Two tiny ICs and a few passive components can yield a capacitive proximity sensor with excellent sensitivity and extremely fast response time. The circuit’s operation leverages the pulse-front-edge propagation delay effect.
A high-frequency oscillator (see the figure), comprising the Schmitt trigger inverter U1A together with timing components R1 and C1, generates square wave pulses. Two independent RC networks, the adjustable reference network R4 with C2 and the sensor network R3 with CS (sensor pad capacitance), act to delay the pulses along different paths.
To set up the sensor, adjust R4 so the oscillator pulse’s rising edges arrive at the C-input of U2 very slightly later than they arrive at the D-input. With this setting, the rising edges at C clock the already high signal at D into the flip-flop. The Out signal will thus normally remain at logic high.
When a conductive or dielectric object comes near the sensor pad, however, the pad’s capacitance to ground increases. This change in capacitance increases the propagation delay of the D-input’s signal. As a result, the D-input will still be low when the clock’s rising edge reaches the C-input, changing the output to logic low to signal object detection.
Sensor pad size is the major factor determining the circuit’s sensing range. For a metal disk, the circuit detects an object’s approach at a distance of about the disk diameter. Response time for detection equals one oscillator period, which in practice can reach a fraction of microsecond. The response time cannot be lower than R3, CS time constant, though.
This circuit can serve as an extremely fast industrial proximity sensor or as part of security system. It can detect open doors or boxes or sense the approach of a human body. It has both lower response time and higher short-range accuracy than ultrasonic proximity sensors.