New displacement current sensor for contactless detection of bio-activity related signals

This article describes a newly developed low-cost displacement current sensor for non-contact, non-invasive sensing of human bio-activity related signals, well suited for applications requiring remote and portable means for detection of human presence. The sensor, which consists of an input electrode forming the sensor head or antenna, an amplification stage, and a filtering stage, operates by detecting the displacement current between the human body and the sensor's antenna. The antenna comprises a thin 5 cm in diameter circular aluminium disc with a 1 cm thick lightly charged dielectric layer attached to its front surface to enhance the sensor's sensitivity. The sensor employs a simple, improvised transimpedance amplifier which uses a resistive feedback T-network to eliminate the need for ultra-high values resistors normally needed in current amplifiers required for this type of measurements. It provides an operational bandwidth of 0.5-250 Hz, and a noise level of 7.8 μV/√Hz at 1 Hz down to 30nV/√Hz at 1 kHz. Reported results, obtained in normal unshielded environment, demonstrate the sensor's remarkable capability in measuring human heart's bio-activity related signals resembling ECG at off-body distance up to 0.4 m, and skeletal muscle's movement related signals resembling EMG within 10 m off-body distance with no obstacle in between, and 5 m off-body distance with a brick wall in between.