch initially in an open circuit state. When the switch is open, all current enters the sensor, causing its voltage to undergo linear conversion. The charging operation continues until the sensor voltage reaches the threshold level of the comparator. The comparator then transitions from low to high, causing the discharge switch to close. The capacitive sensor rapidly discharges to ground through this low impedance path. This process will cause the output of the comparator to transition from high level to low level, and then repeat this cycle. As shown in the following formula, the output frequency (fout) is proportional to the charging current and inversely proportional to the threshold voltage and sensor capacitance. Measure the frequency to determine the sensor capacitance:   diode

When the charging current is 5mA, the comparator threshold is 1.3V, and the sensor capacitance is 30pf, the output frequency is 128kHz. The longer it takes to measure the output frequency, the higher the resolution obtained. The improvement of frequency resolution will enhance the sensitivity of capacitance measurement. Increasing the measurement time will improve the resolution of measuring capacitance. In each application, the measurement time can be adjusted accordingly based on different sensor sizes and transparent plastic film thicknesses. If the above formula is used to solve the capacitance, the following equation will be obtained:

Obviously, what people actually want to measure is the period of the output frequency. Figure 3 shows a block diagram and waveform used for cycle measurement.  diode