The pixel design has two transistors, in contrast to a conventional three-transistor image sensor pixel. During each image capture cycle, the pixels are used first to record and read out the image and then to harvest energy and charge the sensor's power supply – the image sensor continuously toggles between image capture and power harvesting modes. When the camera is not used to capture images, it can be used to generate power for other devices, such as a phone or a watch.
Two-transistor image pixel circuit that can capture images and harvest energy. Source: Columbia University.
This figure shows the pixel circuit for the power-harvesting sensor. The photodiode PD is operated in photovoltaic mode with zero bias (conventional image sensor diodes are operated with back bias). The voltage of the anode of PD increases to a level proportionate to the energy of the incident light. In this case PD draws zero power to produce a voltage proportionate to the incident light, and since it is not biased it does not produce any dark current. An important feature of the design is that emitter of transistor Q1 can be switched between ground (for resetting) and a power supply (for harvesting).
"Digital imaging is expected to enable many emerging fields including wearable devices, sensor networks, smart environments, personalized medicine, and the Internet of Things. A camera that can function as an untethered device forever – without any external power supply – would be incredibly useful," said Professor Nayar in a statement.