Semiconductor Light-Controlled Instrument Transducer with Direct PWM Output for Automatic Control Systems

This work shows that the direct PWM output electric signal, with a duty cycle controlled by light intensity, can be obtained using a circuit containing a saw-tooth voltage generator connected in series with a dc voltage source and a metal (semitransparent gate) oxide semiconductor capacitor (MOS-C). The internal PWM signal conversion occurs by the use of non-equilibrium physical processes in the semiconductor substrate of the MOS-C. The 10-20 V amplitude limited square PWM output signal is obtained by the amplification of the sensor signal with a standard 60 dB transimpedance amplifier. The amplified output signal presents positive and negative PWM waveforms that can be easily separated using diodes. The duty of the positive part is proportional to the light intensity, whereas the negative part is inversely proportional to the intensity. The frequency operating range of this proposed instrument varies from 1 Hz to a few kilohertz. The duty cycle of the PWM output signal varies from 2% to 98% when the incident light intensity varies in the microwatts range. These new transducers or sensors could be useful for automatic control, robotic applications, dimmer systems, feedback electronic systems, and non-contact optical position sensing for nulling and centering measurements.